2018 

Pereira, Filipe Towards Predictive ScaleResolving Simulations of Turbulent External Flows PhD Thesis Instituto Superior Tecnico, 2018. Abstract  Links  BibTeX  Tags: Chow Wing, Circular cylinder, DDES, EARSM, PANS, SRS, Validation, Verification @phdthesis{2018PhD_FilipePereira, title = {Towards Predictive ScaleResolving Simulations of Turbulent External Flows}, author = {Filipe Pereira}, url = { http://www.refresco.org/download/2018phd_filipepereirapdf/}, year = {2018}, date = {20180302}, school = {Instituto Superior Tecnico}, abstract = {This work investigates the requisites and aptitude of SRS methods to achieve modelling accuracies that allow their use for predictive computations of turbulent external flows with practical interest. Selected models are therefore examined through verification and validation exercises using representative flows as validation space: a circular cylinder in crossflow at Reynolds numbers of Re = 3.9E3 and 1.40E5 (statistically unsteady problems); flow around the KVLCC2 tanker at model and fullscale, Re = 4.6E6 and 2.03E9; and flow past a wing at ten degrees of angle of attack and Re = 4.0E6 (statistically steady problems). The analysed models are the DDES, IDDES, XLES, DXLES, and PANS methods. Such exercises are intended to i) estimate the requisites to reduce numerical and input errors below the modelling error; ii) quantify the modelling error; and iii) physically interpret the results to ascertain their validity. Various RANS closures are also evaluated for comparison with the SRS methods. The results illustrate the advantages of SRS methods in the prediction of statistically unsteady flows. Yet, their correct application is rife with challenges: demanding numerical requisites; difficulties setting appropriate boundaryconditions and computational domain; dependence on commutation errors; and the complexity involved in the selection of the physical resolution and closure strategy. A set of guidelines and conditions are proposed to accurately simulate turbulent wake flows driven by coherent structures. The study also shows that EARSM (RANS) closures might be an efficient alternative for some of these flow problems. On the other hand, for statistically steady flows the results indicate that SRS methods are less advantageous. In these cases, advanced RANS closures seem more adequate for predicting meanflow quantities.}, keywords = {Chow Wing, Circular cylinder, DDES, EARSM, PANS, SRS, Validation, Verification}, pubstate = {published}, tppubtype = {phdthesis} } This work investigates the requisites and aptitude of SRS methods to achieve modelling accuracies that allow their use for predictive computations of turbulent external flows with practical interest. Selected models are therefore examined through verification and validation exercises using representative flows as validation space: a circular cylinder in crossflow at Reynolds numbers of Re = 3.9E3 and 1.40E5 (statistically unsteady problems); flow around the KVLCC2 tanker at model and fullscale, Re = 4.6E6 and 2.03E9; and flow past a wing at ten degrees of angle of attack and Re = 4.0E6 (statistically steady problems). The analysed models are the DDES, IDDES, XLES, DXLES, and PANS methods. Such exercises are intended to i) estimate the requisites to reduce numerical and input errors below the modelling error; ii) quantify the modelling error; and iii) physically interpret the results to ascertain their validity. Various RANS closures are also evaluated for comparison with the SRS methods. The results illustrate the advantages of SRS methods in the prediction of statistically unsteady flows. Yet, their correct application is rife with challenges: demanding numerical requisites; difficulties setting appropriate boundaryconditions and computational domain; dependence on commutation errors; and the complexity involved in the selection of the physical resolution and closure strategy. A set of guidelines and conditions are proposed to accurately simulate turbulent wake flows driven by coherent structures. The study also shows that EARSM (RANS) closures might be an efficient alternative for some of these flow problems. On the other hand, for statistically steady flows the results indicate that SRS methods are less advantageous. In these cases, advanced RANS closures seem more adequate for predicting meanflow quantities.  
2017 

Hawkes, James Chaotic Methods for the Strong Scalability of CFD PhD Thesis University of Southampton, 2017. Abstract  Links  BibTeX  Tags: Chaotic Solvers, Exascale, Parallelization, RANS, Solvers @phdthesis{2017PhDJamesHawkes, title = {Chaotic Methods for the Strong Scalability of CFD}, author = {James Hawkes}, url = {http://www.refresco.org/download/2017phdjameshawkespdf/}, year = {2017}, date = {20170824}, school = {University of Southampton}, abstract = {Supercomputing power has been doubling approximately every 14 months for at least three decades, increasing the capabilities of scientic modelling at a similar rate. The first machines capable of one ExaFLOP (1018 foatingpoint operations per second) are expected by 2020. However, architectural changes required to reach `exascale' are significant, with energy efficiency constraints leading to a huge growth in parallelization. A new era of computing has arrived, dubbed the `manycore' era, in which the number of computing cores is increasing faster than CFD simulation sizes { prompting the research question for this thesis: `What limits the strong scalability of CFD and its ability to handle manycore architectures? What can be done to improve the CFD algorithms in this respect?' A number of scalability investigations have been performed from 1 through to 2048 cores, using a semiimplicit, finitevolume CFD code: ReFRESCO; and the University of Southampton supercomputer: Iridis4. The main bottleneck to strong scalability is shown to be the linear equationsystem solvers, occupying up to 95% of total walltime on 2048 cores { where the poor scalability arises from synchronous, global, interprocess communications. Experiments have been performed with alternative, stateoftheart linear solvers and preconditioners, without significant improvements, which motivates novel research into scalable linear solvers for CFD. The theory of `chaotic relaxation' has been used to create a completely asynchronous Jacobilike `chaotic solver', showing almost perfect scalability, and performance far greater than their synchronous counterparts. However, these solvers lack the absolute numerical power to compete with existing solvers, especially as the resolution of the simulations increases. Following this, chaotic relaxation theory has been used to create a novel `chaoticcycle' multigrid solver, combining aspects of the chaotic solver and classical multigrid methods. Both of the solvers have been verfiied and tested using canonical test cases and practical CFD simulations. On 2048 cores, the chaoticcycle multigrid solver performs up to 7:7x faster than a typical Krylov Subspace solver and 13:3x faster than classical Vcycle multigrid. With improvements to the implementation of coarsegrid communications and desynchronized residual computations, it is likely that the chaoticcycle multigrid method will continue scaling to many thousands of cores, thus removing the main bottleneck to the strongscalability of CFD. The novel chaotic solver and chaoticcycle multigrid methods have been implemented as an opensource library, Chaos. It is hoped that work on these scalable solvers can be continued and applied to other disciplines.}, keywords = {Chaotic Solvers, Exascale, Parallelization, RANS, Solvers}, pubstate = {published}, tppubtype = {phdthesis} } Supercomputing power has been doubling approximately every 14 months for at least three decades, increasing the capabilities of scientic modelling at a similar rate. The first machines capable of one ExaFLOP (1018 foatingpoint operations per second) are expected by 2020. However, architectural changes required to reach `exascale' are significant, with energy efficiency constraints leading to a huge growth in parallelization. A new era of computing has arrived, dubbed the `manycore' era, in which the number of computing cores is increasing faster than CFD simulation sizes { prompting the research question for this thesis: `What limits the strong scalability of CFD and its ability to handle manycore architectures? What can be done to improve the CFD algorithms in this respect?' A number of scalability investigations have been performed from 1 through to 2048 cores, using a semiimplicit, finitevolume CFD code: ReFRESCO; and the University of Southampton supercomputer: Iridis4. The main bottleneck to strong scalability is shown to be the linear equationsystem solvers, occupying up to 95% of total walltime on 2048 cores { where the poor scalability arises from synchronous, global, interprocess communications. Experiments have been performed with alternative, stateoftheart linear solvers and preconditioners, without significant improvements, which motivates novel research into scalable linear solvers for CFD. The theory of `chaotic relaxation' has been used to create a completely asynchronous Jacobilike `chaotic solver', showing almost perfect scalability, and performance far greater than their synchronous counterparts. However, these solvers lack the absolute numerical power to compete with existing solvers, especially as the resolution of the simulations increases. Following this, chaotic relaxation theory has been used to create a novel `chaoticcycle' multigrid solver, combining aspects of the chaotic solver and classical multigrid methods. Both of the solvers have been verfiied and tested using canonical test cases and practical CFD simulations. On 2048 cores, the chaoticcycle multigrid solver performs up to 7:7x faster than a typical Krylov Subspace solver and 13:3x faster than classical Vcycle multigrid. With improvements to the implementation of coarsegrid communications and desynchronized residual computations, it is likely that the chaoticcycle multigrid method will continue scaling to many thousands of cores, thus removing the main bottleneck to the strongscalability of CFD. The novel chaotic solver and chaoticcycle multigrid methods have been implemented as an opensource library, Chaos. It is hoped that work on these scalable solvers can be continued and applied to other disciplines.  
2016 

Nanda, Swaraj Flow past a squareprism: A numerical study Masters Thesis TU Delft, 2016. Abstract  Links  BibTeX  Tags: Flow, numerical, Squareprism @mastersthesis{2016Msc_Thesis_SwarajNanda, title = {Flow past a squareprism: A numerical study}, author = {Swaraj Nanda}, url = {http://www.refresco.org/download/2016msc_thesis_swarajnanda/}, year = {2016}, date = {20160517}, address = {Delft}, school = {TU Delft}, abstract = {The ERCOFTAC test case for flow past a squareprism at a Reynolds number of 21,400 was studied numerically using the flow solver ReFRESCO. The main objectives were to evaluate the performance of a Reynolds Averaged NavierStokes (RANS) based turbulence models in Unsteady RANS (URANS) simulations, identify sensitivities to numerical parameters and find out if DetachedEddy Simulation (DES) based turbulence models provide lower modelling errors than in URANS simulations for this practical testcase. In this respect, the effect of the iterative error and the discretization error using k  omega SST2003 model was studied and turbulence models such as the SSTDDES model and the TNTXLES model were also applied. The results using the URANS results have highlighted the importance of small temporal and spatial resolution in the calculation domain in order to reduce the effect of the discretization errors. Moreover, large timeintegration periods (larger than 60 shedding cycles) should be required to ensure that the statistical noise does not affect the solution Verification procedure which would otherwise hamper the estimated numerical uncertainties. As expected from the studies of Pereira et al., the results with the DES based turbulence models have shown that numerical settings for URANS are insufficient for DES and large comparison errors have been found for many quantities such as the timeaveraged pressure distribution at the surface and the integral quantities. The numerical studies with the k  omega SST model have shown that if the numerical uncertainties identified in the study are controlled, Verification & Validation studies using this model could provide low modelling errors for this testcase for the investigated mean flow quantities. For the DES results, since the poor comparisons with experimental results with respect to URANS results using the k  omega SST (2003) turbulence model are caused by poor numerical settings, studies using finer grids, smaller timesteps and lower iterative criteria than the ones used in the present work are recommended.}, keywords = {Flow, numerical, Squareprism}, pubstate = {published}, tppubtype = {mastersthesis} } The ERCOFTAC test case for flow past a squareprism at a Reynolds number of 21,400 was studied numerically using the flow solver ReFRESCO. The main objectives were to evaluate the performance of a Reynolds Averaged NavierStokes (RANS) based turbulence models in Unsteady RANS (URANS) simulations, identify sensitivities to numerical parameters and find out if DetachedEddy Simulation (DES) based turbulence models provide lower modelling errors than in URANS simulations for this practical testcase. In this respect, the effect of the iterative error and the discretization error using k  omega SST2003 model was studied and turbulence models such as the SSTDDES model and the TNTXLES model were also applied. The results using the URANS results have highlighted the importance of small temporal and spatial resolution in the calculation domain in order to reduce the effect of the discretization errors. Moreover, large timeintegration periods (larger than 60 shedding cycles) should be required to ensure that the statistical noise does not affect the solution Verification procedure which would otherwise hamper the estimated numerical uncertainties. As expected from the studies of Pereira et al., the results with the DES based turbulence models have shown that numerical settings for URANS are insufficient for DES and large comparison errors have been found for many quantities such as the timeaveraged pressure distribution at the surface and the integral quantities. The numerical studies with the k  omega SST model have shown that if the numerical uncertainties identified in the study are controlled, Verification & Validation studies using this model could provide low modelling errors for this testcase for the investigated mean flow quantities. For the DES results, since the poor comparisons with experimental results with respect to URANS results using the k  omega SST (2003) turbulence model are caused by poor numerical settings, studies using finer grids, smaller timesteps and lower iterative criteria than the ones used in the present work are recommended.  
Gharraee, Behrad Numerical Simulation of Cavitation on a Tidal Turbine using ReFRESCO Masters Thesis Chalmers University of Technology, 2016. Abstract  Links  BibTeX  Tags: Cavitation, Current Turbines, KSKL, RANS, SST, URANS, Verification @mastersthesis{2016Msc_Thesis_Gharraee, title = {Numerical Simulation of Cavitation on a Tidal Turbine using ReFRESCO}, author = {Behrad Gharraee}, url = { http://www.refresco.org/download/2016msc_thesis_gharraeepdf/}, year = {2016}, date = {20160104}, address = {Gothenburg}, school = {Chalmers University of Technology}, abstract = {As renewable energies continue to grow their share in the global energy landscape, marine resources present an inexhaustible potential to provide the ever increasing human settlements energy demands. Tidal energy conversion technologies enjoy the benefits of the accurately predictable and highly reliable resources, while promising great power to weight ratio due to the relatively small size of the equipment compared with offshore wind for instance. There are various prototypes being tested today and some proposals are employing floating structures as the platform for the energy converters, the design of which is driven by the higher kinetic energy content of the streams close to the water surface. Such concepts increase the turbines susceptibility to cavitation. There has been very little explicit research performed on the cavitation behavior of tidal turbines and this thesis attempts to establish one such study to enable and promote future investigations. The specialized hydrodynamic RANS solver ReFRESCO is used with the builtin Sauer cavitation model. Structured grids have been employed. The effectiveness of an eddyviscosity modification method known as the Reboud correction is also subject of investigation for improving dynamic behavior of cavities. Two different turbulence models used are kOmega SST (SST2003) and kskL. A threebladed model scale Horizontal Axis Tidal Turbine (HATT) is numerically simulated in openwater conditions in an attempt to reproduce previous EFD results from the University of Southampton, thus validating the numerical procedures in use. The simulations are performed through three stages where initially a steady solution is obtained, then the simulation becomes transient and finally the cavitation model is switched on. The results are validated against experiments via nondimensionalized parameters for thrust and torque, which prove satisfactory. General flow shows good agreement with experimental observations and the cavity formation appears to be accurate regarding both its position and blade coverage. Interestingly a cavity is observed near the leading edge on the pressure side. The simulations fail to resolve the details near the closure line of the sheet cavity which is attributed to inadequate meshing resolution. Very little dynamic behavior of the cavity structure is observed specifically where a "horseshoe" cavity structure had been detected during EFD, which will be subject to future work.}, keywords = {Cavitation, Current Turbines, KSKL, RANS, SST, URANS, Verification}, pubstate = {published}, tppubtype = {mastersthesis} } As renewable energies continue to grow their share in the global energy landscape, marine resources present an inexhaustible potential to provide the ever increasing human settlements energy demands. Tidal energy conversion technologies enjoy the benefits of the accurately predictable and highly reliable resources, while promising great power to weight ratio due to the relatively small size of the equipment compared with offshore wind for instance. There are various prototypes being tested today and some proposals are employing floating structures as the platform for the energy converters, the design of which is driven by the higher kinetic energy content of the streams close to the water surface. Such concepts increase the turbines susceptibility to cavitation. There has been very little explicit research performed on the cavitation behavior of tidal turbines and this thesis attempts to establish one such study to enable and promote future investigations. The specialized hydrodynamic RANS solver ReFRESCO is used with the builtin Sauer cavitation model. Structured grids have been employed. The effectiveness of an eddyviscosity modification method known as the Reboud correction is also subject of investigation for improving dynamic behavior of cavities. Two different turbulence models used are kOmega SST (SST2003) and kskL. A threebladed model scale Horizontal Axis Tidal Turbine (HATT) is numerically simulated in openwater conditions in an attempt to reproduce previous EFD results from the University of Southampton, thus validating the numerical procedures in use. The simulations are performed through three stages where initially a steady solution is obtained, then the simulation becomes transient and finally the cavitation model is switched on. The results are validated against experiments via nondimensionalized parameters for thrust and torque, which prove satisfactory. General flow shows good agreement with experimental observations and the cavity formation appears to be accurate regarding both its position and blade coverage. Interestingly a cavity is observed near the leading edge on the pressure side. The simulations fail to resolve the details near the closure line of the sheet cavity which is attributed to inadequate meshing resolution. Very little dynamic behavior of the cavity structure is observed specifically where a "horseshoe" cavity structure had been detected during EFD, which will be subject to future work.  
2015 

Abreu, Hugo Rafael Lopes Aerodynamic characteristics of circular cylinder and squared cylinders with and without rounded corners Masters Thesis Instituto Superior Técnico Lisboa, 2015. Abstract  Links  BibTeX  Tags: 2D square cylinder, boundary conditions, computational domain size, Reynolds number effect, rounded corners, URANS, vortex shedding @mastersthesis{2015HugoAbreu, title = {Aerodynamic characteristics of circular cylinder and squared cylinders with and without rounded corners}, author = {Hugo Rafael Lopes Abreu}, url = {http://www.refresco.org/download/2015msc_thesis_hugoabreu/}, year = {2015}, date = {20151201}, school = {Instituto Superior Técnico Lisboa}, abstract = {Viscous flows around cylinders are a classical research topic in computational fluid dynamics (CFD) with a vast amount of practical applications in the field of aerodynamic and hydrodynamic. Many engineering applications use cylinders that range from circular crosssections to square cylinders with rounded corners. Typical Reynolds numbers of practical applications are in the range of 105 to 106 where the socalled “drag crisis” occurs. Flow simulations in such conditions are extremely challenging because the flow exhibits laminar, transitional and turbulent regions. Furthermore, due to the existence of vortex shedding, the flow is not statistically steady. Although the ReynoldsAveraged NavierStokes (RANS) equations supplemented by eddyviscosity models have evident shortcomings in such complex flows, there are several attempts published in the open literature to simulate this type of flows with such mathematical model. Due to the periodic nature of vortex shedding, ensemble averaging must be used for the definition of the mean flow and for the averaging of the mass and momentum balance. Therefore, the RANS equations are not statistically steady, which is usually designated by URANS. This thesis presents a study of the flow around a square cylinder with rounded corners based on the numerical solution of the URANS equations for bidimensional incompressible flow. We have selected the shear stress transport (SST) komega eddyviscosity model which is widely in practical engineering applications. Two different exercises are presented: an investigation of the influence of the size of the computational domain and of the pressure and turbulence quantities boundary conditions; the simulation of the flow at different Reynolds numbers to identify the flow regimes. All calculations are performed with the solver ReFRESCO. Grid/time refinement and iterative convergence studies are performed for all flow conditions to estimate the numerical uncertainty. The results obtained show a significant dependence on the size of the computational domain and on the pressure and turbulence quantities boundary conditions. For the present level of grid/time refinement there is a significant discretization error and the iterative convergence criteria used at each time step must be a lot more demanding than the usual three orders of magnitude of residual drop. Although the numerical uncertainty is not negligible, this simple mathematical model is able to capture the influence of the Reynolds number on the different flow regimes. }, keywords = {2D square cylinder, boundary conditions, computational domain size, Reynolds number effect, rounded corners, URANS, vortex shedding}, pubstate = {published}, tppubtype = {mastersthesis} } Viscous flows around cylinders are a classical research topic in computational fluid dynamics (CFD) with a vast amount of practical applications in the field of aerodynamic and hydrodynamic. Many engineering applications use cylinders that range from circular crosssections to square cylinders with rounded corners. Typical Reynolds numbers of practical applications are in the range of 105 to 106 where the socalled “drag crisis” occurs. Flow simulations in such conditions are extremely challenging because the flow exhibits laminar, transitional and turbulent regions. Furthermore, due to the existence of vortex shedding, the flow is not statistically steady. Although the ReynoldsAveraged NavierStokes (RANS) equations supplemented by eddyviscosity models have evident shortcomings in such complex flows, there are several attempts published in the open literature to simulate this type of flows with such mathematical model. Due to the periodic nature of vortex shedding, ensemble averaging must be used for the definition of the mean flow and for the averaging of the mass and momentum balance. Therefore, the RANS equations are not statistically steady, which is usually designated by URANS. This thesis presents a study of the flow around a square cylinder with rounded corners based on the numerical solution of the URANS equations for bidimensional incompressible flow. We have selected the shear stress transport (SST) komega eddyviscosity model which is widely in practical engineering applications. Two different exercises are presented: an investigation of the influence of the size of the computational domain and of the pressure and turbulence quantities boundary conditions; the simulation of the flow at different Reynolds numbers to identify the flow regimes. All calculations are performed with the solver ReFRESCO. Grid/time refinement and iterative convergence studies are performed for all flow conditions to estimate the numerical uncertainty. The results obtained show a significant dependence on the size of the computational domain and on the pressure and turbulence quantities boundary conditions. For the present level of grid/time refinement there is a significant discretization error and the iterative convergence criteria used at each time step must be a lot more demanding than the usual three orders of magnitude of residual drop. Although the numerical uncertainty is not negligible, this simple mathematical model is able to capture the influence of the Reynolds number on the different flow regimes.  
Lopes, Rui Miguel Alves Calculation of the flow around hydrofoils at moderate Reynolds numbers Masters Thesis Instituto Superior Técnico Lisboa, 2015. Abstract  Links  BibTeX  Tags: Airfoils, Computational Fluid Dynamics, Reynolds Equations, Transition, Turbulence Models @mastersthesis{2015RuiLopes, title = {Calculation of the flow around hydrofoils at moderate Reynolds numbers}, author = {Rui Miguel Alves Lopes}, url = {http://www.refresco.org/download/2015msc_thesis_ruilopes/}, year = {2015}, date = {20151201}, school = {Instituto Superior Técnico Lisboa}, abstract = {The effect of laminar to turbulent flow transition region, which can be responsible for important features at low Reynolds numbers flows, is not captured by common turbulence models, and thus ignored in most numerical calculations. Recently a new transition model presenting promising results has been proposed  the gammaRe_ model. This model is easily implemented in modern codes which resort to nonstructured grids, unlike the alternatives used until now. The goal of this thesis is to study the behaviour of this model, assessing its numerical robustness and comparing the obtained solution with experimental results and a baseline solution that uses the already extensively validated komega SST turbulence model. As a preliminary calculation, the flow over a zero pressure gradient flat plate was analysed, to study the effect of the inlet turbulent quantities. On a second stage the flow over three different airfoils was simulated, in order to evaluate the improvement of the results obtained due to the additional model. The results obtained show that the new model is capable of capturing laminar separation bubbles and natural transition, improving the modelling accuracy in most cases. However, its numerical behaviour presents poor characteristics due to higher difficulties with iterative convergence and higher influence of the discretization error. The model also suffers from a high sensitivity to the turbulence inlet conditions, aggravated by the overestimated decay of the turbulence model variables. }, keywords = {Airfoils, Computational Fluid Dynamics, Reynolds Equations, Transition, Turbulence Models}, pubstate = {published}, tppubtype = {mastersthesis} } The effect of laminar to turbulent flow transition region, which can be responsible for important features at low Reynolds numbers flows, is not captured by common turbulence models, and thus ignored in most numerical calculations. Recently a new transition model presenting promising results has been proposed  the gammaRe_ model. This model is easily implemented in modern codes which resort to nonstructured grids, unlike the alternatives used until now. The goal of this thesis is to study the behaviour of this model, assessing its numerical robustness and comparing the obtained solution with experimental results and a baseline solution that uses the already extensively validated komega SST turbulence model. As a preliminary calculation, the flow over a zero pressure gradient flat plate was analysed, to study the effect of the inlet turbulent quantities. On a second stage the flow over three different airfoils was simulated, in order to evaluate the improvement of the results obtained due to the additional model. The results obtained show that the new model is capable of capturing laminar separation bubbles and natural transition, improving the modelling accuracy in most cases. However, its numerical behaviour presents poor characteristics due to higher difficulties with iterative convergence and higher influence of the discretization error. The model also suffers from a high sensitivity to the turbulence inlet conditions, aggravated by the overestimated decay of the turbulence model variables.  
Rosetti, Guilherme University of Sao Paulo, USP, Brasil, 2015. Abstract  Links  BibTeX  Tags: 3DoF, Cylinder, Free Motion, FSI, Imposed Motion, LCTM, SAS, SRS, SST, Transition, Turbulence Models, URANS, Validation, Verification, VIV @phdthesis{2015phdgfrosettipdf, title = {Improvements in the Numerical Modeling of Turbulence and FluidStructure Interaction for the VortexInduced Vibrations of a Rigid Cylinder}, author = {Guilherme Rosetti}, url = {http://www.refresco.org/download/2015phdgfrosettipdf/}, year = {2015}, date = {20150902}, school = {University of Sao Paulo, USP, Brasil}, abstract = {This thesis presents the development, implementation and application of turbulence and laminarturbulent transition models and fluidstructure capabilities to address the vortexshedding and vortexinduced vibrations of a rigid cylinder. These numerical developments have been carried out in the computational fluid dynamics (CFD) code ReFRESCO. In the current work, an investigation of the performance of the turbulence modeling with k! SST in a broad range of Reynolds numbers is carried out identifying its modeling deficiencies for this flow. The implementation and systematic application of the scale adaptive simulations (SAS) and the local correlation transition model (LCTM), both combined with the SST, have improved the agreement with experimental results for the cylinder flow, in a novel contribution of this work. The application of verication and validation technique has allowed the estimation of numerical errors and uncertainties for the different models. That is also identified as a contribution of this thesis. The combination of SST modeling with imposed motions is carried out as well as with the SAS and LCTM for moderate Reynolds numbers, different vibration frequencies and amplitudes, which is considered novel, as few publications address this issue in extent. Regarding the freemoving cylinder capabilities, the present work brings contributions with the application of SST and SASSST with freemoving cylinder for the study of VIV of two degreesoffreedom, low mass ratio and moderate Reynolds numbers, higher than commonly seen in the literature. Finally, the investigation of the relative importance of turbulence effects on the freemoving cylinder and the imposedmotions case, with respect to the fixed case is carried out. A natural conjecture that has been raised early on this work and proved correct is that, for engineering applications, the choice of turbulence modeling strategy is less decisive when the cylinder is moving with prescribed motion and even less stringent, for free motions as the body response filters most of the higher order turbulence effects. That is a relevant observation as it might allow modeling simplifications and the application of CFD tools to a range of engineering problems.}, keywords = {3DoF, Cylinder, Free Motion, FSI, Imposed Motion, LCTM, SAS, SRS, SST, Transition, Turbulence Models, URANS, Validation, Verification, VIV}, pubstate = {published}, tppubtype = {phdthesis} } This thesis presents the development, implementation and application of turbulence and laminarturbulent transition models and fluidstructure capabilities to address the vortexshedding and vortexinduced vibrations of a rigid cylinder. These numerical developments have been carried out in the computational fluid dynamics (CFD) code ReFRESCO. In the current work, an investigation of the performance of the turbulence modeling with k! SST in a broad range of Reynolds numbers is carried out identifying its modeling deficiencies for this flow. The implementation and systematic application of the scale adaptive simulations (SAS) and the local correlation transition model (LCTM), both combined with the SST, have improved the agreement with experimental results for the cylinder flow, in a novel contribution of this work. The application of verication and validation technique has allowed the estimation of numerical errors and uncertainties for the different models. That is also identified as a contribution of this thesis. The combination of SST modeling with imposed motions is carried out as well as with the SAS and LCTM for moderate Reynolds numbers, different vibration frequencies and amplitudes, which is considered novel, as few publications address this issue in extent. Regarding the freemoving cylinder capabilities, the present work brings contributions with the application of SST and SASSST with freemoving cylinder for the study of VIV of two degreesoffreedom, low mass ratio and moderate Reynolds numbers, higher than commonly seen in the literature. Finally, the investigation of the relative importance of turbulence effects on the freemoving cylinder and the imposedmotions case, with respect to the fixed case is carried out. A natural conjecture that has been raised early on this work and proved correct is that, for engineering applications, the choice of turbulence modeling strategy is less decisive when the cylinder is moving with prescribed motion and even less stringent, for free motions as the body response filters most of the higher order turbulence effects. That is a relevant observation as it might allow modeling simplifications and the application of CFD tools to a range of engineering problems.  
Rosmarino, Alessandra PREDICTION OF HYDRODYNAMIC PERFORMANCES OF SAILING YACHTS, COMPUTED USING CFD CODES Masters Thesis University of Study of Genova, Italy, 2015. Abstract  Links  BibTeX  Tags: Drag, Lift, Manoeuvring, RANS, SST, Validation, Yachts @mastersthesis{2015Msc_Thesis_ARosmarino, title = {PREDICTION OF HYDRODYNAMIC PERFORMANCES OF SAILING YACHTS, COMPUTED USING CFD CODES}, author = {Alessandra Rosmarino}, url = {http://www.refresco.org/?p=1397}, year = {2015}, date = {20150331}, school = {University of Study of Genova, Italy}, abstract = {The present work deals with the prediction of performance of sailing yachts as computed with inhouse CFD codes. The main part of this study is represented by the process made in order to run steady computations regarding two hulls, sailing with different heel and yaw angles. Since ReFRESCO does not compute the free surface and neither dynamic trim and sinkage, several tools such as RAPID for the free surface generation have been used. Finally the results show that is possible to predict the drag with an accuracy of 2%, and good results can also be obtained for the lift. Further studies are required with a higher refinement level especially close to the keel, and investigations on the influence of different parameters on the quality of the wave pattern obtained from RAPID are necessary. Furthermore a trade off study has to be done in order to assess if the higher accuracy expected from a total RANS approach will compensate the higher computing effort that will be required.}, keywords = {Drag, Lift, Manoeuvring, RANS, SST, Validation, Yachts}, pubstate = {published}, tppubtype = {mastersthesis} } The present work deals with the prediction of performance of sailing yachts as computed with inhouse CFD codes. The main part of this study is represented by the process made in order to run steady computations regarding two hulls, sailing with different heel and yaw angles. Since ReFRESCO does not compute the free surface and neither dynamic trim and sinkage, several tools such as RAPID for the free surface generation have been used. Finally the results show that is possible to predict the drag with an accuracy of 2%, and good results can also be obtained for the lift. Further studies are required with a higher refinement level especially close to the keel, and investigations on the influence of different parameters on the quality of the wave pattern obtained from RAPID are necessary. Furthermore a trade off study has to be done in order to assess if the higher accuracy expected from a total RANS approach will compensate the higher computing effort that will be required.  
Moura, Alceu José Dos Santos CFD Study on Reynolds effects and turbulence modelling for a skewed propeller Masters Thesis 2015. Abstract  Links  BibTeX  Tags: CFD, Propeller, ReynoldsEffects, Turbulence Models @mastersthesis{2015AlceuMoura, title = {CFD Study on Reynolds effects and turbulence modelling for a skewed propeller}, author = { Alceu José Dos Santos Moura }, url = {http://www.refresco.org/download/2015stage_alceumoura/}, year = {2015}, date = {20150301}, abstract = {The performance characteristics of a propeller are frequently determined in model tests that are relatively expensive and time consuming. Furthermore, due to the methods of testing model propellers and the consequent changes in Reynolds number between model and full scale, there can arise a different boundary layer structure to the flow over the blades that affect the performance characteristics. Whilst it is generally recognized that most fullscale propellers will have a primarily turbulent flow over the blade surface this need not be the case for the model where characteristics related to laminar flow can prevail over significant parts of the blade [1]. In order to quantify the effect of scale and predict fullscale performance the most usual way is applying some semiempirical scaling laws. ITTC’78 performance prediction method is such an approach that has been widely used over decades and it is described in [2]. In addition to experiments and with the advancement of computing power, numerical simulations are increasingly being used to obtain enhanced understanding of the flow, extracting detailed flow field quantities in any location of the flow field, and also has the ability to perform analysis of propellers at both model and full scale. In this report the results of the study of the effect of different grid density, Reynolds number and model turbulence on performance prediction of a skewed propeller design using the MARIN inhouse CFD (Computational Fluid Dynamics) solver ReFRESCO is presented. For this the report is structured as follows: Chapter 2 gives the definitions of relevant parameters. Chapter 3 presents the propeller characteristics, shows an overview of the grids used for the simulations and also shows the numerical setup for the open water simulations. The model scale performance prediction is discussed in Chapter 4 including an estimate of the numerical uncertainties, a comparison with experimental results and also the influence of the applied turbulence model. The Chapter 5 shows the variation in performance prediction and boundarylayer flow for the fullscale condition. And finally the Chapter 6 ends with some concluding remarks of the results. }, keywords = {CFD, Propeller, ReynoldsEffects, Turbulence Models}, pubstate = {published}, tppubtype = {mastersthesis} } The performance characteristics of a propeller are frequently determined in model tests that are relatively expensive and time consuming. Furthermore, due to the methods of testing model propellers and the consequent changes in Reynolds number between model and full scale, there can arise a different boundary layer structure to the flow over the blades that affect the performance characteristics. Whilst it is generally recognized that most fullscale propellers will have a primarily turbulent flow over the blade surface this need not be the case for the model where characteristics related to laminar flow can prevail over significant parts of the blade [1]. In order to quantify the effect of scale and predict fullscale performance the most usual way is applying some semiempirical scaling laws. ITTC’78 performance prediction method is such an approach that has been widely used over decades and it is described in [2]. In addition to experiments and with the advancement of computing power, numerical simulations are increasingly being used to obtain enhanced understanding of the flow, extracting detailed flow field quantities in any location of the flow field, and also has the ability to perform analysis of propellers at both model and full scale. In this report the results of the study of the effect of different grid density, Reynolds number and model turbulence on performance prediction of a skewed propeller design using the MARIN inhouse CFD (Computational Fluid Dynamics) solver ReFRESCO is presented. For this the report is structured as follows: Chapter 2 gives the definitions of relevant parameters. Chapter 3 presents the propeller characteristics, shows an overview of the grids used for the simulations and also shows the numerical setup for the open water simulations. The model scale performance prediction is discussed in Chapter 4 including an estimate of the numerical uncertainties, a comparison with experimental results and also the influence of the applied turbulence model. The Chapter 5 shows the variation in performance prediction and boundarylayer flow for the fullscale condition. And finally the Chapter 6 ends with some concluding remarks of the results.  
2014 

Saraiva, Goncalo Solution of Flows Around Airfoils Using RANS with WallFunctions Masters Thesis IST, Lisbon, Portugal, 2014. Abstract  Links  BibTeX  Tags: Eppler, Foils, NACA 0012, RANS, SST, Validation, Verification, Wallfunctions @mastersthesis{2014Msc_Thesis_GoncaloSaraiva, title = {Solution of Flows Around Airfoils Using RANS with WallFunctions}, author = {Goncalo Saraiva}, url = { http://www.refresco.org/?wpdmpro=2014msc_thesis_goncalosaraivapdf}, year = {2014}, date = {20141024}, school = {IST, Lisbon, Portugal}, abstract = {The calculation of the friction forces is essential in hydrodynamic and offshore applications. However, the high gradients that exist in nearwall regions require the use of one of the following approaches: grids that are very fine near the wall to calculate the wall shearstress directly from its definition; or wallfunctions (WF) to calculate indirectly the wall shearstress and provide boundary conditions for the variables of the turbulence models. The objective of this thesis is to assess the validity of WF boundary conditions for the calculation of friction and pressure coefficients, as well as aerodynamic forces coefficients of a conventional and a laminar airfoil. The ReFRESCO solver was used to solve the RANS equations with the SST version of the eddyviscosity turbulence model. The main conclusions obtained were: WF can yield acceptable results if the Reynolds number is high enough to promote transition near the leading edge; if the laminar part of the flow is significant, the results are not realistic because WF lead to a fully turbulent flow; the results for the pressure and lift coefficient are always better than for friction and drag coefficients due to the direct connection of the wall shearstress with the last two; last but not least, the results of the WF approach are strongly dependent on the location of the first interior grid node, even at high Reynolds number.}, keywords = {Eppler, Foils, NACA 0012, RANS, SST, Validation, Verification, Wallfunctions}, pubstate = {published}, tppubtype = {mastersthesis} } The calculation of the friction forces is essential in hydrodynamic and offshore applications. However, the high gradients that exist in nearwall regions require the use of one of the following approaches: grids that are very fine near the wall to calculate the wall shearstress directly from its definition; or wallfunctions (WF) to calculate indirectly the wall shearstress and provide boundary conditions for the variables of the turbulence models. The objective of this thesis is to assess the validity of WF boundary conditions for the calculation of friction and pressure coefficients, as well as aerodynamic forces coefficients of a conventional and a laminar airfoil. The ReFRESCO solver was used to solve the RANS equations with the SST version of the eddyviscosity turbulence model. The main conclusions obtained were: WF can yield acceptable results if the Reynolds number is high enough to promote transition near the leading edge; if the laminar part of the flow is significant, the results are not realistic because WF lead to a fully turbulent flow; the results for the pressure and lift coefficient are always better than for friction and drag coefficients due to the direct connection of the wall shearstress with the last two; last but not least, the results of the WF approach are strongly dependent on the location of the first interior grid node, even at high Reynolds number.  
Corbineau, Erwan Verification of REFRESCO for forced roll oscillations Masters Thesis ENSTA, Brest, Bretagne, France, 2014. Abstract  Links  BibTeX  Tags: Deforminggrids, FreeSurface, Rolldamping, SST, URANS, Validation, Verification @mastersthesis{2014Msc_Thesis_ErwanCorbineau, title = {Verification of REFRESCO for forced roll oscillations}, author = {Erwan Corbineau}, url = {http://www.refresco.org/download/2014msc_thesis_erwancorbineau}, year = {2014}, date = {20140821}, school = {ENSTA, Brest, Bretagne, France}, abstract = {The knowledge of the behavior of a ship in a given sea state is an essential part in ship design. To predict the flow around the hull of a boat, CFD (Computational Fluid Dynamics) is especially appreciated, because it is a good alternative to timeconsuming and expensive experimental studies. MARIN has been developing for the ten last years REFRESCO, which is an inhouse CFD code. It solves the multiphase unsteady incompressible RANS (Reynolds Averaged NavierStokes) equations, which are complemented with turbulence models and volumefraction transport equations for different phases. The objective of this Master’s Thesis is to verify REFRESCO for roll motion. A hull section with bilge keels is tested and a forced oscillating roll motion is imposed to the hull section. The roll damping caused by the bilge keels is a viscous effect, and involves turbulence. It is then necessary to see how accurate is REFRESCO, for a case implying solving nonlinear equations like roll motion. The Master’s Thesis is based on the estimation of numerical uncertainty. The best numerical settings for the study are investigated, and the accuracy of the calculated results is assessed. Extensive sensitivity studies have been performed, such as the effect of the scale, y+, the damping gain, grid and time step refinement, and iterative convergence. Sensitivity studies for different foll parameters are also performed, such as roll period and amplitude and the center of rotation.}, keywords = {Deforminggrids, FreeSurface, Rolldamping, SST, URANS, Validation, Verification}, pubstate = {published}, tppubtype = {mastersthesis} } The knowledge of the behavior of a ship in a given sea state is an essential part in ship design. To predict the flow around the hull of a boat, CFD (Computational Fluid Dynamics) is especially appreciated, because it is a good alternative to timeconsuming and expensive experimental studies. MARIN has been developing for the ten last years REFRESCO, which is an inhouse CFD code. It solves the multiphase unsteady incompressible RANS (Reynolds Averaged NavierStokes) equations, which are complemented with turbulence models and volumefraction transport equations for different phases. The objective of this Master’s Thesis is to verify REFRESCO for roll motion. A hull section with bilge keels is tested and a forced oscillating roll motion is imposed to the hull section. The roll damping caused by the bilge keels is a viscous effect, and involves turbulence. It is then necessary to see how accurate is REFRESCO, for a case implying solving nonlinear equations like roll motion. The Master’s Thesis is based on the estimation of numerical uncertainty. The best numerical settings for the study are investigated, and the accuracy of the calculated results is assessed. Extensive sensitivity studies have been performed, such as the effect of the scale, y+, the damping gain, grid and time step refinement, and iterative convergence. Sensitivity studies for different foll parameters are also performed, such as roll period and amplitude and the center of rotation.  
Make, Michel Predicting scale effects on floating offshore wind turbines Masters Thesis Technical University of Delft, the Netherlands, 2014. Abstract  Links  BibTeX  Tags: BEMT, Foils, MSWT, NREL 5MW, RANS, ScaleEffects, Scaling, SpalartAllmaras, SST, Transition, Turbines, URANS, XFOIL @mastersthesis{2014Msc_Thesis_MichelMake, title = {Predicting scale effects on floating offshore wind turbines}, author = {Michel Make}, url = { http://www.refresco.org/?wpdmpro=2014msc_thesis_michelmakepdf}, year = {2014}, date = {20140428}, school = {Technical University of Delft, the Netherlands}, abstract = {Floating wind turbines are becoming fashionable within the Renewable Energy world. In the last years MARIN has been involved in an increasing number of projects for the offshore wind industry. Model tests are often used for validating and optimizing the floater design before construction starts. A key point of model testing floating wind turbines is that wind and waves are presented simultaneously in the basin. This makes it possible to study the complex motions and interactions between the rotating turbine and the moving platform. However the experiments are done using smaller scaled models. While for the underwater loads Froude scaling laws are used successfully in the Offshore industry, the same should not be done for the aerodynamic loads. Due to the strong Reynolds scale effects, the flow regime on the blades is critical or even subcritical, and therefore laminarturbulent transition and flowseparation effects play an important role. The traditional potentialflow based tools used for design and analysis of turbines (BladeElementMomentumTheory BEMT) were not intended to work in these regimes, nor the inviscidviscous (BoundaryElementMethod BEM) tools, like XFOIL, used to obtain the turbine sections Cl/Cd/Cm input for the BEMT calculations. The complete simulation of a fullscale freefloating wind turbine under waves and winds using viscousflow (UnsteadyReynoldsAveragedNavierStokes URANS) CFD codes is still nowadays very costly, if not impossible. However these CFD theoretically more accurate methods, can be used in an efficient way for aerodynamic analysis. And they can be used rather to generate 2D input for the BEMT design tools or for the real complete analysis of the wind turbine. In the present work CFD URANS code ReFRESCO is used for both purposes, having in mind the design of the new MARIN Stock (not Floating) Wind Turbine (MSWT), based on the 5MW NREL fullscale turbine. Only openwater constant wind, fixed platform conditions are considered here. The objectives of the work presented are therefore threefold: 1) the NREL 5MW baseline turbine is calculated using ReFRESCO both in fullscale and modelscale (Froudescaling) conditions and the scaleeffects studied and quantified; 2) the MSWT designed for thrust and performancescaling is analyzed using CFD and validation against available MARIN experimental data is done; 3) in order to possibly further improve the MSWT design, the aerodynamic characteristics of its sections/foils are scrutinized by means of a full numerical study using ReFRESCO. The poor performance of the NREL 5MW turbine is due to a fully separated flow over the full range of tip speed ratios. Additionally decambering laminar separation bubbles are observed at the pressures side of the blades, further decreasing the aerodynamic performance of the turbine. Although laminar separation bubbles are not observed for the modelscale MSWT, separation does occur over the full span of the suction side of the blades. For the performancescaled MSWT, however, an attached flow region is observed at the blade tips for the higher tip speed ratios, resulting in increased CP /CT values and performance. Flow separation at fullscale conditions is present only for the heavily loaded operating conditions. These separated regions show large radial velocity components, which contradict the assumed 2D flow in BEMT models. The separated flow is also observed for the flow over the 2D airfoil sections of the MSWT. Even for small angles of attack at modelscale Reynolds numbers, separation occurs and URANS computations are necessary for larger angles of attack. For the fullscale Reynolds number regime the flow remains attached up to larger angles of attack and URANS computations are needed only for the extreme angles of attack (AoA > 14deg). The 2D flow phenomena at model and fullscale are in line with those observed for the flow over the 3D turbine. Although the MSWT has already greatly improved modelscale performance characteristics, the present research indicate that more improvements are perhaps possible. An alternative pitch angle distribution can be considered in order to reduce flow separation for even lower TSRs. Furthermore the present work showed the challenge of obtaining accurate numerical solutions for the complex unsteady flow over a wind turbine at these critical Reynolds numbers, which requires: domain studies, grid and timestep studies, good iterative convergence and an adequate turbulence model. All of these aspects were studied in this thesis.}, keywords = {BEMT, Foils, MSWT, NREL 5MW, RANS, ScaleEffects, Scaling, SpalartAllmaras, SST, Transition, Turbines, URANS, XFOIL}, pubstate = {published}, tppubtype = {mastersthesis} } Floating wind turbines are becoming fashionable within the Renewable Energy world. In the last years MARIN has been involved in an increasing number of projects for the offshore wind industry. Model tests are often used for validating and optimizing the floater design before construction starts. A key point of model testing floating wind turbines is that wind and waves are presented simultaneously in the basin. This makes it possible to study the complex motions and interactions between the rotating turbine and the moving platform. However the experiments are done using smaller scaled models. While for the underwater loads Froude scaling laws are used successfully in the Offshore industry, the same should not be done for the aerodynamic loads. Due to the strong Reynolds scale effects, the flow regime on the blades is critical or even subcritical, and therefore laminarturbulent transition and flowseparation effects play an important role. The traditional potentialflow based tools used for design and analysis of turbines (BladeElementMomentumTheory BEMT) were not intended to work in these regimes, nor the inviscidviscous (BoundaryElementMethod BEM) tools, like XFOIL, used to obtain the turbine sections Cl/Cd/Cm input for the BEMT calculations. The complete simulation of a fullscale freefloating wind turbine under waves and winds using viscousflow (UnsteadyReynoldsAveragedNavierStokes URANS) CFD codes is still nowadays very costly, if not impossible. However these CFD theoretically more accurate methods, can be used in an efficient way for aerodynamic analysis. And they can be used rather to generate 2D input for the BEMT design tools or for the real complete analysis of the wind turbine. In the present work CFD URANS code ReFRESCO is used for both purposes, having in mind the design of the new MARIN Stock (not Floating) Wind Turbine (MSWT), based on the 5MW NREL fullscale turbine. Only openwater constant wind, fixed platform conditions are considered here. The objectives of the work presented are therefore threefold: 1) the NREL 5MW baseline turbine is calculated using ReFRESCO both in fullscale and modelscale (Froudescaling) conditions and the scaleeffects studied and quantified; 2) the MSWT designed for thrust and performancescaling is analyzed using CFD and validation against available MARIN experimental data is done; 3) in order to possibly further improve the MSWT design, the aerodynamic characteristics of its sections/foils are scrutinized by means of a full numerical study using ReFRESCO. The poor performance of the NREL 5MW turbine is due to a fully separated flow over the full range of tip speed ratios. Additionally decambering laminar separation bubbles are observed at the pressures side of the blades, further decreasing the aerodynamic performance of the turbine. Although laminar separation bubbles are not observed for the modelscale MSWT, separation does occur over the full span of the suction side of the blades. For the performancescaled MSWT, however, an attached flow region is observed at the blade tips for the higher tip speed ratios, resulting in increased CP /CT values and performance. Flow separation at fullscale conditions is present only for the heavily loaded operating conditions. These separated regions show large radial velocity components, which contradict the assumed 2D flow in BEMT models. The separated flow is also observed for the flow over the 2D airfoil sections of the MSWT. Even for small angles of attack at modelscale Reynolds numbers, separation occurs and URANS computations are necessary for larger angles of attack. For the fullscale Reynolds number regime the flow remains attached up to larger angles of attack and URANS computations are needed only for the extreme angles of attack (AoA > 14deg). The 2D flow phenomena at model and fullscale are in line with those observed for the flow over the 3D turbine. Although the MSWT has already greatly improved modelscale performance characteristics, the present research indicate that more improvements are perhaps possible. An alternative pitch angle distribution can be considered in order to reduce flow separation for even lower TSRs. Furthermore the present work showed the challenge of obtaining accurate numerical solutions for the complex unsteady flow over a wind turbine at these critical Reynolds numbers, which requires: domain studies, grid and timestep studies, good iterative convergence and an adequate turbulence model. All of these aspects were studied in this thesis.  
Burmester, Simon Calculation and Validation of WaveinDeck Loads on a Fixed Platform Deck with CFD Masters Thesis University of DuisburgEssen, Germany, 2014. Abstract  Links  BibTeX  Tags: Damping Zones, FlapMotion, FreeSurface, impacts, Irregular Waves, Regular Waves, URANS, Waves @mastersthesis{2014Msc_Thesis_SimonBurmester, title = {Calculation and Validation of WaveinDeck Loads on a Fixed Platform Deck with CFD}, author = {Simon Burmester}, url = {http://www.refresco.org/?post_type=wpdmpro&p=886}, year = {2014}, date = {20140117}, school = {University of DuisburgEssen, Germany}, abstract = {In this master thesis, a code to generate waves in a numerical domain based on the wave flap motions from a model test basin is presented. These wave flap motions were taken from wave sequences investigated in the ShorTCresT JIP. A transfer function developed by Biésel and Suquet [3] to establish a relation between the wave flap motion and the wave elevation is implemented. Additionally, a velocity field for the inflow is calculated by the flap motion driving signal. This signal is enlarged with the zero padding method (see Engelberg [8]) to calculate various timesteps. The following waves are generated with this code: • regular wave • irregular longcrested wave Furthermore, the possibility to generate shortcrested waves is provided. All needed theories associated with these waves, the used numerical methods and made assumptions are described in this thesis. Numerical studies to obtain the optimal numerical settings are carried out and the results are validated with experimental measurements out of the Seakeeping and Manoeuvring Basin at MARIN. Thus, several twodimensional computations are conducted. These computations are carried out with and without a fixed platform deck. The simulations without platform deck are used to compare the wave propagation. The simulations with platform deck are used for a comparison with the measured vertical deck loads. The test setup of the ShorTCresT JIP is applied in the simulations. }, keywords = {Damping Zones, FlapMotion, FreeSurface, impacts, Irregular Waves, Regular Waves, URANS, Waves}, pubstate = {published}, tppubtype = {mastersthesis} } In this master thesis, a code to generate waves in a numerical domain based on the wave flap motions from a model test basin is presented. These wave flap motions were taken from wave sequences investigated in the ShorTCresT JIP. A transfer function developed by Biésel and Suquet [3] to establish a relation between the wave flap motion and the wave elevation is implemented. Additionally, a velocity field for the inflow is calculated by the flap motion driving signal. This signal is enlarged with the zero padding method (see Engelberg [8]) to calculate various timesteps. The following waves are generated with this code: • regular wave • irregular longcrested wave Furthermore, the possibility to generate shortcrested waves is provided. All needed theories associated with these waves, the used numerical methods and made assumptions are described in this thesis. Numerical studies to obtain the optimal numerical settings are carried out and the results are validated with experimental measurements out of the Seakeeping and Manoeuvring Basin at MARIN. Thus, several twodimensional computations are conducted. These computations are carried out with and without a fixed platform deck. The simulations without platform deck are used to compare the wave propagation. The simulations with platform deck are used for a comparison with the measured vertical deck loads. The test setup of the ShorTCresT JIP is applied in the simulations.  
2013 

Aurelle, Julian Prediction of Wave Loading from Steep Waves on Fixed Offshore Wind Turbines Masters Thesis Ecole Centrale de Nantes, France, 2013. Links  BibTeX  Tags: impacts, Regular Waves, Turbines, Validation, Verification @mastersthesis{2013Stage_Julian_Aurelle, title = {Prediction of Wave Loading from Steep Waves on Fixed Offshore Wind Turbines}, author = {Julian Aurelle}, url = {http://www.refresco.org/?wpdmpro=2013stage_julian_aurellepdf}, year = {2013}, date = {20131231}, school = {Ecole Centrale de Nantes, France}, keywords = {impacts, Regular Waves, Turbines, Validation, Verification}, pubstate = {published}, tppubtype = {mastersthesis} }  
Kuin, Roderick HYDRODYNAMIC IMPROVEMENTS OF A GENERIC SUBMARINE USING VISCOUS FLOW CALCULATIONS Masters Thesis University of Twente, Enschede, the Netherlands, 2013. Abstract  Links  BibTeX  Tags: Design, Drift, Manoeuvring, RANS, Rotation, SST, Submarines, Validation, Verification @mastersthesis{2013Msc_Thesis_RoderickKuin, title = {HYDRODYNAMIC IMPROVEMENTS OF A GENERIC SUBMARINE USING VISCOUS FLOW CALCULATIONS}, author = {Roderick Kuin}, url = {http://www.refresco.org/?wpdmpro=2013msc_thesis_roderickkuinpdf}, year = {2013}, date = {20131231}, school = {University of Twente, Enschede, the Netherlands}, abstract = {Generally, underwater vehicles such as gliders or submarines have a hull or fuselage shape with low drag properties. However, additional appendages are generally required for control or storage of equipment. These appendages induce additional resistance and may be detrimental to the quality of the inflow to the aft control surfaces or propeller. This, in turn, can lead to loss of propulsion performance or increase of vibrations and radiated noise. The underlying hydrodynamic mechanism is the penetration by the appendage of the boundary layer developing on the hull, which causes the formation of a socalled horseshoe vortex in a region of separated flow near the stagnation area on the appendage. Computational Fluid Dynamics (CFD) has matured to a state that it can be applied successfully to investigate and optimise the flow around ships and offshore structures. In this research, CFD is used to study the flow around a typical wingbody junction in order to obtain insight in how to suppress the horseshoe vortex that is wrapped around the appendage. A generic submarine hull shape has been selected and the impact of a range of modifications of the sail (sometimes called fin or fairwater) on the resistance, propulsion, manoeuvring and wake field have been investigated. To quantify the nonuniformity of the wake field, a socalled Wake Object Function (WOF) is used. The WOF is defined such that decreasing its value reduces the chance of (erosive) cavitation and radiated noise. This research presents the results of the CFD computations for a number of sail variants and discusses the changes in the flow in detail. Design guidelines regarding the most promising modifications have been developed. It is shown that a thicker and a tapered sail have a significant negative influence on the main hydrodynamic characteristics, however, quite significant improvements of the resistance as well as the wake quality can be obtained by properly designing the junction between the sail and the hull.}, keywords = {Design, Drift, Manoeuvring, RANS, Rotation, SST, Submarines, Validation, Verification}, pubstate = {published}, tppubtype = {mastersthesis} } Generally, underwater vehicles such as gliders or submarines have a hull or fuselage shape with low drag properties. However, additional appendages are generally required for control or storage of equipment. These appendages induce additional resistance and may be detrimental to the quality of the inflow to the aft control surfaces or propeller. This, in turn, can lead to loss of propulsion performance or increase of vibrations and radiated noise. The underlying hydrodynamic mechanism is the penetration by the appendage of the boundary layer developing on the hull, which causes the formation of a socalled horseshoe vortex in a region of separated flow near the stagnation area on the appendage. Computational Fluid Dynamics (CFD) has matured to a state that it can be applied successfully to investigate and optimise the flow around ships and offshore structures. In this research, CFD is used to study the flow around a typical wingbody junction in order to obtain insight in how to suppress the horseshoe vortex that is wrapped around the appendage. A generic submarine hull shape has been selected and the impact of a range of modifications of the sail (sometimes called fin or fairwater) on the resistance, propulsion, manoeuvring and wake field have been investigated. To quantify the nonuniformity of the wake field, a socalled Wake Object Function (WOF) is used. The WOF is defined such that decreasing its value reduces the chance of (erosive) cavitation and radiated noise. This research presents the results of the CFD computations for a number of sail variants and discusses the changes in the flow in detail. Design guidelines regarding the most promising modifications have been developed. It is shown that a thicker and a tapered sail have a significant negative influence on the main hydrodynamic characteristics, however, quite significant improvements of the resistance as well as the wake quality can be obtained by properly designing the junction between the sail and the hull.  
Willemsen, Christiaan Improving Potential Flow Predictions for Ducted Propellers Masters Thesis University of Twente, Enschede, the Netherlands, 2013. Abstract  Links  BibTeX  Tags: Ducts, Propeller, RANS, RANSBEM Coupling, SST, Validation, Verification @mastersthesis{2013Msc_Thesis_ChrisWillemsen, title = {Improving Potential Flow Predictions for Ducted Propellers}, author = {Christiaan Willemsen}, url = {http://www.refresco.org/?wpdmpro=2013msc_thesis_chriswillemsenpdf}, year = {2013}, date = {20131213}, school = {University of Twente, Enschede, the Netherlands}, abstract = {The advantages of propulsion systems using a thrust generating duct around a propeller are well known in naval architecture. A ducted propeller is often employed to increase the efficiency and thrust of a highly loaded propeller. The flow accelerating duct can contribute to 50 % of the propulsor total thrust at zero ship speed. There are not many fast and accurate hydrodynamic prediction methods for the design phase of ducted propellers. Model tests are expensive, while computations based on the Reynoldsaveraged NavierStokes (RANS) equations require long CPU times. Therefore these approaches are not yet routinely used in the design process of propulsors. Currently the design process is mostly based on the use of potential flow methods, like the MARIN Boundary Element Method (BEM) PROCAL. This method is efficient and is able to deliver accurate predictions of the forces acting on open propellers, but it is less accurate when viscous flow effects become important such as is the case for ducted propellers. The goal of the present research is to investigate the flow around a ducted propeller using the MARIN inhousedeveloped RANS method ReFRESCO, with particular emphasis on the in influence of the viscous flow effects such as boundary layers, tip vortices and flow separation on the outer surface of the duct. The results obtained with RANS are used to improve the prediction of PROCAL. Finally a coupling between PROCAL and ReFRESCO is accomplished to include the viscous flow effects in an efficient way. The viscous flow over the duct is annalyzed using ReFRESCO, in which the propeller action is represented by body forces added to the righthandside of the momentum equations. These body forces are obtained from a PROCAL computation for the ducted propeller in which the propeller is represented as a discrete number of blades. Results of this approach show a good agreement between experiments and the numerical simulations: the forces differ less than 1 % around the design point of the ducted propeller.}, keywords = {Ducts, Propeller, RANS, RANSBEM Coupling, SST, Validation, Verification}, pubstate = {published}, tppubtype = {mastersthesis} } The advantages of propulsion systems using a thrust generating duct around a propeller are well known in naval architecture. A ducted propeller is often employed to increase the efficiency and thrust of a highly loaded propeller. The flow accelerating duct can contribute to 50 % of the propulsor total thrust at zero ship speed. There are not many fast and accurate hydrodynamic prediction methods for the design phase of ducted propellers. Model tests are expensive, while computations based on the Reynoldsaveraged NavierStokes (RANS) equations require long CPU times. Therefore these approaches are not yet routinely used in the design process of propulsors. Currently the design process is mostly based on the use of potential flow methods, like the MARIN Boundary Element Method (BEM) PROCAL. This method is efficient and is able to deliver accurate predictions of the forces acting on open propellers, but it is less accurate when viscous flow effects become important such as is the case for ducted propellers. The goal of the present research is to investigate the flow around a ducted propeller using the MARIN inhousedeveloped RANS method ReFRESCO, with particular emphasis on the in influence of the viscous flow effects such as boundary layers, tip vortices and flow separation on the outer surface of the duct. The results obtained with RANS are used to improve the prediction of PROCAL. Finally a coupling between PROCAL and ReFRESCO is accomplished to include the viscous flow effects in an efficient way. The viscous flow over the duct is annalyzed using ReFRESCO, in which the propeller action is represented by body forces added to the righthandside of the momentum equations. These body forces are obtained from a PROCAL computation for the ducted propeller in which the propeller is represented as a discrete number of blades. Results of this approach show a good agreement between experiments and the numerical simulations: the forces differ less than 1 % around the design point of the ducted propeller.  
Haoran, Yu Analysis of The Effects of Turbulence Models on Cavitation Simulation for NACA0015 Foil Masters Thesis Technical University of Delft, the Netherlands, 2013. Links  BibTeX  Tags: Cavitation, KSKL, NACA 0015, SST, URANS @mastersthesis{2013Msc_Thesis_HaoranYu, title = {Analysis of The Effects of Turbulence Models on Cavitation Simulation for NACA0015 Foil}, author = {Yu Haoran}, url = {http://www.refresco.org/download/2013msc_thesis_haoranyupdf/}, year = {2013}, date = {20130906}, school = {Technical University of Delft, the Netherlands}, keywords = {Cavitation, KSKL, NACA 0015, SST, URANS}, pubstate = {published}, tppubtype = {mastersthesis} }  
2012 

Kamphuis, Nico ANALYSIS OF REFRESCO COMPUTATIONS APPLIED TO A TIP VORTEX OF A WING Masters Thesis University of Twente, Enschede, the Netherlands, 2012. Links  BibTeX  Tags: Chow Wing, EARSM, RANS, SpalartAllmaras, SST, Tipvortex, Turbulence Models @mastersthesis{2011Stage_NicoKamphuis, title = {ANALYSIS OF REFRESCO COMPUTATIONS APPLIED TO A TIP VORTEX OF A WING}, author = {Nico Kamphuis}, url = {http://www.refresco.org/?wpdmpro=2011stage_nicokamphuispdf}, year = {2012}, date = {20121201}, school = {University of Twente, Enschede, the Netherlands}, keywords = {Chow Wing, EARSM, RANS, SpalartAllmaras, SST, Tipvortex, Turbulence Models}, pubstate = {published}, tppubtype = {mastersthesis} }  
Pereira, Filipe Verication of ReFRESCO with the Method of Manufactured Solutions Masters Thesis IST, Lisbon, Portugal, 2012. Abstract  Links  BibTeX  Tags: Code Verification, Convection schemes, Excentricity, MMS, Nonorthogonality, RANS, SST, Validation, Verification @mastersthesis{2012Msc_Thesis_FilipePereira, title = {Verication of ReFRESCO with the Method of Manufactured Solutions}, author = {Filipe Pereira}, url = {http://www.refresco.org/?wpdmpro=2012msc_thesis_filipepereirapdf}, year = {2012}, date = {20121001}, school = {IST, Lisbon, Portugal}, abstract = {The purpose of this Thesis was to Verify the RANS solver ReFRESCO. This analysis was executed over three distinct parts of the code: convection schemes, nonorthogonality and excentricity correctors. Moreover, it was performed the implementation and evaluation of the numerical properties of two nonorthogonality and three excentricity new correction methods. In order to execute the Verification of ReFRESCO, grid refinement studies were performed to check if the numerical error tend to zero with the correct order of grid convergence (theoretical order). The calculation of the numerical error required the use of the Method of Manufactured Solutions to create exact solutions of the RANS equations. Thus, three manufactured solutions were used, each one resembling a different flow. The main conclusions of the present Thesis were: the convection schemes are correctly coded; the Hybrid scheme order of grid convergence did not vary linearly with the blending factor and it tended to a step function with the increase of the ow complexity; the tests performed over the nonorthogonality correctors showed that these methods maintained the secondorder of the code while discarding the correctors originated a constant numerical error in the solution; in grids where the excentricity factor was independent from grid renement, compared to the noncorrected case (constant numerical error), the excentricity correctors (correctly implemented) decreased significantly the magnitude of the numerical error. However, these correctors only guaranteed rstorder accuracy.}, keywords = {Code Verification, Convection schemes, Excentricity, MMS, Nonorthogonality, RANS, SST, Validation, Verification}, pubstate = {published}, tppubtype = {mastersthesis} } The purpose of this Thesis was to Verify the RANS solver ReFRESCO. This analysis was executed over three distinct parts of the code: convection schemes, nonorthogonality and excentricity correctors. Moreover, it was performed the implementation and evaluation of the numerical properties of two nonorthogonality and three excentricity new correction methods. In order to execute the Verification of ReFRESCO, grid refinement studies were performed to check if the numerical error tend to zero with the correct order of grid convergence (theoretical order). The calculation of the numerical error required the use of the Method of Manufactured Solutions to create exact solutions of the RANS equations. Thus, three manufactured solutions were used, each one resembling a different flow. The main conclusions of the present Thesis were: the convection schemes are correctly coded; the Hybrid scheme order of grid convergence did not vary linearly with the blending factor and it tended to a step function with the increase of the ow complexity; the tests performed over the nonorthogonality correctors showed that these methods maintained the secondorder of the code while discarding the correctors originated a constant numerical error in the solution; in grids where the excentricity factor was independent from grid renement, compared to the noncorrected case (constant numerical error), the excentricity correctors (correctly implemented) decreased significantly the magnitude of the numerical error. However, these correctors only guaranteed rstorder accuracy.  
2011 

Otto, William NUMERICAL SIMULATIONS OF FLOW OVER AN AXIAL MARINE CURRENT TURBINE Masters Thesis Technical University of Delft, the Netherlands, 2011. Abstract  Links  BibTeX  Tags: Current Turbines, RANS, SST, Turbines, URANS, Validation, Verification @mastersthesis{2011Msc_Thesis_WilliamOtto, title = {NUMERICAL SIMULATIONS OF FLOW OVER AN AXIAL MARINE CURRENT TURBINE}, author = {William Otto}, url = {http://www.refresco.org/?wpdmpro=2011msc_thesis_williamottopdf}, year = {2011}, date = {20111011}, school = {Technical University of Delft, the Netherlands}, abstract = {The main objective of this Msc. thesis is to obtain and analyze numerical simulations of singlephase flow over an axial marine current turbine. A wide range of operating conditions is simulated. Great attention is paid to verification, validation and uncertainty analysis. As benchmark, a reference turbine with experimental data is used which is found in literature (A.S. Bahaj and W.M.J. Batten, 2005 [17]). The simulations were performed at model scale and scale effects were studied by using the same geometry at full scale Reynolds numbers. This thesis is limited to single phase flows, what means that cavitation and free surface effects are deliberately excluded. Only a uniform inflow is modeled and interaction between the turbine and other objects as walls, floors, mounting rigs or other turbines are not taken into account (’open water condition’). Because these aspects can play a significant roll in practical applications, the numerical method is chosen such that they can be implemented in future work, once verified and validated simulations of noninteracting, singlephase flow have been obtained. Because its ability to include the aforementioned effects, as well its the ability to study scale effects, the MARIN inhouse RANS solver ReFRESCO is used for the simulations. A geometrical description of the reference turbine was received from the original authors. This geometry is modified in order to obtain feasible calculations. First, the trailing edge had to be thickened in order to avoid troubles in the grid generation. Second, a new connection has been constructed between the blades and the hub. The original connection causes an unsteady wake which elongates the calculation time to weeks. With a new constructed blade to hub connection, the flow is less complex, reducing the calculation time to a couple of days per condition. The modeling error caused by the thickened trailing edge is studied by using two dimensional RANS calculations over a radial section of the turbine (r=R = 0:7). It is estimated that the sectional lift is reduced by 3.78% due to the thickened trailing edge. Also an increase in drag is obtained, which is estimated as 6.35%. The turbine power and axial loading is corrected for this effect. The modified blade to hub connection is taken into account as an additional uncertainty in the solutions. A verification and validation procedure is performed to estimate the numerical and modeling uncertainties. The largest component of the numerical uncertainty is the discretization error. This error is hard to quantify due to: 1) the unstructured grid approach what makes it hard to produce a series of geometrical similar grids, 2) the small refinement range limited by the available memory resources. Therefore, a conservative estimation is made by using a safety factor. The numerical uncertainty is estimated as U = 3:6% for the power coefficient CP and U = 4:8% for the axial loading coefficient CT . A cylindrical computational domain is used to represent the open water condition. Initially, the domain size was 8 turbine diameter wide in radial direction. Later it proved that this domain was too small to fully represent an undisturbed flow without (numerical) blockage effects. By systematically increasing the domain size, it is estimated that the modeling error caused by the too small domain is Udomain = 0:5% for CP and Udomain = 2:6% for CT . The calculation results at model scale (Re = 1:4 105) show a very good similarity with the experimental results for the power production as well as the axial loading. Due to the scatter in the experiments, it is not possible to follow an official validation procedure. The flow analysis at model scale shows a large area of laminar flow separation at the suction side of the blades. It can be said that the blades are in stall for a large part. The turbulence intensity shows the boundary layer at the blade is in the transitional region. Roughly half of the chord length has a laminar boundary layer, the second half is turbulent. The stall can be caused by the laminar boundary layer, what makes it a scale effect. The flow analysis at full scale Reynolds numbers Re = 5 106 does not show the large separation areas. A fully turbulent boundary layer is obtained and the flow stays to a great extend attached to the blade. As a consequence, the obtained axial loading and power coefficient is more than 10% higher than at model scale. This is a significant scale effect where designers of marine current turbines should be aware of.}, keywords = {Current Turbines, RANS, SST, Turbines, URANS, Validation, Verification}, pubstate = {published}, tppubtype = {mastersthesis} } The main objective of this Msc. thesis is to obtain and analyze numerical simulations of singlephase flow over an axial marine current turbine. A wide range of operating conditions is simulated. Great attention is paid to verification, validation and uncertainty analysis. As benchmark, a reference turbine with experimental data is used which is found in literature (A.S. Bahaj and W.M.J. Batten, 2005 [17]). The simulations were performed at model scale and scale effects were studied by using the same geometry at full scale Reynolds numbers. This thesis is limited to single phase flows, what means that cavitation and free surface effects are deliberately excluded. Only a uniform inflow is modeled and interaction between the turbine and other objects as walls, floors, mounting rigs or other turbines are not taken into account (’open water condition’). Because these aspects can play a significant roll in practical applications, the numerical method is chosen such that they can be implemented in future work, once verified and validated simulations of noninteracting, singlephase flow have been obtained. Because its ability to include the aforementioned effects, as well its the ability to study scale effects, the MARIN inhouse RANS solver ReFRESCO is used for the simulations. A geometrical description of the reference turbine was received from the original authors. This geometry is modified in order to obtain feasible calculations. First, the trailing edge had to be thickened in order to avoid troubles in the grid generation. Second, a new connection has been constructed between the blades and the hub. The original connection causes an unsteady wake which elongates the calculation time to weeks. With a new constructed blade to hub connection, the flow is less complex, reducing the calculation time to a couple of days per condition. The modeling error caused by the thickened trailing edge is studied by using two dimensional RANS calculations over a radial section of the turbine (r=R = 0:7). It is estimated that the sectional lift is reduced by 3.78% due to the thickened trailing edge. Also an increase in drag is obtained, which is estimated as 6.35%. The turbine power and axial loading is corrected for this effect. The modified blade to hub connection is taken into account as an additional uncertainty in the solutions. A verification and validation procedure is performed to estimate the numerical and modeling uncertainties. The largest component of the numerical uncertainty is the discretization error. This error is hard to quantify due to: 1) the unstructured grid approach what makes it hard to produce a series of geometrical similar grids, 2) the small refinement range limited by the available memory resources. Therefore, a conservative estimation is made by using a safety factor. The numerical uncertainty is estimated as U = 3:6% for the power coefficient CP and U = 4:8% for the axial loading coefficient CT . A cylindrical computational domain is used to represent the open water condition. Initially, the domain size was 8 turbine diameter wide in radial direction. Later it proved that this domain was too small to fully represent an undisturbed flow without (numerical) blockage effects. By systematically increasing the domain size, it is estimated that the modeling error caused by the too small domain is Udomain = 0:5% for CP and Udomain = 2:6% for CT . The calculation results at model scale (Re = 1:4 105) show a very good similarity with the experimental results for the power production as well as the axial loading. Due to the scatter in the experiments, it is not possible to follow an official validation procedure. The flow analysis at model scale shows a large area of laminar flow separation at the suction side of the blades. It can be said that the blades are in stall for a large part. The turbulence intensity shows the boundary layer at the blade is in the transitional region. Roughly half of the chord length has a laminar boundary layer, the second half is turbulent. The stall can be caused by the laminar boundary layer, what makes it a scale effect. The flow analysis at full scale Reynolds numbers Re = 5 106 does not show the large separation areas. A fully turbulent boundary layer is obtained and the flow stays to a great extend attached to the blade. As a consequence, the obtained axial loading and power coefficient is more than 10% higher than at model scale. This is a significant scale effect where designers of marine current turbines should be aware of.  
Montgolfier, Alienor Simulation of Line Vortex Cavitation Masters Thesis ENSTA, Brest, Bretagne, France, 2011. Abstract  Links  BibTeX  Tags: Cavitation, SST, Tipvortex, URANS, Venturi @mastersthesis{2011Msc_Thesis_AlienorMontgolfier, title = {Simulation of Line Vortex Cavitation}, author = {Alienor Montgolfier}, url = {http://www.refresco.org/download/2011msc_thesi…ontgolfierpdf/}, year = {2011}, date = {20110801}, school = {ENSTA, Brest, Bretagne, France}, abstract = {The numerical simulation of cavitation with RANS2 is relatively new and we have a lot to learn about the numerics. But if the computations are carried out carefully they can help to understand and to interpret experimental results. Many studies about cavitation are on sheet cavitation on foils. But there is also the phenomenon of vortex cavitation : cavitation appears in the core of a vortex because of the low local pressure. This kind of vortex can be modelled in a Venturi channel and a swirling inflow. With a proper choice of the conditions cavitation will appear in the channel. The appearance of cavitation will change the flow. The studies are carried out with ReFRESCO3. Results are obtained for two sizes of venturi, one short, one long. The flows are similar in the two venturis, but the size of the cavities are different. Cavities present nodes and antinodes which can be explained by the narrowing of the cross section of liquid due to the presence of cavitation and waves formation on the liquidvapour interface. Calculations with different sizes of viscous core are also studied and compared.}, keywords = {Cavitation, SST, Tipvortex, URANS, Venturi}, pubstate = {published}, tppubtype = {mastersthesis} } The numerical simulation of cavitation with RANS2 is relatively new and we have a lot to learn about the numerics. But if the computations are carried out carefully they can help to understand and to interpret experimental results. Many studies about cavitation are on sheet cavitation on foils. But there is also the phenomenon of vortex cavitation : cavitation appears in the core of a vortex because of the low local pressure. This kind of vortex can be modelled in a Venturi channel and a swirling inflow. With a proper choice of the conditions cavitation will appear in the channel. The appearance of cavitation will change the flow. The studies are carried out with ReFRESCO3. Results are obtained for two sizes of venturi, one short, one long. The flows are similar in the two venturis, but the size of the cavities are different. Cavities present nodes and antinodes which can be explained by the narrowing of the cross section of liquid due to the presence of cavitation and waves formation on the liquidvapour interface. Calculations with different sizes of viscous core are also studied and compared.  
Toxopeus, Serge Practical application of viscousflow calculations for the simulation of manoeuvring ships PhD Thesis Technical University of Delft, the Netherlands, 2011, ISBN: ISBN 9789075757057. Abstract  Links  BibTeX  Tags: DARPA Suboff, Drag, DTMB 5415M, Esso Osaka, HTC, KCS, KVLCC2, Lift, Manoeuvring, RANS, Rotation, Series60, Ships, SST, Submarines, Validation, Verification, Yaw @phdthesis{2011PhDToxopeus, title = {Practical application of viscousflow calculations for the simulation of manoeuvring ships}, author = {Serge Toxopeus}, url = {http://www.refresco.org/?wpdmpro=2011phdtoxopeuspdf}, isbn = {ISBN 9789075757057}, year = {2011}, date = {20110509}, school = {Technical University of Delft, the Netherlands}, abstract = {The present work was initiated in order to improve traditional manoeuvring simulations based on empirical equations to model the forces and moments on the ship. With the evolution of the capability of viscousflow solvers to predict forces and moments on ships, it was decided to develop a practical method to simulate the manoeuvrability of ships in which viscousflow solvers are utilised and to investigate whether this improves the accuracy of manoeuvring predictions. To achieve this goal, the virtual captive test approach is adopted, because of the efficient use of computational resources compared to other methods. This procedure mimics the approach for manoeuvring simulations in which experimental PMM is used to obtain the forces and moments on the ship. This study extends the work of other researchers by providing extensive verification and validation of the predicted forces and moments on the hull and a detailed study of the sensitivity of the manoeuvring characteristics of the ship to changes in the hydrodynamic coefficients in the simulation model. Changes in the flow solvers were required to be able to calculate the flow around ships in rotational motion. These changes are discussed as well as the acceleration techniques that were developed to reduce the effort spent on grid generation and during the computations. In this thesis, it is demonstrated that good predictions of the loads on the hull in manoeuvring motion can be obtained for a wide range of ship types. The trends in the forces and moments as a function of the drift angle or yaw rate are simulated well. The verification studies provide useful insight into the in influence of grid density on the predicted forces and moments. In several cases, validation of the calculations failed, indicating modelling errors in the numerical results. In these cases, it was generally seen that the magnitude of the transverse force was underpredicted, while the magnitude of the yaw moment was overpredicted. For manoeuvring studies in the early design, the comparison errors are within acceptable levels. However, improvements remain desired and may be obtained using finer grids, larger domain sizes, different grid topologies with refinement in the wake of the ship, other turbulence models or incorporating free surface deformation. The manoeuvring prediction program SurSim has been used to simulate the manoeuvrability of the HTC. A procedure is proposed to derive the hydrodynamic coefficients required to model the forces and moments on the bare hull. This procedure is chosen to enable accurate modelling of the linearised behaviour for coursekeeping as well as realistic modelling of the harbour manoeuvring characteristics, and to enable the modelling of nonlinear manoeuvres accurately. To generate validation data for the manoeuvring predictions presented in this thesis, free sailing manoeuvring tests for the HTC were performed. This test campaign resulted in a very valuable data set which can be used for public validation studies. Besides obtaining general characteristics of the manoeuvrability of a singlescrew container ship, unique information has been obtained on the drift angles and rates of turn combined with propeller and rudder forces. Furthermore, repeat tests have been conducted for selected manoeuvres. Based on these tests, the uncertainty in the characteristic manoeuvring properties has been estimated. By using hydrodynamic manoeuvring coefficients derived from the CFD calculations, it has been shown that it is possible to improve the prediction of ship manoeuvres compared to predictions using coefficients based on empirical equations. A considerable improvement in the turning circle predictions was obtained. The prediction of the yaw checking and course keeping and initial turning abilities based on zigzag simulations improved as well, but further improvements are required for more reliable assessment of the manoeuvring performance. The sensitivity of the manoeuvring predictions to changes in the hydrodynamic coefficients was studied. It was found that for accurate predictions of the manoeuvrability using coefficients derived from CFD calculations, accurate predictions of especially the yawing moment must be made.}, keywords = {DARPA Suboff, Drag, DTMB 5415M, Esso Osaka, HTC, KCS, KVLCC2, Lift, Manoeuvring, RANS, Rotation, Series60, Ships, SST, Submarines, Validation, Verification, Yaw}, pubstate = {published}, tppubtype = {phdthesis} } The present work was initiated in order to improve traditional manoeuvring simulations based on empirical equations to model the forces and moments on the ship. With the evolution of the capability of viscousflow solvers to predict forces and moments on ships, it was decided to develop a practical method to simulate the manoeuvrability of ships in which viscousflow solvers are utilised and to investigate whether this improves the accuracy of manoeuvring predictions. To achieve this goal, the virtual captive test approach is adopted, because of the efficient use of computational resources compared to other methods. This procedure mimics the approach for manoeuvring simulations in which experimental PMM is used to obtain the forces and moments on the ship. This study extends the work of other researchers by providing extensive verification and validation of the predicted forces and moments on the hull and a detailed study of the sensitivity of the manoeuvring characteristics of the ship to changes in the hydrodynamic coefficients in the simulation model. Changes in the flow solvers were required to be able to calculate the flow around ships in rotational motion. These changes are discussed as well as the acceleration techniques that were developed to reduce the effort spent on grid generation and during the computations. In this thesis, it is demonstrated that good predictions of the loads on the hull in manoeuvring motion can be obtained for a wide range of ship types. The trends in the forces and moments as a function of the drift angle or yaw rate are simulated well. The verification studies provide useful insight into the in influence of grid density on the predicted forces and moments. In several cases, validation of the calculations failed, indicating modelling errors in the numerical results. In these cases, it was generally seen that the magnitude of the transverse force was underpredicted, while the magnitude of the yaw moment was overpredicted. For manoeuvring studies in the early design, the comparison errors are within acceptable levels. However, improvements remain desired and may be obtained using finer grids, larger domain sizes, different grid topologies with refinement in the wake of the ship, other turbulence models or incorporating free surface deformation. The manoeuvring prediction program SurSim has been used to simulate the manoeuvrability of the HTC. A procedure is proposed to derive the hydrodynamic coefficients required to model the forces and moments on the bare hull. This procedure is chosen to enable accurate modelling of the linearised behaviour for coursekeeping as well as realistic modelling of the harbour manoeuvring characteristics, and to enable the modelling of nonlinear manoeuvres accurately. To generate validation data for the manoeuvring predictions presented in this thesis, free sailing manoeuvring tests for the HTC were performed. This test campaign resulted in a very valuable data set which can be used for public validation studies. Besides obtaining general characteristics of the manoeuvrability of a singlescrew container ship, unique information has been obtained on the drift angles and rates of turn combined with propeller and rudder forces. Furthermore, repeat tests have been conducted for selected manoeuvres. Based on these tests, the uncertainty in the characteristic manoeuvring properties has been estimated. By using hydrodynamic manoeuvring coefficients derived from the CFD calculations, it has been shown that it is possible to improve the prediction of ship manoeuvres compared to predictions using coefficients based on empirical equations. A considerable improvement in the turning circle predictions was obtained. The prediction of the yaw checking and course keeping and initial turning abilities based on zigzag simulations improved as well, but further improvements are required for more reliable assessment of the manoeuvring performance. The sensitivity of the manoeuvring predictions to changes in the hydrodynamic coefficients was studied. It was found that for accurate predictions of the manoeuvrability using coefficients derived from CFD calculations, accurate predictions of especially the yawing moment must be made.  
Crepier, Pierre Validation of URANS CFD Code ReFRESCO Roll Damping Simulations Masters Thesis ENSTA, Brest, Bretagne, France, 2011. Abstract  Links  BibTeX  Tags: Bilgekeels, Rolldamping, ScaleEffects, SpalartAllmaras, SST, URANS, Validation, Verification @mastersthesis{2011Msc_Thesis_PierreCrepier, title = {Validation of URANS CFD Code ReFRESCO Roll Damping Simulations}, author = {Pierre Crepier}, url = {http://www.refresco.org/?wpdmpro=2011msc_thesis_pierrecrepierpdf}, year = {2011}, date = {20110331}, school = {ENSTA, Brest, Bretagne, France}, abstract = {Unsteady calculations of the flow around rolling hull sections have been carried out. Two cases have been considered : a rectangular hull with sharp bilges and a rectangular hull fitted with triangularshaped bilge keels. The doublebody approach has been used for the computations. Sensitivity studies have been carried out on the grid refinement, the timestep size and the iterative convergence. It shows that the dependence on the grid is larger than on the timestep and the convergence threshold. Indeed, in order to have a grid converged, fine meshes have to be used. Results show that the viscous damping coefficient decreases when refining all the parameters. Results have been compared with existing data published by Ikeda and Yeung. Calculations for the case with sharp bilges appear to be in very good agreement with experimental results reported by Ikeda. They confirm the linear behavior of the damping coefficient with the amplitude. Regarding the case with bilge keels, results show a linear behavior of the damping coefficient with the frequency which is a behavior confirmed by Ikeda. For this case, a fairly good agreement is found for nondimensional frequencies lower than 0:7. For these values a deviation lower than 10% is obtained. However we observe larger deviations at high frequencies. Those deviations are believed to be due to freesurface effects and wave making. So, computations involving the free surface leading to a damping coefficient taking the wave damping into account have to be carried out. Preliminary studies on turbulence modeling and scale effects have also been done. Changing the turbulence model from SST to SpalartAllmaras leads to a damping higher by 4:5% explained by a higher pressure on the hull. Generated vortices are observed to be rounder and closer to each other than with SST. For the study on scale effects, a geometrical scale factor of 1=50 have been used and the viscosity have been tuned to reach the full scale conditions. The results obtained do not show large deviation with the modelscale computation. Indeed a damping higher by 1:5% is obtained due to a pressure slightly higher on the hull but the vorticity do not change much between the modelscale and the fullscale calculations}, keywords = {Bilgekeels, Rolldamping, ScaleEffects, SpalartAllmaras, SST, URANS, Validation, Verification}, pubstate = {published}, tppubtype = {mastersthesis} } Unsteady calculations of the flow around rolling hull sections have been carried out. Two cases have been considered : a rectangular hull with sharp bilges and a rectangular hull fitted with triangularshaped bilge keels. The doublebody approach has been used for the computations. Sensitivity studies have been carried out on the grid refinement, the timestep size and the iterative convergence. It shows that the dependence on the grid is larger than on the timestep and the convergence threshold. Indeed, in order to have a grid converged, fine meshes have to be used. Results show that the viscous damping coefficient decreases when refining all the parameters. Results have been compared with existing data published by Ikeda and Yeung. Calculations for the case with sharp bilges appear to be in very good agreement with experimental results reported by Ikeda. They confirm the linear behavior of the damping coefficient with the amplitude. Regarding the case with bilge keels, results show a linear behavior of the damping coefficient with the frequency which is a behavior confirmed by Ikeda. For this case, a fairly good agreement is found for nondimensional frequencies lower than 0:7. For these values a deviation lower than 10% is obtained. However we observe larger deviations at high frequencies. Those deviations are believed to be due to freesurface effects and wave making. So, computations involving the free surface leading to a damping coefficient taking the wave damping into account have to be carried out. Preliminary studies on turbulence modeling and scale effects have also been done. Changing the turbulence model from SST to SpalartAllmaras leads to a damping higher by 4:5% explained by a higher pressure on the hull. Generated vortices are observed to be rounder and closer to each other than with SST. For the study on scale effects, a geometrical scale factor of 1=50 have been used and the viscosity have been tuned to reach the full scale conditions. The results obtained do not show large deviation with the modelscale computation. Indeed a damping higher by 1:5% is obtained due to a pressure slightly higher on the hull but the vorticity do not change much between the modelscale and the fullscale calculations  
Peyro, Guillaume Analysis of Flows on Stabilizer Fins using ReFRESCO: 2D,3D, Static and Dynamic Eects Masters Thesis ENSTA, Brest, Bretagne, France, 2011. Abstract  Links  BibTeX  Tags: Imposed Motion, NACA 0015, RANS, SST, Stabilizer fins, URANS, Validation, Verification @mastersthesis{2011Msc_Thesis_GuilaumePeyro, title = {Analysis of Flows on Stabilizer Fins using ReFRESCO: 2D,3D, Static and Dynamic Eects}, author = {Guillaume Peyro}, url = { http://www.refresco.org/?wpdmpro=2011msc_thesis_guilaumepeyropdf}, year = {2011}, date = {20110301}, school = {ENSTA, Brest, Bretagne, France}, abstract = {Nowadays, Computational Fluid Dynamic (CFD) is becoming more and more important in the maritime field to investigate complex hydrodynamic phenomena, especially when combined with model tests. Conscious about this, the Maritime Research Institute of Netherlands (MARIN) developed its own CFD code called ReFRESCO. The aim of this study is to use CFD to investigate the flow on a NACA 0015 hydrofoil which represents a stabilizer n, and to compare CFD results with model tests of this n. For this study, first, 2D static computations are done with ReFRESCO which allows us to know the effects of numerical parameters (boundary conditions, domain dimensions, domain shape...) on the results. Using these results, similar 3D computations are performed and compared to the results of the model tests. Finally, preliminary dynamic computations are done in order to test some tools simulating the oscillation of the foil.}, keywords = {Imposed Motion, NACA 0015, RANS, SST, Stabilizer fins, URANS, Validation, Verification}, pubstate = {published}, tppubtype = {mastersthesis} } Nowadays, Computational Fluid Dynamic (CFD) is becoming more and more important in the maritime field to investigate complex hydrodynamic phenomena, especially when combined with model tests. Conscious about this, the Maritime Research Institute of Netherlands (MARIN) developed its own CFD code called ReFRESCO. The aim of this study is to use CFD to investigate the flow on a NACA 0015 hydrofoil which represents a stabilizer n, and to compare CFD results with model tests of this n. For this study, first, 2D static computations are done with ReFRESCO which allows us to know the effects of numerical parameters (boundary conditions, domain dimensions, domain shape...) on the results. Using these results, similar 3D computations are performed and compared to the results of the model tests. Finally, preliminary dynamic computations are done in order to test some tools simulating the oscillation of the foil.  
2010 

LeGarrec, Morgan Verification and Validation of Viscous FreeSurface Flows around Ships with ReFRESCO Masters Thesis Ecole Centrale de Nantes, France, 2010. Abstract  Links  BibTeX  Tags: Fixed WavySurface, FreeSurface, Resistance, Series60, Validation, Verification, WigleyHull @mastersthesis{2010Msc_Thesis_MorganLeGarrec, title = {Verification and Validation of Viscous FreeSurface Flows around Ships with ReFRESCO}, author = {Morgan LeGarrec }, url = {http://www.refresco.org/?wpdmpro=2010msc_morganlegarrecpdf}, year = {2010}, date = {20101001}, school = {Ecole Centrale de Nantes, France}, abstract = {Viscousflow RANS solvers usually called Computational Fluid Dynamics (CFD) tools are being widely used nowadays to compute the flow around ship hulls. They permit to capture both potential and viscous components of the flow, and even to model the freesurface formed by the ship displacement. Additionally, contrary to the modelbasin modelscale experiments, CFD tools permit to compute the flow around the model and the prototype, i.e. the modelscale and the fullscale real ship. ReFRESCO is an inhouse RANS MARIN tool under development. It is very versatile regarding the nature of the problem as well as the grids used for the computation. The following thesis aims to check the accuracy and to test general numerical aspects of ReFRESCO calculations for two wellknown ships test cases: WigleyHull and Series 60 hull. Verification procedures are used whenever possible in order to ensure that the numerical errors are under control, and to consequently permit correct validation exercises. A thorough bibliography research on available experimental and numerical data has been done for these two test cases and it is shown in tabular form in this thesis. Three approaches are considered for tackling the flow around these ships including freesurface: 1. a doublebody model where the freesurface modeling is neglected and a symmetry surface is considered instead; 2. a composite method where the freesurface is computed using a potentialflow solver, RAPID, and afterwards considered as a wavy boundary surface in the viscousflow computation; 3. a fully freesurface capturing RANS approach where the freesurface is solved using ReFRESCO by means of additional modeling equations. For this last method, the one to be used in the future, only a preliminary first attempt has been done. For the WigleyHull, HO grids targeted for the other MARIN RANS solver PARNASSOS are used. Calculations using these grids are usually difficult to converge by general codes. In this thesis we show that it is possible, being however the computational demands high. For this ship only a doublebody approach has been used and several numerical studies performed: computational domain size, grid layout, grid stretching, grid size, discretization schemes, influence of turbulence models, influence of turbulence model boundary conditions (with and without socalled wallfunctions). In general, the results show that the domain size has a large influence on the results, that wallfunctions should not be used, that fluxlimiters decreased the order of accuracy of the higherorder QUICK convection scheme. The comparison of the numerically verified results with the ITTC57 estimate showed a small underprediction of the friction resistance. The numerical tests also showed a large discrepancy on the forces when using k }, keywords = {Fixed WavySurface, FreeSurface, Resistance, Series60, Validation, Verification, WigleyHull}, pubstate = {published}, tppubtype = {mastersthesis} } Viscousflow RANS solvers usually called Computational Fluid Dynamics (CFD) tools are being widely used nowadays to compute the flow around ship hulls. They permit to capture both potential and viscous components of the flow, and even to model the freesurface formed by the ship displacement. Additionally, contrary to the modelbasin modelscale experiments, CFD tools permit to compute the flow around the model and the prototype, i.e. the modelscale and the fullscale real ship. ReFRESCO is an inhouse RANS MARIN tool under development. It is very versatile regarding the nature of the problem as well as the grids used for the computation. The following thesis aims to check the accuracy and to test general numerical aspects of ReFRESCO calculations for two wellknown ships test cases: WigleyHull and Series 60 hull. Verification procedures are used whenever possible in order to ensure that the numerical errors are under control, and to consequently permit correct validation exercises. A thorough bibliography research on available experimental and numerical data has been done for these two test cases and it is shown in tabular form in this thesis. Three approaches are considered for tackling the flow around these ships including freesurface: 1. a doublebody model where the freesurface modeling is neglected and a symmetry surface is considered instead; 2. a composite method where the freesurface is computed using a potentialflow solver, RAPID, and afterwards considered as a wavy boundary surface in the viscousflow computation; 3. a fully freesurface capturing RANS approach where the freesurface is solved using ReFRESCO by means of additional modeling equations. For this last method, the one to be used in the future, only a preliminary first attempt has been done. For the WigleyHull, HO grids targeted for the other MARIN RANS solver PARNASSOS are used. Calculations using these grids are usually difficult to converge by general codes. In this thesis we show that it is possible, being however the computational demands high. For this ship only a doublebody approach has been used and several numerical studies performed: computational domain size, grid layout, grid stretching, grid size, discretization schemes, influence of turbulence models, influence of turbulence model boundary conditions (with and without socalled wallfunctions). In general, the results show that the domain size has a large influence on the results, that wallfunctions should not be used, that fluxlimiters decreased the order of accuracy of the higherorder QUICK convection scheme. The comparison of the numerically verified results with the ITTC57 estimate showed a small underprediction of the friction resistance. The numerical tests also showed a large discrepancy on the forces when using k  
Pengam, Benjamin NUMERICAL ACCURACY IN RANS SIMULATIONS OF THE FLOW AROUND A CYLINDER Masters Thesis ENSTA, Brest, Bretagne, France, 2010. Links  BibTeX  Tags: Cylinder, Separation, Transition, URANS, Validation, Verification, Vortexshedding @mastersthesis{2010Msc_Thesis_BenjaminPengam, title = {NUMERICAL ACCURACY IN RANS SIMULATIONS OF THE FLOW AROUND A CYLINDER}, author = {Benjamin Pengam}, url = {http://www.refresco.org/?wpdmpro=2010msc_thesis_benjaminpengampdf}, year = {2010}, date = {20100806}, school = {ENSTA, Brest, Bretagne, France}, keywords = {Cylinder, Separation, Transition, URANS, Validation, Verification, Vortexshedding}, pubstate = {published}, tppubtype = {mastersthesis} }  
Gubler, Roman Numerical Simulation of the Flow Around a Ship Hull Including Interaction Effects between Hull and Propeller Masters Thesis Ecole Polytechnique Federale de Lausanne, Switzerland, 2010. Abstract  Links  BibTeX  Tags: HTC, RANSBEM Coupling, Ships, Validation, Verification, Wakefield @mastersthesis{2010MSc_Thesis_RomanGubler, title = {Numerical Simulation of the Flow Around a Ship Hull Including Interaction Effects between Hull and Propeller}, author = {Roman Gubler}, url = {http://www.refresco.org/?wpdmpro=2010msc_thesis_romangublerpdf}, year = {2010}, date = {20100301}, school = {Ecole Polytechnique Federale de Lausanne, Switzerland}, abstract = {Propeller effects are usually neglected in computations of the flow around ship hulls since the inclusion of the complex rotating propeller geometry in a numerical viscous flow simulation is very time consuming in terms of preprocessing and requires a lot of computational resources. Various methods have emerged to approach the highly complex problem of including propeller action in the description of the flow field to allow for a more accurate prediction of forces and moments acting on the ship hull. The present work describes the implementation of a body force approach to model effects of the propeller action on the viscous flow around a ship hull, that is, the forces exerted by the propeller are included in the body force terms in the Reynolds averaged NavierStokes equations incorporated in a viscous flow solver. The propeller forces are computed in a separate potential flow code. Nonlinear interaction effects between the flow around the ship hull and the propeller are taken into account by an iterative solution procedure between the two sequentially coupled flow solvers. The implementation is tested in an open water condition associated with a uniform inflow field to assess the reliability of the approach in terms of the velocity induction caused be the propeller. Application to a benchmark ship results in an expected alteration of the flow structure when the propeller effects are included. The subsequent comparison to experimental measurements reveals encouraging results in terms of global and local quantities.}, keywords = {HTC, RANSBEM Coupling, Ships, Validation, Verification, Wakefield}, pubstate = {published}, tppubtype = {mastersthesis} } Propeller effects are usually neglected in computations of the flow around ship hulls since the inclusion of the complex rotating propeller geometry in a numerical viscous flow simulation is very time consuming in terms of preprocessing and requires a lot of computational resources. Various methods have emerged to approach the highly complex problem of including propeller action in the description of the flow field to allow for a more accurate prediction of forces and moments acting on the ship hull. The present work describes the implementation of a body force approach to model effects of the propeller action on the viscous flow around a ship hull, that is, the forces exerted by the propeller are included in the body force terms in the Reynolds averaged NavierStokes equations incorporated in a viscous flow solver. The propeller forces are computed in a separate potential flow code. Nonlinear interaction effects between the flow around the ship hull and the propeller are taken into account by an iterative solution procedure between the two sequentially coupled flow solvers. The implementation is tested in an open water condition associated with a uniform inflow field to assess the reliability of the approach in terms of the velocity induction caused be the propeller. Application to a benchmark ship results in an expected alteration of the flow structure when the propeller effects are included. The subsequent comparison to experimental measurements reveals encouraging results in terms of global and local quantities.  
2009 

Glebart, Matthieu Validation and verification of FreSCo for loads on moving cylinders Masters Thesis ENSTA, Brest, Bretagne, France, 2009. Links  BibTeX  Tags: Cylinder, Imposed Motion, SST, Validation, Verification @mastersthesis{2009Stage_MatthieuGlebart, title = {Validation and verification of FreSCo for loads on moving cylinders}, author = {Matthieu Glebart}, url = {http://www.refresco.org/?wpdmpro=2009stage_matthieuglebartpdf }, year = {2009}, date = {20091103}, school = {ENSTA, Brest, Bretagne, France}, keywords = {Cylinder, Imposed Motion, SST, Validation, Verification}, pubstate = {published}, tppubtype = {mastersthesis} }  
Chanony, Francois Validation and verification of FreSCo for viscous flows around oscillating bodies. Roll motion Masters Thesis ENSTA, Brest, Bretagne, France, 2009. Links  BibTeX  Tags: Rolldamping, SST, URANS, Validation, Verification @mastersthesis{2009Stage_FrancoisChanony, title = {Validation and verification of FreSCo for viscous flows around oscillating bodies. Roll motion}, author = {Francois Chanony}, url = {http://www.refresco.org/?wpdmpro=2009stage_francoischanonypdf}, year = {2009}, date = {20090901}, school = {ENSTA, Brest, Bretagne, France}, keywords = {Rolldamping, SST, URANS, Validation, Verification}, pubstate = {published}, tppubtype = {mastersthesis} }  
Delvoye, Simon Simulation and analysis of the flow around an underwater exhaust with FreSCo Masters Thesis ISITV, Toulon, France, 2009. Links  BibTeX  Tags: Design, Multiphase, RANS, Scoops, SpalartAllmaras, SST, URANS, Verification @mastersthesis{2009Msc_Thesis_SimonDelvoye, title = {Simulation and analysis of the flow around an underwater exhaust with FreSCo}, author = {Delvoye, Simon}, url = {http://www.refresco.org/?wpdmpro=2009msc_thesis_simondelvoyepdf }, year = {2009}, date = {20090807}, school = {ISITV, Toulon, France}, keywords = {Design, Multiphase, RANS, Scoops, SpalartAllmaras, SST, URANS, Verification}, pubstate = {published}, tppubtype = {mastersthesis} }  
2008 

Rijpkema, Douwe Numerical Simulation of SinglePhase and MultiPhase Flow over a NACA 0015 Hydrofoil Masters Thesis Technical University of Delft, the Netherlands, 2008. Abstract  Links  BibTeX  Tags: Cavitation, Drag, Lift, NACA 0015, RANS, URANS, Validation, Verification @mastersthesis{2008Msc_Thesis_DouweRijpkema, title = {Numerical Simulation of SinglePhase and MultiPhase Flow over a NACA 0015 Hydrofoil}, author = {Douwe Rijpkema}, url = {http://www.refresco.org/?wpdmpro=2008msc_thesis_douwerijpkemapdf}, year = {2008}, date = {20081107}, school = {Technical University of Delft, the Netherlands}, abstract = {In the design of marine propellers, cavitation  the phenomenon of vapour formation due to a pressure reduction at constant temperature  is associated with negative effects on the performance and lifespan of the propeller. Additionally cavitation can be a source of inboard and underwater noise. Therefore insight in the occurence of cavitation and the development of the cavity on a propeller is essential. The numerical simulation of this phenomenon with computational fluid dynamics (CFD) tools may play an important role in this analysis. In this study a numerical simulation of both wetted and cavitating flow over a NACA 0015 hydrofoil is performed. The foil is placed at an angle of attack of 6 degrees for a Reynolds number of 1.5E6. The CFD code FreSCo is used for the numerical simulations. FreSCo is an unsteady RANS solver actively being developed by a cooperation of Maritime Research Institute Netherlands (MARIN), Hamburgische SchiffbauVersuchsanstalt (HSVA) and Technische Universitaet HamburgHarburg (TUHH). In the computations, a MenterSST turbulence model is applied and a volume of fluid approach is used for the modelling of multiple phases. The influence of various numerical parameters on the hydrodynamic forces and cavitation behaviour is investigated. For wetted flow a variation in grid topology showed a significant influence on the lift. Computations with the Otype grid resulted in an increase in lift in comparison to the Ctype grid results. The Otype grid showed a better agreement with the pressure distributions obtained by other numerical methods. A refinement of the grid produced less variation in results for the QUICK scheme compared to the blending scheme for the convective flux term in the momentum equations. Therefore an Otype grid combined with a QUICK convection scheme is preferred for this type of flow. The comparison of the pressure distributions between FreSCo and two different boundary element methods showed a good agreement in results. In the case of cavitating flow, the cavitation model accounts for the creation and destruction of the vapour in the liquid. It gives an expression for the source term in the transport equation of the vapour volume fraction. A comparison was made between the Sauer, Zwart and Kunz cavitation models. The different formulations for the source term of the three models, led to large deviations in results and affected the numerical stability. It was observed that for the same cavitation model a reduction in tuning coefficient resulted in a smaller cavity and more numerically stable behaviour. For the high cavitation numbers ( = 1.75, = 1.5 and = 1.25) a steady attached cavity was found on the foil for all cavitation models. The different formulation of the various cavitation models resulted in a difference in hydrodynamic forces and cavity characteristics, becoming more pronounced with decreasing cavitation number. Due to the large source terms of the cavitation models, large pressure oscillations and consequently large lift and drag peaks were observed at low cavitation numbers ( = 1.0). A reduction of the timestep or of the condensation tuning coefficient resulted in a more stable computation. For = 1.0 shedding of the cavity was observed in the initial phase of the computation, eventually leading to an attached cavity on the foil that periodically varied in size}, keywords = {Cavitation, Drag, Lift, NACA 0015, RANS, URANS, Validation, Verification}, pubstate = {published}, tppubtype = {mastersthesis} } In the design of marine propellers, cavitation  the phenomenon of vapour formation due to a pressure reduction at constant temperature  is associated with negative effects on the performance and lifespan of the propeller. Additionally cavitation can be a source of inboard and underwater noise. Therefore insight in the occurence of cavitation and the development of the cavity on a propeller is essential. The numerical simulation of this phenomenon with computational fluid dynamics (CFD) tools may play an important role in this analysis. In this study a numerical simulation of both wetted and cavitating flow over a NACA 0015 hydrofoil is performed. The foil is placed at an angle of attack of 6 degrees for a Reynolds number of 1.5E6. The CFD code FreSCo is used for the numerical simulations. FreSCo is an unsteady RANS solver actively being developed by a cooperation of Maritime Research Institute Netherlands (MARIN), Hamburgische SchiffbauVersuchsanstalt (HSVA) and Technische Universitaet HamburgHarburg (TUHH). In the computations, a MenterSST turbulence model is applied and a volume of fluid approach is used for the modelling of multiple phases. The influence of various numerical parameters on the hydrodynamic forces and cavitation behaviour is investigated. For wetted flow a variation in grid topology showed a significant influence on the lift. Computations with the Otype grid resulted in an increase in lift in comparison to the Ctype grid results. The Otype grid showed a better agreement with the pressure distributions obtained by other numerical methods. A refinement of the grid produced less variation in results for the QUICK scheme compared to the blending scheme for the convective flux term in the momentum equations. Therefore an Otype grid combined with a QUICK convection scheme is preferred for this type of flow. The comparison of the pressure distributions between FreSCo and two different boundary element methods showed a good agreement in results. In the case of cavitating flow, the cavitation model accounts for the creation and destruction of the vapour in the liquid. It gives an expression for the source term in the transport equation of the vapour volume fraction. A comparison was made between the Sauer, Zwart and Kunz cavitation models. The different formulations for the source term of the three models, led to large deviations in results and affected the numerical stability. It was observed that for the same cavitation model a reduction in tuning coefficient resulted in a smaller cavity and more numerically stable behaviour. For the high cavitation numbers ( = 1.75, = 1.5 and = 1.25) a steady attached cavity was found on the foil for all cavitation models. The different formulation of the various cavitation models resulted in a difference in hydrodynamic forces and cavity characteristics, becoming more pronounced with decreasing cavitation number. Due to the large source terms of the cavitation models, large pressure oscillations and consequently large lift and drag peaks were observed at low cavitation numbers ( = 1.0). A reduction of the timestep or of the condensation tuning coefficient resulted in a more stable computation. For = 1.0 shedding of the cavity was observed in the initial phase of the computation, eventually leading to an attached cavity on the foil that periodically varied in size  
Jaouen, Frederick The Validation of the FreeSurface Modelling of FRESCO Masters Thesis ENSTA, Brest, Bretagne, France, 2008. Abstract  Links  BibTeX  Tags: Duncan Foil, FreeSurface, Regular Waves, TransomStern Flows, Validation, Verification @mastersthesis{2008Msc_Thesis_FrederickJaouen, title = {The Validation of the FreeSurface Modelling of FRESCO}, author = {Frederick Jaouen}, url = {http://www.refresco.org/?wpdmpro=2008msc_thesis_frederickjaouenpdf}, year = {2008}, date = {20080801}, school = {ENSTA, Brest, Bretagne, France}, abstract = {The CFD code FRESCO – FREe Surface COde – is the result from a cooperation between HSVA (Hamburgische Schiffbau Versuch Anstalt), TUHH (Technische Universit¨at HamburgHarburg) and MARIN (MAritime Research Institute of the Netherlands). This code, whose mathematical formulation is based on the RANS equations (Reynolds Averaged NavierStokes), focuses on the resolution of multiphase flows, ie. free surface flows, cavitating flows, multispecies flows. In order to identify the modelling errors and reduce the iteration and discretization errors, any CFD code has to be validated. The purpose of this master’s thesis is the validation of the freesurface modelling of FRESCO, especially regarding wave generation and propagation. The study of the numerical dissipation and dispersion of Airy and Stokes waves is carried out. Finally, the Duncan experiments and Transom Stern flows are used for the validation. Basically the numerical results show good agreements with analytical solutions or experimental tests and conclusions are so quite optimistic. However, some specific boundary conditions seem to be required for wave problems and a more consistent way of discretizing the hydrostatic law at the interface has to be implemented.}, keywords = {Duncan Foil, FreeSurface, Regular Waves, TransomStern Flows, Validation, Verification}, pubstate = {published}, tppubtype = {mastersthesis} } The CFD code FRESCO – FREe Surface COde – is the result from a cooperation between HSVA (Hamburgische Schiffbau Versuch Anstalt), TUHH (Technische Universit¨at HamburgHarburg) and MARIN (MAritime Research Institute of the Netherlands). This code, whose mathematical formulation is based on the RANS equations (Reynolds Averaged NavierStokes), focuses on the resolution of multiphase flows, ie. free surface flows, cavitating flows, multispecies flows. In order to identify the modelling errors and reduce the iteration and discretization errors, any CFD code has to be validated. The purpose of this master’s thesis is the validation of the freesurface modelling of FRESCO, especially regarding wave generation and propagation. The study of the numerical dissipation and dispersion of Airy and Stokes waves is carried out. Finally, the Duncan experiments and Transom Stern flows are used for the validation. Basically the numerical results show good agreements with analytical solutions or experimental tests and conclusions are so quite optimistic. However, some specific boundary conditions seem to be required for wave problems and a more consistent way of discretizing the hydrostatic law at the interface has to be implemented.  
Janssen, Bram A numerical and semianalytical study of the structure of stationary vortices Masters Thesis University of Twente, Enschede, the Netherlands, 2008. Abstract  Links  BibTeX  Tags: Cavitation, SST, URANS, Venturi @mastersthesis{2008Msc_Thesis_BramJanssen, title = {A numerical and semianalytical study of the structure of stationary vortices}, author = {Bram Janssen}, url = {http://www.refresco.org/?wpdmpro=2008msc_thesis_bramjanssenpdf}, year = {2008}, date = {20080306}, school = {University of Twente, Enschede, the Netherlands}, abstract = {Vortex cavitation is a source of broadband noise experienced onboard ships. A clear understanding of the physical phenomenon underlying the broadband noise does not exist, due to the lack of accurate theoretical models of cavitating vortices. To study the structure of a cavitating vortex, two existing semianalytical noncavitating vortex models have been adapted to include cavitation. The first model describes a leadingedge vortex, as observed above highly skewed blades and slender delta wings. The second vortex model describes a trailing vortex as can be observed several chords downstream of a propeller. To describe a cavitating vortex core, boundary conditions at the liquid/vapour interface have been derived based on jump relations that have to hold at a interface in viscous flow including surface tension. For the leadingedge vortex model, similarity solutions can be generated in which the radius of the outer edge of the cavity is selfsimilar with the vortex core. It can be shown that only for parabolic vortex cores selfsimilar solutions exist. The solutions are only valid for cavity radii much smaller than the vortex core radius. The radius of the outer edge of the cavity, depending on viscosity and cavitation number, decreases up to 50% when compared to the inviscid and viscous noncavitating flow solutions. The presence of a cavity gives a considerable change in the distribution of vorticity, leading to a local maximum in vorticity in the core, close to the cavity. For the trailing vortex core model no similarity solutions can be generated for cavitating vortices, since for parabolic vortex core growth the pressure becomes a function of the axial position. If similarity solutions are generated these solutions become very local assuming the pressure to be constant. These local solutions predict a decrease in cavity size compared to the inviscid cavity size. For increasing cavity size, the difference with the estimated size based on the inviscid flow solution decreases, depending on cavitation number and viscosity. At MARIN a new RANS method, FreSCo, has been developed which is capable of modeling cavitating flows. As a first test case a venturi with a swirling flow is considered. The flow solution exhibits an unsteady (oscillating) cavitating vortex in the throat of the venturi. At the tail of this cavity a second cavity, which consists of a mixture of liquid water and water vapour, appears. When observing the structure of the vortex close to the cavity, it appears that the numerical solution shows the same trends as the semianalytical similarity solutions.}, keywords = {Cavitation, SST, URANS, Venturi}, pubstate = {published}, tppubtype = {mastersthesis} } Vortex cavitation is a source of broadband noise experienced onboard ships. A clear understanding of the physical phenomenon underlying the broadband noise does not exist, due to the lack of accurate theoretical models of cavitating vortices. To study the structure of a cavitating vortex, two existing semianalytical noncavitating vortex models have been adapted to include cavitation. The first model describes a leadingedge vortex, as observed above highly skewed blades and slender delta wings. The second vortex model describes a trailing vortex as can be observed several chords downstream of a propeller. To describe a cavitating vortex core, boundary conditions at the liquid/vapour interface have been derived based on jump relations that have to hold at a interface in viscous flow including surface tension. For the leadingedge vortex model, similarity solutions can be generated in which the radius of the outer edge of the cavity is selfsimilar with the vortex core. It can be shown that only for parabolic vortex cores selfsimilar solutions exist. The solutions are only valid for cavity radii much smaller than the vortex core radius. The radius of the outer edge of the cavity, depending on viscosity and cavitation number, decreases up to 50% when compared to the inviscid and viscous noncavitating flow solutions. The presence of a cavity gives a considerable change in the distribution of vorticity, leading to a local maximum in vorticity in the core, close to the cavity. For the trailing vortex core model no similarity solutions can be generated for cavitating vortices, since for parabolic vortex core growth the pressure becomes a function of the axial position. If similarity solutions are generated these solutions become very local assuming the pressure to be constant. These local solutions predict a decrease in cavity size compared to the inviscid cavity size. For increasing cavity size, the difference with the estimated size based on the inviscid flow solution decreases, depending on cavitation number and viscosity. At MARIN a new RANS method, FreSCo, has been developed which is capable of modeling cavitating flows. As a first test case a venturi with a swirling flow is considered. The flow solution exhibits an unsteady (oscillating) cavitating vortex in the throat of the venturi. At the tail of this cavity a second cavity, which consists of a mixture of liquid water and water vapour, appears. When observing the structure of the vortex close to the cavity, it appears that the numerical solution shows the same trends as the semianalytical similarity solutions.  
2007 

Monroy, Charles A RANSE BASED STUDY OF THE FLOW BEHIND A CYLINDER. A FIRST STEP TOWARDS RISER FLOW Masters Thesis Ecole Centrale de Nantes, France, 2007. Abstract  Links  BibTeX  Tags: Cylinder, SST, URANS, Validation, Verification, Vortexshedding @mastersthesis{2007Msc_Thesis_CharlesMonroy, title = {A RANSE BASED STUDY OF THE FLOW BEHIND A CYLINDER. A FIRST STEP TOWARDS RISER FLOW}, author = {Charles Monroy}, url = {http://www.refresco.org/?wpdmpro=2007msc_thesis_charlesmonroypdf }, year = {2007}, date = {20070927}, school = {Ecole Centrale de Nantes, France}, abstract = {The objective of this thesis is to validate the RANS solver FreSCo, developed by MARIN, in the case of the flow around a fixed smooth circular cylinder. Despite the simple geometry of the problem, the computation of the flow around a cylinder is perhaps one of the most challenging problems of fluid dynamics. It is also of prime interest for offshore applications, especially for riser flows. The study focuses on 2D computations and deals with the different types of flows for several Reynolds numbers. The main features of the flow, such as velocity, pressure distribution on the cylinder and vorticity, are presented. The results are compared with experimental data and other computational results from different sources. The study shows that FreSCo provides excellent results for the steady laminar flow (up to Re ∼ 47), and satisfying ones for the unsteady laminar flow (from Re ∼ 47 to Re ∼ 1000). For turbulent flows, although there are significant differences with the experiment, FreSCo results are comparable with the performances of other CFD codes}, keywords = {Cylinder, SST, URANS, Validation, Verification, Vortexshedding}, pubstate = {published}, tppubtype = {mastersthesis} } The objective of this thesis is to validate the RANS solver FreSCo, developed by MARIN, in the case of the flow around a fixed smooth circular cylinder. Despite the simple geometry of the problem, the computation of the flow around a cylinder is perhaps one of the most challenging problems of fluid dynamics. It is also of prime interest for offshore applications, especially for riser flows. The study focuses on 2D computations and deals with the different types of flows for several Reynolds numbers. The main features of the flow, such as velocity, pressure distribution on the cylinder and vorticity, are presented. The results are compared with experimental data and other computational results from different sources. The study shows that FreSCo provides excellent results for the steady laminar flow (up to Re ∼ 47), and satisfying ones for the unsteady laminar flow (from Re ∼ 47 to Re ∼ 1000). For turbulent flows, although there are significant differences with the experiment, FreSCo results are comparable with the performances of other CFD codes  
Manzke, Manuel Using FreSCo for the Determination of Frictional Forces Masters Thesis TUHH, Hamburg, Germany, 2007. Links  BibTeX  Tags: 1eq Menter, Drag, Flatplate, kepsilon, komega, SpalartAllmaras, SST, Turbulence Models, Validation, Verification, Wallfunctions @mastersthesis{2007Stage_ManuelManzke, title = {Using FreSCo for the Determination of Frictional Forces}, author = {Manuel Manzke}, url = {http://www.refresco.org/?wpdmpro=2007stage_manuelmanzkepdf}, year = {2007}, date = {20070504}, school = {TUHH, Hamburg, Germany}, keywords = {1eq Menter, Drag, Flatplate, kepsilon, komega, SpalartAllmaras, SST, Turbulence Models, Validation, Verification, Wallfunctions}, pubstate = {published}, tppubtype = {mastersthesis} }  
Abeil, Bastien Validation of a RANS Code in the Handling of FreeSurface Flows Masters Thesis Chalmers University, Gothenburg, Sweden, 2007. Abstract  Links  BibTeX  Tags: 2D dambreak, 3D dambreak, CICSAM, Code Verification, Convection schemes, FreeSurface, Validation, Verification @mastersthesis{2007Msc_Thesis_BastienAbeil, title = {Validation of a RANS Code in the Handling of FreeSurface Flows}, author = {Bastien Abeil}, url = {http://www.refresco.org/?wpdmpro=2007msc_thesis_bastienabeilpdf}, year = {2007}, date = {20070305}, school = {Chalmers University, Gothenburg, Sweden}, abstract = {The ever increasing demand for performances of CFD codes in terms of free surface flows has yielded developers of new RANS code FRESCO to focus on the possible ways to retain interfaces between two fluids as accurate as possible. To that extent, a fairly complete study of the existing discretization schemes designed to handle free surface flows has led to dress up a list of the most relevant ones in a view of numerical implementation within the code. The emphasis will then be put on the performances of the CICSAM scheme (developed by O. Ubbink). A wide range of computations carried out with this scheme on several test cases will lead to a conclusion regarding his performance. Finally, two different dambreak test cases will be performed, and the results from FRESCO will be compared with the ones obtained by a commercial code currently used (COMFLOW) and also with experimental data.}, keywords = {2D dambreak, 3D dambreak, CICSAM, Code Verification, Convection schemes, FreeSurface, Validation, Verification}, pubstate = {published}, tppubtype = {mastersthesis} } The ever increasing demand for performances of CFD codes in terms of free surface flows has yielded developers of new RANS code FRESCO to focus on the possible ways to retain interfaces between two fluids as accurate as possible. To that extent, a fairly complete study of the existing discretization schemes designed to handle free surface flows has led to dress up a list of the most relevant ones in a view of numerical implementation within the code. The emphasis will then be put on the performances of the CICSAM scheme (developed by O. Ubbink). A wide range of computations carried out with this scheme on several test cases will lead to a conclusion regarding his performance. Finally, two different dambreak test cases will be performed, and the results from FRESCO will be compared with the ones obtained by a commercial code currently used (COMFLOW) and also with experimental data. 
Cavitation Chow Wing Code Verification Cylinder Drag EARSM FreeSurface HTC impacts Imposed Motion Lift Manoeuvring NACA 0015 Propeller RANS RANSBEM Coupling Regular Waves Rolldamping Ships SpalartAllmaras SRS SST Tipvortex Transition Turbines Turbulence Models URANS Validation Verification Wallfunctions