1.  Fernandes, Gerson; Kapsenberg, Geert; Kerkvliet, Maarten; van Walree, Frans: Numerical Investigation of Dynamic Stall, with Application to Stabilizer Fins of Ships. NUTTS 2015, September, Cortona, Italy, 2015. (Type: Conference  Links  BibTeX) @conference{2015NUTTS_Fernandes_StabilizerFins, title = {Numerical Investigation of Dynamic Stall, with Application to Stabilizer Fins of Ships}, author = {Gerson Fernandes and Geert Kapsenberg and Maarten Kerkvliet and Frans van Walree}, url = {http://www.refresco.org/?p=1554}, year = {2015}, date = {20150930}, booktitle = {NUTTS 2015, September, Cortona, Italy}, journal = {In }, keywords = {}, pubstate = {published}, tppubtype = {conference} } 
2.  Eca, Luis; Saraiva, Goncalo; Vaz, Guilherme; Abreu, Hugo: The PROS and CONS of Wall Functions. Proceedings of the ASME 2015 34st International Conference on Ocean, Offshore and Arctic Engineering, May 31stJune 5th, St. John’s, Canada, 2015. (Type: Conference  Abstract  Links  BibTeX) @conference{OMAE2015_Eca_WallFunctions, title = {The PROS and CONS of Wall Functions}, author = {Luis Eca and Goncalo Saraiva and Guilherme Vaz and Hugo Abreu}, url = {http://www.refresco.org/?wpdmpro=2015omae41518_ecavaz_et_al_wallfunctionspdf http://www.asmeconferences.org/}, year = {2015}, date = {20150601}, booktitle = {Proceedings of the ASME 2015 34st International Conference on Ocean, Offshore and Arctic Engineering, May 31stJune 5th, St. John’s, Canada}, journal = {Proceedings of the ASME 2015 34st International Conference on Ocean, Offshore and Arctic Engineering, May 31stJune 5th, 2015, St. John’s, Canada}, abstract = {Averaged NavierStokes (RANS) equations have become an engineering tool used on a daily basis. One of the main goals of such calculations is to determine friction forces, which are a consequence of the shearstress at solid walls. In RANS (and other more sophisticated mathematical models), there are two main approaches for the determination of the shearstress at a wall: direct application of the noslip condition, i.e. the velocity gradient is determined directly at the surface; wall functions which determine the shearstress at the wall from semiempirical equations applicable up to the outer edge of the socalled ”wall layer/log layer”. Although the first option is physically preferable, its numerical requirements may lead to iterative convergence problems and/or excessive calculation times. Therefore, especially at high Reynolds numbers, it is not unusual to use the latter approach. In this paper we discuss the advantages and disadvantages of wallfunction boundary conditions. To this end we have calculated the flow around a flat plate, conventional and laminar airfoils and a circular cylinder. The influence of the location where wall functions are applied (distance to the wall) and the effect of the Reynolds number (ranging from model to full scale applications) are discussed. Griding requirements for wallfunction boundary conditions are also addressed. The results obtained with wall functions are compared with those obtained from the direct application of the no slip at the wall. The results obtained in this study show that the use of wall functions in viscous flow calculations may be justifiable or completely unacceptable depending on the flow conditions. Furthermore, it is also shown that wallfunction boundary conditions also require clustering of grid nodes close to the wall, but obviously less demanding than the direct application of no slip condition.}, keywords = {}, pubstate = {published}, tppubtype = {conference} } Averaged NavierStokes (RANS) equations have become an engineering tool used on a daily basis. One of the main goals of such calculations is to determine friction forces, which are a consequence of the shearstress at solid walls. In RANS (and other more sophisticated mathematical models), there are two main approaches for the determination of the shearstress at a wall: direct application of the noslip condition, i.e. the velocity gradient is determined directly at the surface; wall functions which determine the shearstress at the wall from semiempirical equations applicable up to the outer edge of the socalled ”wall layer/log layer”. Although the first option is physically preferable, its numerical requirements may lead to iterative convergence problems and/or excessive calculation times. Therefore, especially at high Reynolds numbers, it is not unusual to use the latter approach. In this paper we discuss the advantages and disadvantages of wallfunction boundary conditions. To this end we have calculated the flow around a flat plate, conventional and laminar airfoils and a circular cylinder. The influence of the location where wall functions are applied (distance to the wall) and the effect of the Reynolds number (ranging from model to full scale applications) are discussed. Griding requirements for wallfunction boundary conditions are also addressed. The results obtained with wall functions are compared with those obtained from the direct application of the no slip at the wall. The results obtained in this study show that the use of wall functions in viscous flow calculations may be justifiable or completely unacceptable depending on the flow conditions. Furthermore, it is also shown that wallfunction boundary conditions also require clustering of grid nodes close to the wall, but obviously less demanding than the direct application of no slip condition. 
3.  Fernandes, Gerson; Make, Michel; Gueydon, Sebastien; Vaz, Guilherme: Sensitivity to Aerodynamic Forces for the Accurate Modelling of Floating Offshore Wind Turbines. Renewable2014, November, Lisbon, Portugal., 2014. (Type: Conference  Abstract  Links  BibTeX) @conference{Renewable2014_Fernandes_BEMT+MSWT, title = {Sensitivity to Aerodynamic Forces for the Accurate Modelling of Floating Offshore Wind Turbines}, author = {Gerson Fernandes and Michel Make and Sebastien Gueydon and Guilherme Vaz }, url = {http://www.refresco.org/?wpdmpro=2014renewable2014_fernandes_et_alpdf}, year = {2014}, date = {20141120}, booktitle = {Renewable2014, November, Lisbon, Portugal.}, abstract = {In order to accurately study floating offshore wind turbines modelscaled experimental campaigns are crucial. Parallel to it, numerical tools play an important role. They permit to: aid the preparation of the experiments, to make faster and more costeffective design variations, and to obtain more physical insight on the flow. Within this context, in this paper the simulation of MARIN Stock Wind Turbine (MSWT) at modelscale (1/50) conditions has been addressed. Standard (BEMT, XFOIL) and advanced (CFD) methods have been used. Both the panel method code XFOIL and the BEMT tool FAST are engineering tools used successfully for the study of airfoils and wind turbines at typical fullscale conditions, e.g. Reynolds numbers > 1E6. However, for the current work at modelscale the characteristic Reynolds numbers are lower than 1E5. The twodimensional input data for the BEMT code was calculated in the common way using XFOIL and also with the viscousflow ReFRESCO CFD code. The aerodynamic characteristics of the flow over the airfoil are shown to be highly unsteady, with extensive regions of separation beyond moderate angles of attack. BEMT calculations showed a poor comparison of the CP and a better match in the CT. Threedimensional CFD calculations of the MSWT at model and fullscale were performed, in which the flow over the blade at modelscale was seen to be highly separated with extensive crossflow components, contrary to what is seen at computations at fullscale. The agreement of the 3D CFD calculations with the experimental results was remarkably good. These results show that these BEMT tools are very sensitive to the input provided, and approach their limits for flows at modelscale lowReynolds number conditions.}, keywords = {}, pubstate = {published}, tppubtype = {conference} } In order to accurately study floating offshore wind turbines modelscaled experimental campaigns are crucial. Parallel to it, numerical tools play an important role. They permit to: aid the preparation of the experiments, to make faster and more costeffective design variations, and to obtain more physical insight on the flow. Within this context, in this paper the simulation of MARIN Stock Wind Turbine (MSWT) at modelscale (1/50) conditions has been addressed. Standard (BEMT, XFOIL) and advanced (CFD) methods have been used. Both the panel method code XFOIL and the BEMT tool FAST are engineering tools used successfully for the study of airfoils and wind turbines at typical fullscale conditions, e.g. Reynolds numbers > 1E6. However, for the current work at modelscale the characteristic Reynolds numbers are lower than 1E5. The twodimensional input data for the BEMT code was calculated in the common way using XFOIL and also with the viscousflow ReFRESCO CFD code. The aerodynamic characteristics of the flow over the airfoil are shown to be highly unsteady, with extensive regions of separation beyond moderate angles of attack. BEMT calculations showed a poor comparison of the CP and a better match in the CT. Threedimensional CFD calculations of the MSWT at model and fullscale were performed, in which the flow over the blade at modelscale was seen to be highly separated with extensive crossflow components, contrary to what is seen at computations at fullscale. The agreement of the 3D CFD calculations with the experimental results was remarkably good. These results show that these BEMT tools are very sensitive to the input provided, and approach their limits for flows at modelscale lowReynolds number conditions. 
4.  Saraiva, Goncalo: Solution of Flows Around Airfoils Using RANS with WallFunctions. IST, Lisbon, Portugal, 2014. (Type: Masters Thesis  Abstract  Links  BibTeX) @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 = {}, 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. 
5.  Make, Michel: Predicting scale effects on floating offshore wind turbines. Technical University of Delft, the Netherlands, 2014. (Type: Masters Thesis  Abstract  Links  BibTeX) @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 = {}, 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. 
2015 

Fernandes, Gerson; Kapsenberg, Geert; Kerkvliet, Maarten; van Walree, Frans Numerical Investigation of Dynamic Stall, with Application to Stabilizer Fins of Ships Conference NUTTS 2015, September, Cortona, Italy, 2015. Links  BibTeX  Tags: dynamic stall, Foils, KSKL, NACA0012, RANS, SpalartAllmaras, SST, Stabilizer fins, stall, URANS, Validation, Verification @conference{2015NUTTS_Fernandes_StabilizerFins, title = {Numerical Investigation of Dynamic Stall, with Application to Stabilizer Fins of Ships}, author = {Gerson Fernandes and Geert Kapsenberg and Maarten Kerkvliet and Frans van Walree}, url = {http://www.refresco.org/?p=1554}, year = {2015}, date = {20150930}, booktitle = {NUTTS 2015, September, Cortona, Italy}, journal = {In }, keywords = {dynamic stall, Foils, KSKL, NACA0012, RANS, SpalartAllmaras, SST, Stabilizer fins, stall, URANS, Validation, Verification}, pubstate = {published}, tppubtype = {conference} }  
Eca, Luis; Saraiva, Goncalo; Vaz, Guilherme; Abreu, Hugo The PROS and CONS of Wall Functions Conference Proceedings of the ASME 2015 34st International Conference on Ocean, Offshore and Arctic Engineering, May 31stJune 5th, St. John’s, Canada, 2015. Abstract  Links  BibTeX  Tags: Cylinder, Flatplate, Foils, RANS, SST, URANS, Validation, Verification, Wallfunctions, yplus @conference{OMAE2015_Eca_WallFunctions, title = {The PROS and CONS of Wall Functions}, author = {Luis Eca and Goncalo Saraiva and Guilherme Vaz and Hugo Abreu}, url = {http://www.refresco.org/?wpdmpro=2015omae41518_ecavaz_et_al_wallfunctionspdf http://www.asmeconferences.org/}, year = {2015}, date = {20150601}, booktitle = {Proceedings of the ASME 2015 34st International Conference on Ocean, Offshore and Arctic Engineering, May 31stJune 5th, St. John’s, Canada}, journal = {Proceedings of the ASME 2015 34st International Conference on Ocean, Offshore and Arctic Engineering, May 31stJune 5th, 2015, St. John’s, Canada}, abstract = {Averaged NavierStokes (RANS) equations have become an engineering tool used on a daily basis. One of the main goals of such calculations is to determine friction forces, which are a consequence of the shearstress at solid walls. In RANS (and other more sophisticated mathematical models), there are two main approaches for the determination of the shearstress at a wall: direct application of the noslip condition, i.e. the velocity gradient is determined directly at the surface; wall functions which determine the shearstress at the wall from semiempirical equations applicable up to the outer edge of the socalled ”wall layer/log layer”. Although the first option is physically preferable, its numerical requirements may lead to iterative convergence problems and/or excessive calculation times. Therefore, especially at high Reynolds numbers, it is not unusual to use the latter approach. In this paper we discuss the advantages and disadvantages of wallfunction boundary conditions. To this end we have calculated the flow around a flat plate, conventional and laminar airfoils and a circular cylinder. The influence of the location where wall functions are applied (distance to the wall) and the effect of the Reynolds number (ranging from model to full scale applications) are discussed. Griding requirements for wallfunction boundary conditions are also addressed. The results obtained with wall functions are compared with those obtained from the direct application of the no slip at the wall. The results obtained in this study show that the use of wall functions in viscous flow calculations may be justifiable or completely unacceptable depending on the flow conditions. Furthermore, it is also shown that wallfunction boundary conditions also require clustering of grid nodes close to the wall, but obviously less demanding than the direct application of no slip condition.}, keywords = {Cylinder, Flatplate, Foils, RANS, SST, URANS, Validation, Verification, Wallfunctions, yplus}, pubstate = {published}, tppubtype = {conference} } Averaged NavierStokes (RANS) equations have become an engineering tool used on a daily basis. One of the main goals of such calculations is to determine friction forces, which are a consequence of the shearstress at solid walls. In RANS (and other more sophisticated mathematical models), there are two main approaches for the determination of the shearstress at a wall: direct application of the noslip condition, i.e. the velocity gradient is determined directly at the surface; wall functions which determine the shearstress at the wall from semiempirical equations applicable up to the outer edge of the socalled ”wall layer/log layer”. Although the first option is physically preferable, its numerical requirements may lead to iterative convergence problems and/or excessive calculation times. Therefore, especially at high Reynolds numbers, it is not unusual to use the latter approach. In this paper we discuss the advantages and disadvantages of wallfunction boundary conditions. To this end we have calculated the flow around a flat plate, conventional and laminar airfoils and a circular cylinder. The influence of the location where wall functions are applied (distance to the wall) and the effect of the Reynolds number (ranging from model to full scale applications) are discussed. Griding requirements for wallfunction boundary conditions are also addressed. The results obtained with wall functions are compared with those obtained from the direct application of the no slip at the wall. The results obtained in this study show that the use of wall functions in viscous flow calculations may be justifiable or completely unacceptable depending on the flow conditions. Furthermore, it is also shown that wallfunction boundary conditions also require clustering of grid nodes close to the wall, but obviously less demanding than the direct application of no slip condition.  
2014 

Fernandes, Gerson; Make, Michel; Gueydon, Sebastien; Vaz, Guilherme Sensitivity to Aerodynamic Forces for the Accurate Modelling of Floating Offshore Wind Turbines Conference Renewable2014, November, Lisbon, Portugal., 2014. Abstract  Links  BibTeX  Tags: BEMT, Foils, MSWT, RANS, SpalartAllmaras, SST, URANS @conference{Renewable2014_Fernandes_BEMT+MSWT, title = {Sensitivity to Aerodynamic Forces for the Accurate Modelling of Floating Offshore Wind Turbines}, author = {Gerson Fernandes and Michel Make and Sebastien Gueydon and Guilherme Vaz }, url = {http://www.refresco.org/?wpdmpro=2014renewable2014_fernandes_et_alpdf}, year = {2014}, date = {20141120}, booktitle = {Renewable2014, November, Lisbon, Portugal.}, abstract = {In order to accurately study floating offshore wind turbines modelscaled experimental campaigns are crucial. Parallel to it, numerical tools play an important role. They permit to: aid the preparation of the experiments, to make faster and more costeffective design variations, and to obtain more physical insight on the flow. Within this context, in this paper the simulation of MARIN Stock Wind Turbine (MSWT) at modelscale (1/50) conditions has been addressed. Standard (BEMT, XFOIL) and advanced (CFD) methods have been used. Both the panel method code XFOIL and the BEMT tool FAST are engineering tools used successfully for the study of airfoils and wind turbines at typical fullscale conditions, e.g. Reynolds numbers > 1E6. However, for the current work at modelscale the characteristic Reynolds numbers are lower than 1E5. The twodimensional input data for the BEMT code was calculated in the common way using XFOIL and also with the viscousflow ReFRESCO CFD code. The aerodynamic characteristics of the flow over the airfoil are shown to be highly unsteady, with extensive regions of separation beyond moderate angles of attack. BEMT calculations showed a poor comparison of the CP and a better match in the CT. Threedimensional CFD calculations of the MSWT at model and fullscale were performed, in which the flow over the blade at modelscale was seen to be highly separated with extensive crossflow components, contrary to what is seen at computations at fullscale. The agreement of the 3D CFD calculations with the experimental results was remarkably good. These results show that these BEMT tools are very sensitive to the input provided, and approach their limits for flows at modelscale lowReynolds number conditions.}, keywords = {BEMT, Foils, MSWT, RANS, SpalartAllmaras, SST, URANS}, pubstate = {published}, tppubtype = {conference} } In order to accurately study floating offshore wind turbines modelscaled experimental campaigns are crucial. Parallel to it, numerical tools play an important role. They permit to: aid the preparation of the experiments, to make faster and more costeffective design variations, and to obtain more physical insight on the flow. Within this context, in this paper the simulation of MARIN Stock Wind Turbine (MSWT) at modelscale (1/50) conditions has been addressed. Standard (BEMT, XFOIL) and advanced (CFD) methods have been used. Both the panel method code XFOIL and the BEMT tool FAST are engineering tools used successfully for the study of airfoils and wind turbines at typical fullscale conditions, e.g. Reynolds numbers > 1E6. However, for the current work at modelscale the characteristic Reynolds numbers are lower than 1E5. The twodimensional input data for the BEMT code was calculated in the common way using XFOIL and also with the viscousflow ReFRESCO CFD code. The aerodynamic characteristics of the flow over the airfoil are shown to be highly unsteady, with extensive regions of separation beyond moderate angles of attack. BEMT calculations showed a poor comparison of the CP and a better match in the CT. Threedimensional CFD calculations of the MSWT at model and fullscale were performed, in which the flow over the blade at modelscale was seen to be highly separated with extensive crossflow components, contrary to what is seen at computations at fullscale. The agreement of the 3D CFD calculations with the experimental results was remarkably good. These results show that these BEMT tools are very sensitive to the input provided, and approach their limits for flows at modelscale lowReynolds number conditions.  
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.  
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. 