2016 

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.  
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.  
2014 

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 

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} }  
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.  
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 

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} }  
2009 

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  
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 
ReFRESCO related MSc and Phd Thesesrefrescoadmin20190619T14:01:23+01:00