2014 

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.  
2013 

Aurelle, Julian Prediction of Wave Loading from Steep Waves on Fixed Offshore Wind Turbines Masters Thesis Ecole Centrale de Nantes, France, 2013. Links  BibTeX  Tags: impacts, Regular Waves, Turbines, Validation, Verification @mastersthesis{2013Stage_Julian_Aurelle, title = {Prediction of Wave Loading from Steep Waves on Fixed Offshore Wind Turbines}, author = {Julian Aurelle}, url = {http://www.refresco.org/?wpdmpro=2013stage_julian_aurellepdf}, year = {2013}, date = {20131231}, school = {Ecole Centrale de Nantes, France}, keywords = {impacts, Regular Waves, Turbines, Validation, Verification}, pubstate = {published}, tppubtype = {mastersthesis} }  
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. 
ReFRESCO related MSc and Phd Thesesrefrescoadmin20190619T14:01:23+01:00
2014 

Predicting scale effects on floating offshore wind turbines Masters Thesis Technical University of Delft, the Netherlands, 2014.  
2013 

Prediction of Wave Loading from Steep Waves on Fixed Offshore Wind Turbines Masters Thesis Ecole Centrale de Nantes, France, 2013.  
2011 

NUMERICAL SIMULATIONS OF FLOW OVER AN AXIAL MARINE CURRENT TURBINE Masters Thesis Technical University of Delft, the Netherlands, 2011. 