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The most effective way to decrease the cost of energy of wind turbines is to increase the rotor diameter. Therefore, this calls for the need of aerodynamic research on large wind turbines. This thesis project will tackle the topics addressed by the IEA Wind Task 47 which is focused on the aerodynamic analysis and comparison of the newly proposed IEA 15-MW reference wind turbine using varying fidelity tools. Therefore, the main goal of this work is to analyze whether a low-fidelity approach is still feasible for aerodynamic analysis in comparison with a high-fidelity approach. It was found that there are quantitative differences between the different fidelity methods but also qualitative similarities. It can be concluded that the low-fidelity methods are relatively less suitable for a large turbine design, especially for high TSR values, which should be confirmed by further research including different fidelity tools and turbines., Aerospace Engineering

This paper compares three different numerical models to evaluate their accuracy for predicting the performance of an H-rotor vertical-axis wind turbine (VAWT) considering the influence of struts. The strut of VAWTs is one factor that makes the flow feature around the turbine more complex and thus influences the rotor performance. The focus of this study is placed on analyzing how accurately three different numerical approaches are able to reproduce the force distribution and the resulting power, taking the strut effect into account. For the 12 kW straight-bladed VAWT, the blade force is simulated at three tip speed ratios by the full computational fluid dynamics (CFD) model based on the Reynolds-averaged Navier-Stokes (RANS) equations, the actuator line model (ALM), and the vortex model. The results show that all the models do not indicate a significant influence of the struts in the total force over one revolution at low tip speed ratio. However, at middle and high tip speed ratio, the RANS model reproduces the significant decrease of the total tangential force that is caused due to the strut. Additionally, the RANS and vortex models present a clear influence of the struts in the force distribution along the blade at all three tip speed ratios investigated. The prediction by the ALM does not show such distinctive features of the strut impact. The RANS model is superior to the other two models for predicting the power coefficient considering the strut effect, especially at high tip speed ratio.

This paper compares three different numerical models to evaluate their accuracy for predicting the performance of an H-rotor vertical-axis wind turbine (VAWT) considering the influence of struts. The strut of VAWTs is one factor that makes the flow feature around the turbine more complex and thus influences the rotor performance. The focus of this study is placed on analyzing how accurately three different numerical approaches are able to reproduce the force distribution and the resulting power, taking the strut effect into account. For the 12 kW straight-bladed VAWT, the blade force is simulated at three tip speed ratios by the full computational fluid dynamics (CFD) model based on the Reynolds-averaged Navier-Stokes (RANS) equations, the actuator line model (ALM), and the vortex model. The results show that all the models do not indicate a significant influence of the struts in the total force over one revolution at low tip speed ratio. However, at middle and high tip speed ratio, the RANS model reproduces the significant decrease of the total tangential force that is caused due to the strut. Additionally, the RANS and vortex models present a clear influence of the struts in the force distribution along the blade at all three tip speed ratios investigated. The prediction by the ALM does not show such distinctive features of the strut impact. The RANS model is superior to the other two models for predicting the power coefficient considering the strut effect, especially at high tip speed ratio.

This paper compares three different numerical models to evaluate their accuracy for predicting the performance of an H-rotor vertical-axis wind turbine (VAWT) considering the influence of struts. The strut of VAWTs is one factor that makes the flow feature around the turbine more complex and thus influences the rotor performance. The focus of this study is placed on analyzing how accurately three different numerical approaches are able to reproduce the force distribution and the resulting power, taking the strut effect into account. For the 12 kW straight-bladed VAWT, the blade force is simulated at three tip speed ratios by the full computational fluid dynamics (CFD) model based on the Reynolds-averaged Navier-Stokes (RANS) equations, the actuator line model (ALM), and the vortex model. The results show that all the models do not indicate a significant influence of the struts in the total force over one revolution at low tip speed ratio. However, at middle and high tip speed ratio, the RANS model reproduces the significant decrease of the total tangential force that is caused due to the strut. Additionally, the RANS and vortex models present a clear influence of the struts in the force distribution along the blade at all three tip speed ratios investigated. The prediction by the ALM does not show such distinctive features of the strut impact. The RANS model is superior to the other two models for predicting the power coefficient considering the strut effect, especially at high tip speed ratio.

This paper compares three different numerical models to evaluate their accuracy for predicting the performance of an H-rotor vertical-axis wind turbine (VAWT) considering the influence of struts. The strut of VAWTs is one factor that makes the flow feature around the turbine more complex and thus influences the rotor performance. The focus of this study is placed on analyzing how accurately three different numerical approaches are able to reproduce the force distribution and the resulting power, taking the strut effect into account. For the 12 kW straight-bladed VAWT, the blade force is simulated at three tip speed ratios by the full computational fluid dynamics (CFD) model based on the Reynolds-averaged Navier-Stokes (RANS) equations, the actuator line model (ALM), and the vortex model. The results show that all the models do not indicate a significant influence of the struts in the total force over one revolution at low tip speed ratio. However, at middle and high tip speed ratio, the RANS model reproduces the significant decrease of the total tangential force that is caused due to the strut. Additionally, the RANS and vortex models present a clear influence of the struts in the force distribution along the blade at all three tip speed ratios investigated. The prediction by the ALM does not show such distinctive features of the strut impact. The RANS model is superior to the other two models for predicting the power coefficient considering the strut effect, especially at high tip speed ratio.

The manuscript presents a novel numerical investigation on the impact of the wake of a floating IEA wind 15-MW reference wind turbine (RWT) on downstream machines using a state-of-the-art vortex solver coupled to a multi-body code. The coupling enables to account for the flexibility of the different turbine components as well as to include the effect of the controller and the dynamics of the floating support structure. First, the turbine is mounted on the WindCrete spar-buoy platform, and the wake impact on a second turbine positioned at different downstream positions is investigated and compared with the impact of the wake generated by a bottom-fixed machine. It is found that the faster breakdown of the vortex structures triggered by the motion of the floater in the upstream turbine increases the power production of a downstream machine, as well as its mean thrust level relative to normal operation downstream a bottom-fixed machine. It is demonstrated that this effect is drastically reduced with the increase of the turbulence intensity (TI). Further, simulations with a prescribed harmonic motion of the upstream turbine in surge and pitch under different turbulence levels are presented. It is found that the motion of the floater has a strong impact in the generated wake and consequently in the operation of downstream machines. In particular, downstream turbines experienced considerably higher blade loading that led to an increase of the aerodynamic power. Finally, aero-hydro-servo-elastic simulations of five turbines in a row have shown that the interaction between multiple floating machines is more dynamic than between bottom-fixed turbines. This has been mainly observed at high wind speeds, where the pitch and surge motions of the floater in turbines located deep inside the farm can be resonantly excited by the interaction with the wind farm flow. In practice, this means that the power and thrust variations increase with the turbine location depth inside the farm.

The most effective way to decrease the cost of energy of wind turbines is to increase the rotor diameter. Therefore, this calls for the need of aerodynamic research on large wind turbines. This thesis project will tackle the topics addressed by the IEA Wind Task 47 which is focused on the aerodynamic analysis and comparison of the newly proposed IEA 15-MW reference wind turbine using varying fidelity tools. Therefore, the main goal of this work is to analyze whether a low-fidelity approach is still feasible for aerodynamic analysis in comparison with a high-fidelity approach. It was found that there are quantitative differences between the different fidelity methods but also qualitative similarities. It can be concluded that the low-fidelity methods are relatively less suitable for a large turbine design, especially for high TSR values, which should be confirmed by further research including different fidelity tools and turbines., Aerospace Engineering

This paper compares three different numerical models to evaluate their accuracy for predicting the performance of an H-rotor vertical-axis wind turbine (VAWT) considering the influence of struts. The strut of VAWTs is one factor that makes the flow feature around the turbine more complex and thus influences the rotor performance. The focus of this study is placed on analyzing how accurately three different numerical approaches are able to reproduce the force distribution and the resulting power, taking the strut effect into account. For the 12 kW straight-bladed VAWT, the blade force is simulated at three tip speed ratios by the full computational fluid dynamics (CFD) model based on the Reynolds-averaged Navier-Stokes (RANS) equations, the actuator line model (ALM), and the vortex model. The results show that all the models do not indicate a significant influence of the struts in the total force over one revolution at low tip speed ratio. However, at middle and high tip speed ratio, the RANS model reproduces the significant decrease of the total tangential force that is caused due to the strut. Additionally, the RANS and vortex models present a clear influence of the struts in the force distribution along the blade at all three tip speed ratios investigated. The prediction by the ALM does not show such distinctive features of the strut impact. The RANS model is superior to the other two models for predicting the power coefficient considering the strut effect, especially at high tip speed ratio.

About a century ago, two different rotor models were proposed by Joukowsky (1912–1918) and Betz (1919). Both models were based on assumptions regarding the vortex structures of the wake and its induction on the rotor plane. However, due to complications of formulating the wake behavior into actual guidelines for the aerodynamic design of the rotor geometry, the models have until now not been used to design actual industrial wind turbine rotors. In this article, we propose a technique to solve analytically the induction problem of the two models, which enables the design of wind turbine rotors. We briefly present the theory behind the two rotor models, and show how this theory can be exploited to make actual designs of rotor planforms, i.e. chord- and twist-distributions. The designs are for three-bladed rotors optimized for different tip speed ratios and analyzed by comparing their performance to results using blade-element/momentum technique and lifting line theory.

Floating wind turbines have the potential to enable global exploitation of offshore wind energy, but there is a need to further understand the complex aerodynamic phenomena they can encounter due to floater induced rotor motion. Aerodynamic models traditionally used in the wind energy sector, like the Blade Element Momentum (BEM) theory, may not be capable of capturing the dynamic phenomena that occur when the rotor moves in and out of its own wake. In the present paper, we therefore compare an industry standard BEM-based code to a state of the art vortex solver, to investigate the phenomena in detail and further clarify the capabilities and limitations of both methods. An initial benchmark of the two codes using the IEA Wind 15 MW RWT mounted on the WindCrete spar-buoy floater is carried out. Three different scenarios are taken into account: a bottom-fixed, a floating case, and a floating case subject to regular waves. Growing discrepancies between the codes have been observed with the increasing complexity of the simulations. Moreover, large differences between the wake generated by a bottom fixed and a floating turbine have been observed, with the latter one experiencing a faster recovery. To further explore the floating turbines behaviors that can affect the rotor performance and wake, a systematic investigation of the mean tilt angle influence in the wake development has been carried out. Further, to account for the oscillatory motion of a floating turbine, a parametric study where the floater motion is prescribed in both pitch and surge degrees of freedom (DoF) is designed. The study covers a large variety of scenarios; a wide range of relevant frequencies and amplitudes are taken into account in under and above-rated wind conditions. A total of more than 28 unique cases have been defined and simulated with both fidelity models. The results include the downstream evolution of the wake recovery and intensity of the turbulent disturbances induced by the rotor. T

High fidelity interactions between game characters and gaseous effects like smoke, fire and explosions are often neglected in real-time applications due to the high computational cost of simulating fluids. In addition, the pose of game characters is only known at run-time as it depends on input from the user. Thus simulation-suitable representations of surface geometry must be generated on the fly. Common approaches like conversion into signed distance fields are not feasible for high-resolution geometry due to the computational cost and the amount of memory required on the GPU to store these fields. We present a purely vortex particle based fluid model for games which is capable of resolving the collision between fluids and complex objects such as moving game characters in real time. To handle collisions, we use a collocation method which only require a set of disassociated particles stuck to collision surfaces. Contrary to most other vorticity based methods, we use a simple inversion free approach to obtain the collision velocity field on surfaces while at the same time avoiding the expensive pressure projection step associated with pressure based fluid solvers.

High fidelity interactions between game characters and gaseous effects like smoke, fire and explosions are often neglected in real-time applications due to the high computational cost of simulating fluids. In addition, the pose of game characters is only known at run-time as it depends on input from the user. Thus simulation-suitable representations of surface geometry must be generated on the fly. Common approaches like conversion into signed distance fields are not feasible for high-resolution geometry due to the computational cost and the amount of memory required on the GPU to store these fields. We present a purely vortex particle based fluid model for games which is capable of resolving the collision between fluids and complex objects such as moving game characters in real time. To handle collisions, we use a collocation method which only require a set of disassociated particles stuck to collision surfaces. Contrary to most other vorticity based methods, we use a simple inversion free approach to obtain the collision velocity field on surfaces while at the same time avoiding the expensive pressure projection step associated with pressure based fluid solvers.

This study investigates the wake interaction of four full-scale three-bladed tidal turbines with different ambient turbulence conditions, in straight and yawed flows. A three-dimensional unsteady Lagrangian Vortex Blob software is used for the numerical simulations of the turbines’ wakes. In order to model the ambient turbulence in the Lagrangian Vortex Method formalism, a Synthetic Eddy Method is used. With this method, turbulent structures are added in the computational domain to generate a velocity field which statistically reproduces any ambient turbulence intensity and integral length scale. The influence of the size of the structures and their density (within the study volume) on the wake of a single turbine is studied. Good agreement is obtained between numerical and experimental results for a high turbulence intensity but too many structures can increase the numerical dissipation and reduce the wake extension. Numerical simulations of the four turbine array with the layout initially proposed for the NEPTHYD pilot farm are then presented. Two ambient turbulence intensities encountered in the Alderney Race and two integral length scales are tested with a straight flow. Finally, the wakes obtained for yawed flows with different angles are presented, highlighting turbine interactions.

Midway between a vortex method and a grid-based CFD, the Vortex Particle-Mesh method with Immersed Lifting Lines is intended to provide medium-fidelity results on rotor loadings, together with a realistic representation of the 3-D vortical wakes and their dynamics over long distances. We assess the potential of this hybrid approach for the Large-Eddy Simulation of helicopter rotors in hover. A novel Poisson solver with mixed unbounded-outflow boundary conditions here further enables the computation of turbulent hovering scenarios in tight domains. Considering the Knight and Hefner experiment and the S-76 test case as references, we present and compare blade integrated and distributed loads, induction velocities, and wake characteristics. While the quality of the performance predictions highly depends on external polar data, the obtained wakes exhibit similar characteristics to those recently identified in other CFD analyses, here at a moderate computational cost. Based on these results, we further investigate the secondary vortex structures forming between the main tip vortices, and we bring additional clues on their relation to the phenomenon of wake breakdown. We finally discuss the strengths and difficulties of hybrid vortex methods for the challenging analysis of hovering rotors.

This study investigates the wake interaction of four full-scale three-bladed tidal turbines with different ambient turbulence conditions, in straight and yawed flows. A three-dimensional unsteady Lagrangian Vortex Blob software is used for the numerical simulations of the turbines’ wakes. In order to model the ambient turbulence in the Lagrangian Vortex Method formalism, a Synthetic Eddy Method is used. With this method, turbulent structures are added in the computational domain to generate a velocity field which statistically reproduces any ambient turbulence intensity and integral length scale. The influence of the size of the structures and their density (within the study volume) on the wake of a single turbine is studied. Good agreement is obtained between numerical and experimental results for a high turbulence intensity but too many structures can increase the numerical dissipation and reduce the wake extension. Numerical simulations of the four turbine array with the layout initially proposed for the NEPTHYD pilot farm are then presented. Two ambient turbulence intensities encountered in the Alderney Race and two integral length scales are tested with a straight flow. Finally, the wakes obtained for yawed flows with different angles are presented, highlighting turbine interactions.

This study investigates the wake interaction of four full-scale three-bladed tidal turbines with different ambient turbulence conditions, in straight and yawed flows. A three-dimensional unsteady Lagrangian Vortex Blob software is used for the numerical simulations of the turbines’ wakes. In order to model the ambient turbulence in the Lagrangian Vortex Method formalism, a Synthetic Eddy Method is used. With this method, turbulent structures are added in the computational domain to generate a velocity field which statistically reproduces any ambient turbulence intensity and integral length scale. The influence of the size of the structures and their density (within the study volume) on the wake of a single turbine is studied. Good agreement is obtained between numerical and experimental results for a high turbulence intensity but too many structures can increase the numerical dissipation and reduce the wake extension. Numerical simulations of the four turbine array with the layout initially proposed for the NEPTHYD pilot farm are then presented. Two ambient turbulence intensities encountered in the Alderney Race and two integral length scales are tested with a straight flow. Finally, the wakes obtained for yawed flows with different angles are presented, highlighting turbine interactions.

The manuscript presents a novel aero‐hydro‐servo‐elastic coupling framework, MIRAS‐HAWC2. In this coupling, the wind turbine blades and rotor‐wake aerodynamics are modeled using a modified lifting‐line theory which accounts for blade curvature, combined with a hybrid vortex method. The wind turbine structure and foundation are modeled using a finite‐element and multi‐body system approach. Last, hydrodynamics are modeled using Airy wave theory together with Morison's equation. An initial assessment of the performance of the aeroelastic coupling framework has been performed for steady rotor‐only cases, assuming laminar inflow without shear. This included a comparison against fully resolved computational fluid dynamics, for both stiff and flexible blades showing an excellent agreement. In a second stage, the aero‐hydro‐servo‐elastic coupling is used, comparing MIRAS‐HAWC2 as well as blade‐element momentum‐based simulations with selected results from the Offshore Code Comparison Collaboration projects (OC3 and OC4), which study the NREL 5 MW turbine mounted on different offshore support structures. A good agreement has been obtained for the simulations of a monopile with rigid foundation, a tripod, and jacket support structures.

Vertical Axis Wind Turbines (VAWTs) are characterized by complex and unsteady flow patterns resulting in considerable challenges for both numerical simulations and measurements describing the phenomena involved. In this study, a 3D Actuator Line Model (ALM) is compared to a 2D and a 3D Vortex Model, and they are validated using the normal forces measurements on a blade of an operating 12 kW VAWT, which is located in an open site in the north of Uppsala, Sweden. First, the coefficient power ( Cp ) curve of the device has been simulated and compared against the experimental one. Then, a wide range of operational conditions for different tip speed ratios (TSRs), with λ = 1.84, 2.55, 3.06, 3.44, 4.09 and 4.57 were investigated. The results showed descent agreement with the experimental data for both models in terms of the trend and magnitudes. On one side, a slight improvement for representing the normal forces was achieved by the ALM, while the vortex code performs better in the simulation of the Cp curve. Similarities and discrepancies between numerical and experimental results are discussed.

Vertical Axis Wind Turbines (VAWTs) are characterized by complex and unsteady flow patterns resulting in considerable challenges for both numerical simulations and measurements describing the phenomena involved. In this study, a 3D Actuator Line Model (ALM) is compared to a 2D and a 3D Vortex Model, and they are validated using the normal forces measurements on a blade of an operating 12 kW VAWT, which is located in an open site in the north of Uppsala, Sweden. First, the coefficient power ( Cp ) curve of the device has been simulated and compared against the experimental one. Then, a wide range of operational conditions for different tip speed ratios (TSRs), with λ = 1.84, 2.55, 3.06, 3.44, 4.09 and 4.57 were investigated. The results showed descent agreement with the experimental data for both models in terms of the trend and magnitudes. On one side, a slight improvement for representing the normal forces was achieved by the ALM, while the vortex code performs better in the simulation of the Cp curve. Similarities and discrepancies between numerical and experimental results are discussed.

Vertical Axis Wind Turbines (VAWTs) are characterized by complex and unsteady flow patterns resulting in considerable challenges for both numerical simulations and measurements describing the phenomena involved. In this study, a 3D Actuator Line Model (ALM) is compared to a 2D and a 3D Vortex Model, and they are validated using the normal forces measurements on a blade of an operating 12 kW VAWT, which is located in an open site in the north of Uppsala, Sweden. First, the coefficient power ( Cp ) curve of the device has been simulated and compared against the experimental one. Then, a wide range of operational conditions for different tip speed ratios (TSRs), with λ = 1.84, 2.55, 3.06, 3.44, 4.09 and 4.57 were investigated. The results showed descent agreement with the experimental data for both models in terms of the trend and magnitudes. On one side, a slight improvement for representing the normal forces was achieved by the ALM, while the vortex code performs better in the simulation of the Cp curve. Similarities and discrepancies between numerical and experimental results are discussed.

This paper describes a detailed implementation of the Synthetic Eddy Method (SEM) initially presented in Jarrin et al. (2006) applied to the Lagrangian Vortex simulation. While the treatment of turbulent diffusion is already extensively covered in scientific literature, this is one of the first attempts to represent ambient turbulence in a fully Lagrangian framework. This implementation is well suited to the integration of PSE (Particle Strength Exchange) or DVM (Diffusion Velocity Method), often used to account for molecular and turbulent diffusion in Lagrangian simulations. The adaptation and implementation of the SEM into a Lagrangian method using the PSE diffusion model is presented, and the turbulent velocity fields produced by this method are then analysed. In this adaptation, SEM turbulent structures are simply advected, without stretching or diffusion of their own, over the flow domain. This implementation proves its ability to produce turbulent velocity fields in accordance with any desired turbulent flow parameters. As the SEM is a purely mathematical and stochastic model, turbulent spectra and turbulent length scales are also investigated. With the addition of variation in the turbulent structures sizes, a satisfying representation of turbulent spectra is recovered, and a linear relation is obtained between the turbulent structures sizes and the Taylor macroscale. Lastly, the model is applied to the computation of a tidal turbine wake for different ambient turbulence levels, demonstrating the ability of this new implementation to emulate experimentally observed tendencies.

This paper describes a detailed implementation of the Synthetic Eddy Method (SEM) initially presented in Jarrin et al. (2006) applied to the Lagrangian Vortex simulation. While the treatment of turbulent diffusion is already extensively covered in scientific literature, this is one of the first attempts to represent ambient turbulence in a fully Lagrangian framework. This implementation is well suited to the integration of PSE (Particle Strength Exchange) or DVM (Diffusion Velocity Method), often used to account for molecular and turbulent diffusion in Lagrangian simulations. The adaptation and implementation of the SEM into a Lagrangian method using the PSE diffusion model is presented, and the turbulent velocity fields produced by this method are then analysed. In this adaptation, SEM turbulent structures are simply advected, without stretching or diffusion of their own, over the flow domain. This implementation proves its ability to produce turbulent velocity fields in accordance with any desired turbulent flow parameters. As the SEM is a purely mathematical and stochastic model, turbulent spectra and turbulent length scales are also investigated. With the addition of variation in the turbulent structures sizes, a satisfying representation of turbulent spectra is recovered, and a linear relation is obtained between the turbulent structures sizes and the Taylor macroscale. Lastly, the model is applied to the computation of a tidal turbine wake for different ambient turbulence levels, demonstrating the ability of this new implementation to emulate experimentally observed tendencies.

This paper describes a detailed implementation of the Synthetic Eddy Method (SEM) initially presented in Jarrin et al. (2006) applied to the Lagrangian Vortex simulation. While the treatment of turbulent diffusion is already extensively covered in scientific literature, this is one of the first attempts to represent ambient turbulence in a fully Lagrangian framework. This implementation is well suited to the integration of PSE (Particle Strength Exchange) or DVM (Diffusion Velocity Method), often used to account for molecular and turbulent diffusion in Lagrangian simulations. The adaptation and implementation of the SEM into a Lagrangian method using the PSE diffusion model is presented, and the turbulent velocity fields produced by this method are then analysed. In this adaptation, SEM turbulent structures are simply advected, without stretching or diffusion of their own, over the flow domain. This implementation proves its ability to produce turbulent velocity fields in accordance with any desired turbulent flow parameters. As the SEM is a purely mathematical and stochastic model, turbulent spectra and turbulent length scales are also investigated. With the addition of variation in the turbulent structures sizes, a satisfying representation of turbulent spectra is recovered, and a linear relation is obtained between the turbulent structures sizes and the Taylor macroscale. Lastly, the model is applied to the computation of a tidal turbine wake for different ambient turbulence levels, demonstrating the ability of this new implementation to emulate experimentally observed tendencies.

The thesis presented in this document deals with the caracterisation of the behaviour of an undulating membrane tidal energy converter. This kind of device uses the flutter instabilities occurring between a semi-rigid pre-strained membrane and a fluid flow in order to convert the sea currents energy. Above a certain critical flow speed, the structure ondulates, thus activating the linear converters fixed on it. In order to study this system, an experimental model is developped and tested in a flume tank. The power conversion system is simulated by hydraulic dampers. The membrane’s dynamics is analysed in many configurations through trajectory and force measurements, and leads to a parametric study. More realistic flow conditions are also studied, such as the impact of current direction influence and the influence of surface waves on the behaviour of the system. Wake caracterisation is carried out with two-dimensional PIV measurements. Scale effects and confinement are also studied through a comparison with a bigger scale prototype tested in tank and at sea. In addition to the experimental study, an analytical model and a numerical model are developped and compared with experiments. The linear analytical model is based on Euler-Bernouili’s beam theory and on Lighthill’s slender body theory. It is solved in the frequency domain and gives good undulation frequency and critical speed results. The numerical model uses strong interactions between a fluid code using the vortex method and a structure code based on corotationnal finite elements. This model is validated on an experimental case., La thèse présentée dans ce document concerne la caractérisation du comportement d’une hydrolienne à membrane ondulante. Ce dispositif novateur utilise les instabilités de flottement d’une membrane semi-rigide précontrainte dans un écoulement pour capter l’énergie des courants marins. À partir d’une certaine vitesse critique de l’écoulement, une onde se propage le long de la structure, ce qui actionne des convertisseurs linéaires fixés sur celleci. Un modèle expérimental à échelle réduite 1/20eme est développé et testé en bassin d’essai. Le système de conversion est simulé par des amortisseurs hydrauliques. Les mesures de trajectoire et d’efforts permettent d’analyser la dynamique de la membrane pour un grand nombre de configurations et d’aboutir à une étude paramétrique. Des conditions réalistes d’écoulement sont étudiées, notamment l’influence de la direction du courant et celle de la houle sur le fonctionnement du système. La caractérisation du sillage est effectuée par mesures de Particle Image Velocimetry en 2D. Les effets d’échelle sont présentés à travers une comparaison des essais en bassin et en mer. Un modèle analytique et un modèle numérique sont développés et comparés avec les résultats d’expériences. Le modèle analytique linéaire est basé sur la théorie des poutres de Euler-Bernouilli et la théorie des profils minces de Lighthill. Il est résolu dans le domaine fréquentiel et donne de bons résultats en termes de fréquence d’ondulation et de vitesse critique. Le modèle numérique est fondé sur le couplage fort entre un code fluide basé sur la méthode vortex et un code structure utilisant les éléments finis en corotationnel. Ce modèle est validé sur un cas expérimental.

The thesis presented in this document deals with the caracterisation of the behaviour of an undulating membrane tidal energy converter. This kind of device uses the flutter instabilities occurring between a semi-rigid pre-strained membrane and a fluid flow in order to convert the sea currents energy. Above a certain critical flow speed, the structure ondulates, thus activating the linear converters fixed on it. In order to study this system, an experimental model is developped and tested in a flume tank. The power conversion system is simulated by hydraulic dampers. The membrane’s dynamics is analysed in many configurations through trajectory and force measurements, and leads to a parametric study. More realistic flow conditions are also studied, such as the impact of current direction influence and the influence of surface waves on the behaviour of the system. Wake caracterisation is carried out with two-dimensional PIV measurements. Scale effects and confinement are also studied through a comparison with a bigger scale prototype tested in tank and at sea. In addition to the experimental study, an analytical model and a numerical model are developped and compared with experiments. The linear analytical model is based on Euler-Bernouili’s beam theory and on Lighthill’s slender body theory. It is solved in the frequency domain and gives good undulation frequency and critical speed results. The numerical model uses strong interactions between a fluid code using the vortex method and a structure code based on corotationnal finite elements. This model is validated on an experimental case., La thèse présentée dans ce document concerne la caractérisation du comportement d’une hydrolienne à membrane ondulante. Ce dispositif novateur utilise les instabilités de flottement d’une membrane semi-rigide précontrainte dans un écoulement pour capter l’énergie des courants marins. À partir d’une certaine vitesse critique de l’écoulement, une onde se propage le long de la structure, ce qui actionne des convertisseurs linéaires fixés sur celleci. Un modèle expérimental à échelle réduite 1/20eme est développé et testé en bassin d’essai. Le système de conversion est simulé par des amortisseurs hydrauliques. Les mesures de trajectoire et d’efforts permettent d’analyser la dynamique de la membrane pour un grand nombre de configurations et d’aboutir à une étude paramétrique. Des conditions réalistes d’écoulement sont étudiées, notamment l’influence de la direction du courant et celle de la houle sur le fonctionnement du système. La caractérisation du sillage est effectuée par mesures de Particle Image Velocimetry en 2D. Les effets d’échelle sont présentés à travers une comparaison des essais en bassin et en mer. Un modèle analytique et un modèle numérique sont développés et comparés avec les résultats d’expériences. Le modèle analytique linéaire est basé sur la théorie des poutres de Euler-Bernouilli et la théorie des profils minces de Lighthill. Il est résolu dans le domaine fréquentiel et donne de bons résultats en termes de fréquence d’ondulation et de vitesse critique. Le modèle numérique est fondé sur le couplage fort entre un code fluide basé sur la méthode vortex et un code structure utilisant les éléments finis en corotationnel. Ce modèle est validé sur un cas expérimental.

The battle against climate change begins with maximizing the share of renewable energy in the total energy mix, such that the cost of energy will be minimum. Wind energy will play a vital role in maximizing the share of renewables in the total energy mix. In order to reach the LCOE projection of 0.09 Euro/KWh a decrease in the material cost involved in the manufacturing of blades should play an important role. The drive to make wind energy cheaper pushed me to dive deeper into the subject of wind turbine aerodynamics and thus, I chose to work on this thesis project. The research will help the wind energy community to make the wind turbine design process robust, safe and cost-optimal. The thesis focuses on improving the general practice (based on the IEC standard) for wind turbine blade design by incorporating a more advanced aerodynamic model, called as the vortex wake model. The objective of this thesis is to evaluate differences in BEM and vortex wake models for IEC load case calculation. With respect to these differences, the thesis also aims to recommend a practice for fast and more accurate results. Initially, the IEC load cases are simulated with the Focus6 software making use of BEM based model for aerodynamic load calculations. A selection of most relevant load cases is to be done on the basis of results for the load cases which violate the BEM model. Finally, the selected load cases are further simulated with a vortex wake model coupled to Focus6, and a recommended practice for more accurate and fast results is devised., TKI Vortex loads Project

The thesis presented in this document deals with the caracterisation of the behaviour of an undulating membrane tidal energy converter. This kind of device uses the flutter instabilities occurring between a semi-rigid pre-strained membrane and a fluid flow in order to convert the sea currents energy. Above a certain critical flow speed, the structure ondulates, thus activating the linear converters fixed on it. In order to study this system, an experimental model is developped and tested in a flume tank. The power conversion system is simulated by hydraulic dampers. The membrane’s dynamics is analysed in many configurations through trajectory and force measurements, and leads to a parametric study. More realistic flow conditions are also studied, such as the impact of current direction influence and the influence of surface waves on the behaviour of the system. Wake caracterisation is carried out with two-dimensional PIV measurements. Scale effects and confinement are also studied through a comparison with a bigger scale prototype tested in tank and at sea. In addition to the experimental study, an analytical model and a numerical model are developped and compared with experiments. The linear analytical model is based on Euler-Bernouili’s beam theory and on Lighthill’s slender body theory. It is solved in the frequency domain and gives good undulation frequency and critical speed results. The numerical model uses strong interactions between a fluid code using the vortex method and a structure code based on corotationnal finite elements. This model is validated on an experimental case., La thèse présentée dans ce document concerne la caractérisation du comportement d’une hydrolienne à membrane ondulante. Ce dispositif novateur utilise les instabilités de flottement d’une membrane semi-rigide précontrainte dans un écoulement pour capter l’énergie des courants marins. À partir d’une certaine vitesse critique de l’écoulement, une onde se propage le long de la structure, ce qui actionne des convertisseurs linéaires fixés sur celleci. Un modèle expérimental à échelle réduite 1/20eme est développé et testé en bassin d’essai. Le système de conversion est simulé par des amortisseurs hydrauliques. Les mesures de trajectoire et d’efforts permettent d’analyser la dynamique de la membrane pour un grand nombre de configurations et d’aboutir à une étude paramétrique. Des conditions réalistes d’écoulement sont étudiées, notamment l’influence de la direction du courant et celle de la houle sur le fonctionnement du système. La caractérisation du sillage est effectuée par mesures de Particle Image Velocimetry en 2D. Les effets d’échelle sont présentés à travers une comparaison des essais en bassin et en mer. Un modèle analytique et un modèle numérique sont développés et comparés avec les résultats d’expériences. Le modèle analytique linéaire est basé sur la théorie des poutres de Euler-Bernouilli et la théorie des profils minces de Lighthill. Il est résolu dans le domaine fréquentiel et donne de bons résultats en termes de fréquence d’ondulation et de vitesse critique. Le modèle numérique est fondé sur le couplage fort entre un code fluide basé sur la méthode vortex et un code structure utilisant les éléments finis en corotationnel. Ce modèle est validé sur un cas expérimental.

The thesis presented in this document deals with the caracterisation of the behaviour of an undulating membrane tidal energy converter. This kind of device uses the flutter instabilities occurring between a semi-rigid pre-strained membrane and a fluid flow in order to convert the sea currents energy. Above a certain critical flow speed, the structure ondulates, thus activating the linear converters fixed on it. In order to study this system, an experimental model is developped and tested in a flume tank. The power conversion system is simulated by hydraulic dampers. The membrane’s dynamics is analysed in many configurations through trajectory and force measurements, and leads to a parametric study. More realistic flow conditions are also studied, such as the impact of current direction influence and the influence of surface waves on the behaviour of the system. Wake caracterisation is carried out with two-dimensional PIV measurements. Scale effects and confinement are also studied through a comparison with a bigger scale prototype tested in tank and at sea. In addition to the experimental study, an analytical model and a numerical model are developped and compared with experiments. The linear analytical model is based on Euler-Bernouili’s beam theory and on Lighthill’s slender body theory. It is solved in the frequency domain and gives good undulation frequency and critical speed results. The numerical model uses strong interactions between a fluid code using the vortex method and a structure code based on corotationnal finite elements. This model is validated on an experimental case., La thèse présentée dans ce document concerne la caractérisation du comportement d’une hydrolienne à membrane ondulante. Ce dispositif novateur utilise les instabilités de flottement d’une membrane semi-rigide précontrainte dans un écoulement pour capter l’énergie des courants marins. À partir d’une certaine vitesse critique de l’écoulement, une onde se propage le long de la structure, ce qui actionne des convertisseurs linéaires fixés sur celleci. Un modèle expérimental à échelle réduite 1/20eme est développé et testé en bassin d’essai. Le système de conversion est simulé par des amortisseurs hydrauliques. Les mesures de trajectoire et d’efforts permettent d’analyser la dynamique de la membrane pour un grand nombre de configurations et d’aboutir à une étude paramétrique. Des conditions réalistes d’écoulement sont étudiées, notamment l’influence de la direction du courant et celle de la houle sur le fonctionnement du système. La caractérisation du sillage est effectuée par mesures de Particle Image Velocimetry en 2D. Les effets d’échelle sont présentés à travers une comparaison des essais en bassin et en mer. Un modèle analytique et un modèle numérique sont développés et comparés avec les résultats d’expériences. Le modèle analytique linéaire est basé sur la théorie des poutres de Euler-Bernouilli et la théorie des profils minces de Lighthill. Il est résolu dans le domaine fréquentiel et donne de bons résultats en termes de fréquence d’ondulation et de vitesse critique. Le modèle numérique est fondé sur le couplage fort entre un code fluide basé sur la méthode vortex et un code structure utilisant les éléments finis en corotationnel. Ce modèle est validé sur un cas expérimental.

The battle against climate change begins with maximizing the share of renewable energy in the total energy mix, such that the cost of energy will be minimum. Wind energy will play a vital role in maximizing the share of renewables in the total energy mix. In order to reach the LCOE projection of 0.09 Euro/KWh a decrease in the material cost involved in the manufacturing of blades should play an important role. The drive to make wind energy cheaper pushed me to dive deeper into the subject of wind turbine aerodynamics and thus, I chose to work on this thesis project. The research will help the wind energy community to make the wind turbine design process robust, safe and cost-optimal. The thesis focuses on improving the general practice (based on the IEC standard) for wind turbine blade design by incorporating a more advanced aerodynamic model, called as the vortex wake model. The objective of this thesis is to evaluate differences in BEM and vortex wake models for IEC load case calculation. With respect to these differences, the thesis also aims to recommend a practice for fast and more accurate results. Initially, the IEC load cases are simulated with the Focus6 software making use of BEM based model for aerodynamic load calculations. A selection of most relevant load cases is to be done on the basis of results for the load cases which violate the BEM model. Finally, the selected load cases are further simulated with a vortex wake model coupled to Focus6, and a recommended practice for more accurate and fast results is devised., TKI Vortex loads Project

A prescribed velocity‐vorticity boundary layer model for the vorticity transport equation is proposed, which corrects the unphysical upward deflection of the wake seen in a simpler prescribed velocity shear approach. A Lagrangian implementation of the boundary layer model has been investigated using our in‐house vortex solver MIRAS. The MIRAS code contains both an aerodynamic part and a structural‐mechanical part taking into account aeroelastic phenomena. The solver is employed to simulate flows around wind turbines and uses a combination of filaments and particles in order to mimic the vorticity released by the wind turbine blades. The vorticity is interpolated onto a uniform Cartesian mesh, where the interaction is efficiently calculated by an fast Fourier transform‐based method. Simulations of wind turbines operating in an atmospheric boundary layer flow are carried out and analysed in detail for a range of scenarios. The manuscript focuses on studying the influence of wind shear and turbulence, which is varied to mimic natural atmospheric conditions. A traverse virtual probe up to 30 diameters downstream of the rotor plane is used to investigate the properties of the turbulent wake flow for the different cases. This includes mean and standard deviation of the streamwise velocity component, wake deficit, Reynolds stresses, and power spectral density of the velocity signal. The results show that combining a prescribed boundary layer approach with a vortex method gives consistent and physically correct results if properly implemented.

Extracting energy from wind is a contemporary, still actively developing topic. When designing wind turbine blades, as the key components of the HAWT system, the greatest cost reductions can be made in aerodynamic (geometrical modelling, production techniques) and structural (material) sectors. The paper presents one possible approach to the integrated, both aerodynamic and structural, constrained optimization process by particle swarm optimization (PSO) algorithm. The goal of the study is achieving maximal annual energy production (AEP) while minimizing blade mass. Inlet parameters include complete blade geometry (radius, airfoils, twist and chord distributions) as well as its structural definition (ply numbers and orientations). Aerodynamic performances of the blade were estimated by vortex method, while subsequent structural analyses (by importing aerodynamic loads) were realized by finite element method. Presented results include possible wind turbine designs as well as experiences gained while performing the study.

We present a Brinkman penalization method for three-dimensional (3D) flows using particle vortex methods, improving the existing technique by means of an iterative process. We perform simulations to study the impulsively started flow past a sphere at Re=1000and normal to a circular disc at Re=500. The simulation results obtained for the flow past a sphere are found in qualitative good agreement with previously published results obtained using respectively a 3D vortex penalization method and a 3D vortex method combined with an accurate boundary element method. From the results obtained for the flow normal to a circular disc it is found that the iterative method enables the use of atime step that is one order of magnitude larger than required by the standard non-iterative Brinkman penalization method.

In the present work, an in-house UnSteady Double Wake Model (USDWM) is developed for simulating general flow problems behind bodies. The model is presented and used to simulate flows past a circular cylinder at subcritical, supercritical, and transcritical flows. The flow model is a two-dimensional panel method which uses the unsteady double wake technique to model flow separation and its dynamics. In the present work the separation location is obtained from experimental data and fixed in time. The highly unsteady flow field behind the cylinder is analyzed in detail. The results are compared with experiments and Unsteady Reynolds-Averaged Navier Stokes (URANS) simulations and show good agreement in terms of the vortex shedding characteristics, drag, and pressure coefficients for the different flow regimes.

In the present work, the in-house UnSteady Double Wake Model (USDWM) is used to simulate flows past a cylinder at subcritical, supercritical, and transcritical Reynolds numbers. The flow model is a two-dimensional panel method which uses the unsteady double wake technique to model flow separation and its dynamics. In the present work the separation location is obtained from experimental data and fixed in time. The highly unsteady flow field behind the cylinder is analyzed in detail, comparing the vortex shedding charactericts under the different flow conditions.

We present an iterative Brinkman penalization scheme to enforce the no-slip condition onsolid boundaries in three-dimensional ow simulations. We use a high-order particle-meshvortex method, where the velocity field is obtained from the vorticity field by solving a Poisson equation on a Cartesian mesh as a convolution of the vorticity field with a regularized Green's function [2]. By doing this we can enforce free-space boundary conditions allowing us to consider a minimal computational domain. The Brinkman penalization method [1] is an immersed body method that allows the treatment of solids having complex geometries on a Cartesian mesh. Thereby we avoid the use of unstructured meshes that conventional ow solvers rely on. In the presented iterative scheme the penalization term is only active in the solid region and in its immediate neighborhood thus the computational costs required for the solution of the penalization problem is kept at a minimum.We apply our method for the simulation of the impulsively started flow past a sphere atRe = 1000 and normal to a circular disc at Re = 500, respectively. Our results for theunsteady sphere flow are found to be in qualitative good agreement with results obtainedby Ploumhans et. al [3] using a boundary element method. The flow is illustrated by a volume rendering of the vorticity field cf. Fig. 1. The figure shows that the ow is highly unsteady and a challenging problem for accurate analysis. Furthermore we show that the iterative scheme allows a significantly larger time step than the iterative scheme (more than a factor of 10) in the simulation of the impulsively started flow normal to a circular disc.

We present the treatment of lifting lines with a Vortex Particle-Mesh (VPM) methodology. The VPM method relies on the Lagrangian discretization of the Navier-Stokes equations in vorticity-velocity formulation. The use of this hybrid discretization offers several advantages. The particles are used solely for the advection, thereby waiving classical time stability constraints. They also exploit the compactness of vorticity support, leading to high computational gains for external flow simulations. The mesh, on the other hand, handles all the other computationally intensive tasks, such as the evaluation of the differential operators and the use of fast Fourier-based Poisson solvers, which allow the combination of unbounded directions and inlet/outlet boundaries. Both discretizations communicate through high order interpolation. The mesh and the interpolation also allow for additional advances; they are used to handle Lagrangian distortion by reinitializing the particle positions onto a regular grid. This crucial step, referred to as remeshing, guarantees the accuracy of the method. In addition, the resulting methodology provides computational efficiency and scalability to massively parallel architectures. Sources of vorticity are accounted for through a lifting line approach. This line handles the attached and shed vorticity contributions in a Lagrangian manner. Its immersed treatment efficiently captures the development of vorticity from thin sheets into a three-dimensional field. We apply this approach to the simulation of wake flows encountered in aeronautical and wind energy applications. An important aspect in these fields is the handling of turbulent inflows. We have developed a technique for the introduction of pre-computed or synthetic turbulent flow fields in vorticity form. Our treatment is based on particles as well and consistent with the Lagrangian character of the method. We apply here our method to the investigation of wind turbine wakes over very large

This paper aims at presenting an alternative and more consistent way to account for bodies in the framework of computing incompressible external flows in aerodynamics using vortex particle-mesh methods (VPM). More specifically, we present a purely grid-based numerical technique using an immersed interface method and the James-Lackner algorithm to compute the 2-D potential flow in an unbounded domain, including irregular interior boundaries and also the presence of a given vorticity field. This ingredient is one of the main components that are required for the computation of general viscous flows. It is therefore the first step towards an integrated unbounded immersed interface VPM method.

A helicopter is considered as an alternative candidate of fire engines in the case of fire in high-rise buildings. To investigate the safety, efficiency, and the operational limitation of the fire helicopter, research was conducted at NAL in collaboration with the Tokyo Fire Department. After a theoretical wind model was developed based on the discrete vortex method, simulations were carried out using a flight simulator of the NAL. The following results were drawn: (1) The fire helicopter based on the Super Puma, which is a twin-engine helicopter, was shown to have enough capability to perform safe and efficient fire fighting in wind speed up to 7 m/s. (2) During the simulation tests, cooperation between the pilot, copilot, and the ejection boom operator had great importance on the safety and efficiency of the fire fighting operation. (3) A mathematical model of the wind around a high-rise building has the ability to produce wind velocity in real time depending on time and position., ヘリコプタは高層ビルディング火災での消防車の代替と考えられている。消火ヘリコプタの安全性、効率性、および操作上の制限を調べるために、東京消防庁との共同で、NALにおいて研究を行った。風の理論モデルを離散渦法に基づいて開発した後、シミュレーションをNALの飛行シミュレータを用いて行った。結果は次の通りである。(1)ツインエンジンヘリコプタであるスーパプーマを改造した消火ヘリコプタは、7m/sまでの風速までは、消火活動を安全かつ効率よく行うことが充分可能なことが示された。(2)シミュレーション試験中は、操縦士、副操縦士および放水ブーム操作士との間の協力が消火活動作業の安全と効率に非常に重要である。(3)高層ビルデング周辺の風の数学的モデルは、時間と場所により即時に風速を割り出すことが出来る。

The power coefficient of a turbine increases according to the predictions from streamtube theory for sites with a confined fluid flow. Here, a vertical axis turbine (optimized for free flow) has been simulated by a two-dimensional vortex method, both in a channel and in free flow. The first part of the study concerns the numerical parameters of channel simulations. It is found that for free flow and wide channels, a large number of revolutions is required for convergence (around 100 at the optimal tip speed ratio and increasing with higher tip speed ratio), while for smaller channels, the required number of revolutions decreases. The second part analyses changes in turbine performance by the channel boundaries. The turbine performance increases when the channel width is decreased, although the results are below the predictions from streamtube theory, and this difference increases with decreasing channel width. It is also observed that the optimal tip speed ratio increases with decreasing channel width. By increasing the chord, which decreases the optimal tip speed ratio, the power coefficient can be increased somewhat.

The prediction of roll motion of a ship with bilge keels is particularly difficult because of the nonlinear characteristics of viscous damping. Flow separation and vortex shedding caused by bilge keels significantly affect the roll damping and the magnitude of the roll response. To predict roll damping and motion of a ship, the Slender-Ship Free-Surface Random Vortex Method (SSFSRVM) was employed. It is a free-surface viscous-flow solver with low computational cost so that it can run on a standard desktop computer. It features a quasi-three dimensional formulation that allows the decomposition of the three-dimensional hull problem into a sequence of two-dimensional computational planes, in which the two-dimensional free-surface Navier-Stokes solver FSRVM can be applied. In this work, the SSFSRVM methodology has been further developed to model multi-degrees of freedom of free-body motion in the time domain. This version of SSFSRVM model does not require the assumption of small amplitude motion, and is capable of having viscosity turned on or off in the solution procedure. Because FSRVM uses a grid-free formulation, there is no issue with numerical viscosity. We validated the SSFSRVM in simulating the free roll decay motion of a naval vessel without forward speed. The numerically predicted vorticity distributions at different time instants near a bilge keel closely matched experimental PIV images. We found that the SSFSRVM model is capable of predicting the roll motion of a hull, and capturing the behavior of the vortical structures in the fluid. Further, we examined how the roll decay coefficients and the flow field were altered by the span of the bilge keels, based on the time-domain simulation of the coupled hull and fluid motion. Plots of vorticity contours and iso-surfaces along the three-dimensional hull were presented to reveal the motion of fluid particles and vortex filaments near the keels. In addition, the generation of the quadratic roll damping was invest

This paper presents a novel algorithm to solve the 2-D potential flow past complex geometries with circulation in unbounded domain and in the presence of a given vorticity field. It is based on a Poisson solver that combines two components: the immersed interface method to enforce the boundary condition on each inner boundary and the James–Lackner algorithm to compute the outer boundary condition consistent with the unbounded domain solution. The algorithm is here based on second order finite differences and it requires solely 1-D stencil corrections; this makes the immersed interface part of the present method easily extendable to higher dimensional problems. The treatment of the outer boundaries requires an iterative boundary potential method. The algorithm is validated, by means of grid convergence studies, on the flow past multiple bodies. The results confirm the second order accuracy everywhere. The algorithm is also self consistent as ‘‘all is done on the grid’’ (thus without using a vortex panel boundary element method in addition to the grid). For cusped airfoils, a consistent way to enforce the Kutta–Joukowsky condition is also presented. The present algorithm constitutes a crucial building block towards an immersed interface-enabled vortex particle-mesh method for the computation of unsteady viscous flows, with boundary layers, detached shear layers and wakes. A possible extension to 3-D problems is also briefly discussed.

The wake geometry of a Vertical Axis Wind Turbine (VAWT) is unlike the standard Horizontal Axis Wind Turbine (HAWT). The blades of the turbine continuously passes through its own wake, creating complex wake-body interactions such as flow separation and dynamic stall, and convectional grid-based numerical method which is capable of describing such near-body phenomena fails at efficiently resolving the wake geometry. However, as these phenomena have a direct impact on the performance of the VAWT, it is paramount that there exists a numerical method that is not only capable of accurately resolving the small-scale near-body phenomena but also excels at efficiently resolving the unsteady large-scale wake geometry. This was the goal of the research and the numerical method that satisfied these requirements was the domain decomposition method known as the Hybrid Eulerian-Lagrangian Vortex Particle Method (HELVPM), based on the doctoral thesis of Daeninck [24] and the additional study performed by Stock [61]. In the present study, we coupled an Eulerian Finite Element Method, which only resolves the near-body domain, with a Lagrangian Vortex Particle Method, which resolves the entire wake. The advantage of such fluid domain segregation was that the Eulerian method could focus on accurately describing the near-body features whereas the Lagrangian method could focus on efficiently evolving the wake using simulation acceleration methods such as Fast Multipole Method (FMM) and parallel computation in Graphical Processing Units (GPU). The present study initially developed, verified and validated the Finite Element method and the Vortex Particle method separately ensuring it performs according to the theory. These methods were then coupled using the algorithm of Daeninck [24] and Lagrangian correction strategy developed by Stock [61]. However, during the study we determined that additional modifications to the coupling strategy is required to ensure conservation of circulation. F, Aerodynamics & Wind Energy, Aerospace Engineering

We developed bio-inspired reduced-order models of swimmers, consisting of a self-propelling pair of rotating cylinders. The aim of the project is twofold. First, simplified and non-deforming geometries can more easily be employed in small-scale robotic applications to solve relevant engineering problems. Second, they can serve as reduced physics models to efficiently simulate fluid-mediated interactions in schools of swimmers and perform learning studies involving multiple swimmers. In the first half of the thesis, we investigate the self-propulsion regimes of a pair of counter-rotating cylinders. For low rotation rates, the cylinders behave like a vortex dipole and the flow is characterized by an elliptical closed streamline surrounding the cylinders. For intermediate rotation rates, the cylinders move in the opposite direction and each has a different set of closed streamlines. Further increasing the rotation rate, the motion of the pair becomes unstable. We systematically explore the phase space defined by the non-dimensional centre-to-centre distance and the rotational Reynolds number, and find inverted exponential correlations that describe the transition between states. In the second half, we design three different locomotory modes of the cylinder pair, with few degrees of freedom, inspired by the movement of undulatory fish and jellyfish. The parameter spaces were explored with the CMA-ES stochastic optimization algorithm in order to find the best solutions in terms of maximum speed and efficiency. The undulatory fish-inspired motion achieves propulsion by shedding vorticity in a sequence of alternating sign vortices, similarly to its biologic counterpart. The jellyfish-inspired motion during each period sheds a vortex dipole, which generates a strong momentum flux in its wake and thus a large thrust, the definition of the swimming mode of jet-propelled oblate medusae., Solid and Fluid Mechanics, Process and Energy, Mechanical, Maritime and Materials Engineering

The prediction of roll motion of a ship with bilge keels is particularly difficult because of the nonlinear characteristics of viscous damping. Flow separation and vortex shedding caused by bilge keels significantly affect the roll damping and the magnitude of the roll response. To predict roll damping and motion of a ship, the Slender-Ship Free-Surface Random Vortex Method (SSFSRVM) was employed. It is a free-surface viscous-flow solver with low computational cost so that it can run on a standard desktop computer. It features a quasi-three dimensional formulation that allows the decomposition of the three-dimensional hull problem into a sequence of two-dimensional computational planes, in which the two-dimensional free-surface Navier-Stokes solver FSRVM can be applied. In this work, the SSFSRVM methodology has been further developed to model multi-degrees of freedom of free-body motion in the time domain. This version of SSFSRVM model does not require the assumption of small amplitude motion, and is capable of having viscosity turned on or off in the solution procedure. Because FSRVM uses a grid-free formulation, there is no issue with numerical viscosity. We validated the SSFSRVM in simulating the free roll decay motion of a naval vessel without forward speed. The numerically predicted vorticity distributions at different time instants near a bilge keel closely matched experimental PIV images. We found that the SSFSRVM model is capable of predicting the roll motion of a hull, and capturing the behavior of the vortical structures in the fluid. Further, we examined how the roll decay coefficients and the flow field were altered by the span of the bilge keels, based on the time-domain simulation of the coupled hull and fluid motion. Plots of vorticity contours and iso-surfaces along the three-dimensional hull were presented to reveal the motion of fluid particles and vortex filaments near the keels. In addition, the generation of the quadratic roll damping was invest

The wake geometry of a Vertical Axis Wind Turbine (VAWT) is unlike the standard Horizontal Axis Wind Turbine (HAWT). The blades of the turbine continuously passes through its own wake, creating complex wake-body interactions such as flow separation and dynamic stall, and convectional grid-based numerical method which is capable of describing such near-body phenomena fails at efficiently resolving the wake geometry. However, as these phenomena have a direct impact on the performance of the VAWT, it is paramount that there exists a numerical method that is not only capable of accurately resolving the small-scale near-body phenomena but also excels at efficiently resolving the unsteady large-scale wake geometry. This was the goal of the research and the numerical method that satisfied these requirements was the domain decomposition method known as the Hybrid Eulerian-Lagrangian Vortex Particle Method (HELVPM), based on the doctoral thesis of Daeninck [24] and the additional study performed by Stock [61]. In the present study, we coupled an Eulerian Finite Element Method, which only resolves the near-body domain, with a Lagrangian Vortex Particle Method, which resolves the entire wake. The advantage of such fluid domain segregation was that the Eulerian method could focus on accurately describing the near-body features whereas the Lagrangian method could focus on efficiently evolving the wake using simulation acceleration methods such as Fast Multipole Method (FMM) and parallel computation in Graphical Processing Units (GPU). The present study initially developed, verified and validated the Finite Element method and the Vortex Particle method separately ensuring it performs according to the theory. These methods were then coupled using the algorithm of Daeninck [24] and Lagrangian correction strategy developed by Stock [61]. However, during the study we determined that additional modifications to the coupling strategy is required to ensure conservation of circulation. F, Aerodynamics & Wind Energy, Aerospace Engineering

We developed bio-inspired reduced-order models of swimmers, consisting of a self-propelling pair of rotating cylinders. The aim of the project is twofold. First, simplified and non-deforming geometries can more easily be employed in small-scale robotic applications to solve relevant engineering problems. Second, they can serve as reduced physics models to efficiently simulate fluid-mediated interactions in schools of swimmers and perform learning studies involving multiple swimmers. In the first half of the thesis, we investigate the self-propulsion regimes of a pair of counter-rotating cylinders. For low rotation rates, the cylinders behave like a vortex dipole and the flow is characterized by an elliptical closed streamline surrounding the cylinders. For intermediate rotation rates, the cylinders move in the opposite direction and each has a different set of closed streamlines. Further increasing the rotation rate, the motion of the pair becomes unstable. We systematically explore the phase space defined by the non-dimensional centre-to-centre distance and the rotational Reynolds number, and find inverted exponential correlations that describe the transition between states. In the second half, we design three different locomotory modes of the cylinder pair, with few degrees of freedom, inspired by the movement of undulatory fish and jellyfish. The parameter spaces were explored with the CMA-ES stochastic optimization algorithm in order to find the best solutions in terms of maximum speed and efficiency. The undulatory fish-inspired motion achieves propulsion by shedding vorticity in a sequence of alternating sign vortices, similarly to its biologic counterpart. The jellyfish-inspired motion during each period sheds a vortex dipole, which generates a strong momentum flux in its wake and thus a large thrust, the definition of the swimming mode of jet-propelled oblate medusae., Solid and Fluid Mechanics, Process and Energy, Mechanical, Maritime and Materials Engineering

A control system for a vertical axis wind turbine is presented.The control strategy is to determine the electric load solely from the rotational velocity and the characteristics of the turbine, thus measurement of the incoming wind velocity is not required.The control system is evaluated with an aerodynamic vortex model coupled with an electrical model.Three different sets of control system parameters are tested, due to the trade off between high power absorption and achieving a fast system with high stability.The control systems are tested against a reference strategy where the wind speed is known.The simulations show that the three control systems provide a similar power absorption as the reference case.For dynamic cases, with fast changes in wind speed, the fast control strategies are beneficial.All control strategies are stable throughout the simulations when proper power absorption characteristics of the turbine are used. It is also shown that if peak power absorption is estimated at a too low tip speed ratio, the control strategies may inadvertently stop the turbine.

The understanding of interaction effects between marine energy converters represents the next step in the research process that should eventually lead to the deployment of such devices. Although some a priori considerations have been suggested recently, very few real condition studies have been carried out concerning this issue. Trials were run on 1/30 th scale models of three-bladed marine current turbine prototypes in a flume tank. The present work focuses on the case where a turbine is placed at different locations in the wake of a first one. The interaction effects in terms of performance and wake of the second turbine are examined and compared to the results obtained on the case of one single turbine. Besides, a three-dimensional software, based on a vortex method is currently being developed, and will be used in the near future to model more complex layouts. The experimental study shows that the second turbine is deeply affected by the presence of an upstream device and that a compromise between individual device performance and inter-device spacing is necessary. Numerical results show good agreement with the experiment and are promising for the future modelling of turbine farms.

This manuscript deals with the numerical and experimental characterisation of the behaviour of marine current turbines. In the experimental part, trials were run at IFREMER’s wave and current flume tank in Boulogne-Sur-Mer, on prototypes of three-bladed horizontal axis turbines. Configurations with one single turbine on the one hand, and two turbines aligned with the incoming flow on the other hand, were considered for a large range of TSRs and, when relevant, of inter-device distances. The behaviour of the turbines is analysed in terms of performances (power and thrust coefficients) and development of the wake. The effects of the ambient turbulence intensity rate are also considered. Besides, numerical computations, obtained from a tridimensional unsteady software, based on the Vortex particle method and developed at the LOMC (UMR 6294, CNRS – University of Le Havre) in partnership with IFREMER, are presented. The numerical tool also enables to study the performances and the wake of a turbine. It was rewritten during this PhD and its technical and theoretical support is available in the manuscript, where the Vortex method, as it is used in the software, is described in details., Ce manuscrit traite de la caractérisation numérique et expérimentale du comportement d’hydroliennes. D’un point de vue expérimental, des essais ont été réalisés au bassin à houle et courant de l’IFREMER de Boulolgne-Sur-Mer, sur des maquettes d’hydroliennes tri-pales à axe horizontal. Des configurations comprenant une seule hydrolienne d’une part, et deux hydroliennes alignées avec l’écoulement d’autre part, ont été considérées pour une large gamme de TSR et, le cas échéant, de distances inter-hydroliennes. Le comportement des hydroliennes est analysé à la fois en termes de performances (coefficients de puissance et de traînée) et de développement du sillage. Les effets du taux de turbulence ambiantes ont également été examinés. Par ailleurs, des simulations numériques, obtenues à l’aide d’un code tridimensionnel instationnaire, fondé sur la méthode Vortex particulaire et développé au LOMC (UMR 6294, CNRS – Université du Havre) en partenariat avec l’IFREMER, sont présentées. Le code de calcul permet également d’étudier les performances et le sillage d’une hydrolienne. Ce dernier a été complètement réécrit dans le cadre de cette thèse et le support théorique et technique des différents aspects du code est fourni dans ce manuscrit, où la méthode Vortex telle qu’elle est utilisée dans le code est exposée en détail.

The implantation and deployment of marine current turbine arrays depend on the understanding of their interactions. Based on a priori suggestions about the layout of marine energy converter arrays, we propose to highlight interactions effects between two horizontal axis marine current turbines, in open water. experimental trials were run in IFREMER’s wave and current flume tank in Boulogne-sur-Mer (France), on marine current turbine models. Our study focuses on setups were the second turbine is axially aligned, at different distances, in the wake of the first one. Interaction effects are pointed out both in terms of performance and wake characterisation by means of a comparison with results on a single turbine. This study shows that the downstream turbine behaviour is deeply influenced by the upstream turbine wake.We also present numerical results obtained on a single turbine from our tri-dimensional numerical software, which is developed at the Laboratoire Ondes et Milieux Complexes,Le Havre (France). Those numerical results are validated by the comparison with the experiments, which allows us to foresee the modelling of several turbines and thus of turbine farms with a more complex layout.