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http://hdl.handle.net/10985/8836
Analysis of added mass in cavitating flow
BENAOUICHA, Mustapha; ASTOLFI, Jacques Andre
The paper addresses a theoretical study of the added mass effect in cavitating flow.The cavitation is considered to induce a strong time–space variation of the fluid density at the interface between an inviscid fluid and a three-degree-of-freedom rigid section. The coupled problem is then simplified to a Laplace equation written for the pressure with a boundary condition at the fluid–structure interface depending on the acceleration, the velocity of the structure and on the rate of change of flow density. It is shown that contrary to the homogeneous flow, the added mass operator is not symmetrical and depends on the flow through fluid density variation. The added mass coefficients decrease as the cavitation increases which should induce an increase of the natural structural frequencies. The model shows also an added damping operator related to the rate of change of flow density. Added damping coefficients are found to be positive or negative according to the rate of change of the fluid density, indicating the possibility of instability development between flexible structures and unsteady cavitating flows.
Sun, 01 Jan 2012 00:00:00 GMThttp://hdl.handle.net/10985/88362012-01-01T00:00:00ZBENAOUICHA, MustaphaASTOLFI, Jacques AndreThe paper addresses a theoretical study of the added mass effect in cavitating flow.The cavitation is considered to induce a strong time–space variation of the fluid density at the interface between an inviscid fluid and a three-degree-of-freedom rigid section. The coupled problem is then simplified to a Laplace equation written for the pressure with a boundary condition at the fluid–structure interface depending on the acceleration, the velocity of the structure and on the rate of change of flow density. It is shown that contrary to the homogeneous flow, the added mass operator is not symmetrical and depends on the flow through fluid density variation. The added mass coefficients decrease as the cavitation increases which should induce an increase of the natural structural frequencies. The model shows also an added damping operator related to the rate of change of flow density. Added damping coefficients are found to be positive or negative according to the rate of change of the fluid density, indicating the possibility of instability development between flexible structures and unsteady cavitating flows.Discontinuity of lift on a hydrofoil in reversed flow for tidal turbine Application
http://hdl.handle.net/10985/12715
Discontinuity of lift on a hydrofoil in reversed flow for tidal turbine Application
MARCHAND, Jean-Baptiste; ASTOLFI, Jacques Andre; BOT, Patrick
This work presents an experimental investigation of a hydrofoil in reversed flow configuration in the context of marine current turbine development. Experiments consist in hydrodynamic force measurements and PIV flow observations on a NACA 0015 hydrofoil, at 5 × 105 Reynolds number. The hydrofoil in reversed flow produces a higher lift than in the classical forward flow for very low angles of attack and proved to be relatively efficient for an angle of attack lower than 10°, despite a much higher drag than the same foil in direct flow. Moreover, the lift coefficient shows a discontinuity with an hysteresis effect when the angle of attack is varied up and down around zero-degree. It is shown that the sharp leading edge generates an early Leading Edge Separation Bubble on one side (suction side) even for vanishing angles of attack. This separation bubble triggers the transition to turbulence of the boundary layer on the suction side while the pressure side boundary layer remains laminar. As a consequence, separation on the rounded trailing edge occurs farther downstream on the (turbulent) suction side compared to the (laminar) pressure side. The Leading Edge Separation Bubble and the inherent up–down asymmetry in the boundary layer regime are responsible for the lift singularity.
Sun, 01 Jan 2017 00:00:00 GMThttp://hdl.handle.net/10985/127152017-01-01T00:00:00ZMARCHAND, Jean-BaptisteASTOLFI, Jacques AndreBOT, PatrickThis work presents an experimental investigation of a hydrofoil in reversed flow configuration in the context of marine current turbine development. Experiments consist in hydrodynamic force measurements and PIV flow observations on a NACA 0015 hydrofoil, at 5 × 105 Reynolds number. The hydrofoil in reversed flow produces a higher lift than in the classical forward flow for very low angles of attack and proved to be relatively efficient for an angle of attack lower than 10°, despite a much higher drag than the same foil in direct flow. Moreover, the lift coefficient shows a discontinuity with an hysteresis effect when the angle of attack is varied up and down around zero-degree. It is shown that the sharp leading edge generates an early Leading Edge Separation Bubble on one side (suction side) even for vanishing angles of attack. This separation bubble triggers the transition to turbulence of the boundary layer on the suction side while the pressure side boundary layer remains laminar. As a consequence, separation on the rounded trailing edge occurs farther downstream on the (turbulent) suction side compared to the (laminar) pressure side. The Leading Edge Separation Bubble and the inherent up–down asymmetry in the boundary layer regime are responsible for the lift singularity.An experimental analysis of fluid structure interaction on a flexible hydrofoil in various flow regimes including cavitating flow
http://hdl.handle.net/10985/8998
An experimental analysis of fluid structure interaction on a flexible hydrofoil in various flow regimes including cavitating flow
DUCOIN, Antoine; ASTOLFI, Jacques Andre; SIGRIST, Jean-François
The structural response of a rectangular cantilevered flexible hydrofoil submitted to various flow regimes is analyzed through an original experiment carried out in a hydrodynamic tunnel at a Reynolds number of 0.75 × 10 6 . The experiment considers static and transient regimes. The latter consists of transient pitching motions at low and fast pitching velocities. The experiments are also performed for cavitating flow. The structural response is analyzed through the measurement of the free foil tip section displacement using a high speed video camera and surface velocity vibrations using a laser doppler vibrometer. In non cavitating flows, it is shown that the structural response is linked to the hydrodynamic loading, which is governed by viscous effects such as laminar to turbulent transition induced by Laminar Separation Bubble (LSB), and stall. It is also observed that the foil elastic displacement depends strongly on the pitching velocity. Large overshoots and hysteresis effect are observed as the pitching velocity increases. Cavitation induces a large increase of the vibration level due to hydrodynamic loading unsteadiness and change of modal response for specific frequencies. The experimental results presented in this paper will help to develop high fidelity fluid–structure interaction models in naval applications.
The authors gratefully acknowledge the technical staff of IRENav for its contribution to the experimental set up.
Sun, 01 Jan 2012 00:00:00 GMThttp://hdl.handle.net/10985/89982012-01-01T00:00:00ZDUCOIN, AntoineASTOLFI, Jacques AndreSIGRIST, Jean-FrançoisThe structural response of a rectangular cantilevered flexible hydrofoil submitted to various flow regimes is analyzed through an original experiment carried out in a hydrodynamic tunnel at a Reynolds number of 0.75 × 10 6 . The experiment considers static and transient regimes. The latter consists of transient pitching motions at low and fast pitching velocities. The experiments are also performed for cavitating flow. The structural response is analyzed through the measurement of the free foil tip section displacement using a high speed video camera and surface velocity vibrations using a laser doppler vibrometer. In non cavitating flows, it is shown that the structural response is linked to the hydrodynamic loading, which is governed by viscous effects such as laminar to turbulent transition induced by Laminar Separation Bubble (LSB), and stall. It is also observed that the foil elastic displacement depends strongly on the pitching velocity. Large overshoots and hysteresis effect are observed as the pitching velocity increases. Cavitation induces a large increase of the vibration level due to hydrodynamic loading unsteadiness and change of modal response for specific frequencies. The experimental results presented in this paper will help to develop high fidelity fluid–structure interaction models in naval applications.Influence of the Cavitation Model on the Simulation of Cloud Cavitation on 2D Foil Section
http://hdl.handle.net/10985/6688
Influence of the Cavitation Model on the Simulation of Cloud Cavitation on 2D Foil Section
FRIKHA, Sobhi; COUTIER-DELGOSHA, Olivier; ASTOLFI, Jacques Andre
For numerical simulations of cavitating flows, many physical models are currently used. One approach is the void fraction transport equation-based model including source terms for vaporization and condensation processes. Various source terms have been proposed by different researchers. However, they have been tested only in different flow configurations, which make direct comparisons between the results difficult. A comparative study, based on the expression of the source terms as a function of the pressure, is presented in the present paper. This analytical approach demonstrates a large resemblance between the models, and it also clarifies the influence of the model parameters on the vaporization and condensation terms and, therefore, on the cavity shape and behavior. Some of the models were also tested using a 2D CFD code in configurations of cavitation on two-dimensional foil sections. Void fraction distributions and frequency of the cavity oscillations were compared to existing experimental measurements. These numerical results confirm the analytical study.
Tue, 01 Jan 2008 00:00:00 GMThttp://hdl.handle.net/10985/66882008-01-01T00:00:00ZFRIKHA, SobhiCOUTIER-DELGOSHA, OlivierASTOLFI, Jacques AndreFor numerical simulations of cavitating flows, many physical models are currently used. One approach is the void fraction transport equation-based model including source terms for vaporization and condensation processes. Various source terms have been proposed by different researchers. However, they have been tested only in different flow configurations, which make direct comparisons between the results difficult. A comparative study, based on the expression of the source terms as a function of the pressure, is presented in the present paper. This analytical approach demonstrates a large resemblance between the models, and it also clarifies the influence of the model parameters on the vaporization and condensation terms and, therefore, on the cavity shape and behavior. Some of the models were also tested using a 2D CFD code in configurations of cavitation on two-dimensional foil sections. Void fraction distributions and frequency of the cavity oscillations were compared to existing experimental measurements. These numerical results confirm the analytical study.Cavity induced vibration of flexible hydrofoils
http://hdl.handle.net/10985/8992
Cavity induced vibration of flexible hydrofoils
AKCABAY, Deniz Tolga; CHAE, Eun Jung; YOUNG, Yin Lu; DUCOIN, Antoine; ASTOLFI, Jacques Andre
The objective of this work is to investigate the influence of cavity-induced vibrations on the dynamic response and stability of a NACA66 hydrofoil at 8° angle of attack at Re=750 000 via combined experimental measurements and numerical simulations. The rectangular, cantilevered hydrofoil is assumed to be rigid in the chordwise direction, while the spanwise bending and twisting deformations are represented using a two-degrees-of-freedom structural model. The multiphase flow is modeled with an incompressible, unsteady Reynolds Averaged Navier–Stokes solver with the k–ω Shear Stress Transport (SST) turbulence closure model, while the phase evolutions are modeled with a mass-transport equation based cavitation model. The numerical predictions are compared with experimental measurements across a range of cavitation numbers for a rigid and a flexible hydrofoil with the same undeformed geometries. The results showed that foil flexibility can lead to: (1) focusing – locking – of the frequency content of the vibrations to the nearest sub-harmonics of the foil׳s wetted natural frequencies, and (2) broadening of the frequency content of the vibrations in the unstable cavitation regime, where amplifications are observed in the sub-harmonics of the foil natural frequencies. Cavitation was also observed to cause frequency modulation, as the fluid density, and hence fluid induced (inertial, damping, and disturbing) forces fluctuated with unsteady cavitation.
Wed, 01 Jan 2014 00:00:00 GMThttp://hdl.handle.net/10985/89922014-01-01T00:00:00ZAKCABAY, Deniz TolgaCHAE, Eun JungYOUNG, Yin LuDUCOIN, AntoineASTOLFI, Jacques AndreThe objective of this work is to investigate the influence of cavity-induced vibrations on the dynamic response and stability of a NACA66 hydrofoil at 8° angle of attack at Re=750 000 via combined experimental measurements and numerical simulations. The rectangular, cantilevered hydrofoil is assumed to be rigid in the chordwise direction, while the spanwise bending and twisting deformations are represented using a two-degrees-of-freedom structural model. The multiphase flow is modeled with an incompressible, unsteady Reynolds Averaged Navier–Stokes solver with the k–ω Shear Stress Transport (SST) turbulence closure model, while the phase evolutions are modeled with a mass-transport equation based cavitation model. The numerical predictions are compared with experimental measurements across a range of cavitation numbers for a rigid and a flexible hydrofoil with the same undeformed geometries. The results showed that foil flexibility can lead to: (1) focusing – locking – of the frequency content of the vibrations to the nearest sub-harmonics of the foil׳s wetted natural frequencies, and (2) broadening of the frequency content of the vibrations in the unstable cavitation regime, where amplifications are observed in the sub-harmonics of the foil natural frequencies. Cavitation was also observed to cause frequency modulation, as the fluid density, and hence fluid induced (inertial, damping, and disturbing) forces fluctuated with unsteady cavitation.URANSE simulation of an active variable-pitch cross-flow Darrieus tidal turbine: Sinusoidal pitch function investigation
http://hdl.handle.net/10985/9496
URANSE simulation of an active variable-pitch cross-flow Darrieus tidal turbine: Sinusoidal pitch function investigation
PAILLARD, Benoît; ASTOLFI, Jacques Andre; HAUVILLE, Frédéric
This article describes a 2D CFD simulation implementation of a crossflow tidal turbine, the blades of which have their pitch modified during revolution. Unsteady flow around the turbine is computed with an URANSE method, using the solver ANSYS-CFX. Spatial and temporal discretizations have been studied. The pitch motion of the blades is obtained through mesh deformation, and the main rotation is implemented through sliding boundaries, with general grid interface model. The turbulence model used is kx SST. Langtry Menter transition model was tried but showed high discrepancies with experimental results. Five experimental cases were used to assess the accuracy of the simulation. It provided accurate computed forces for a wide range of tip speed ratios, and proved to be suitable for exploratory simulations. Harmonic pitch control was thus implemented for a tip speed ratio of 5, close to an operational value for a crossflow turbine. First, second and third harmonics pitch function were tested. It was shown that an improvement of more than 50% could be achieved with the second harmonics, with a large reduction in thrust. The flow inside the turbine and close to the blade was examined so that the case of performance improvement due to pitch control could be clearly understood. It was observed that turbine efficiency improvement requires a very slight recirculation and an angle of attack decrease on the upstream part of the turbine, and an angle of attack increase on the downstream part. The flow deceleration through the turbine was found to be a primary factor in pitch function as well. Moreover the hydrodynamic torque and thus the energy required to control the pitch were found to be insignificant.
Thu, 01 Jan 2015 00:00:00 GMThttp://hdl.handle.net/10985/94962015-01-01T00:00:00ZPAILLARD, BenoîtASTOLFI, Jacques AndreHAUVILLE, FrédéricThis article describes a 2D CFD simulation implementation of a crossflow tidal turbine, the blades of which have their pitch modified during revolution. Unsteady flow around the turbine is computed with an URANSE method, using the solver ANSYS-CFX. Spatial and temporal discretizations have been studied. The pitch motion of the blades is obtained through mesh deformation, and the main rotation is implemented through sliding boundaries, with general grid interface model. The turbulence model used is kx SST. Langtry Menter transition model was tried but showed high discrepancies with experimental results. Five experimental cases were used to assess the accuracy of the simulation. It provided accurate computed forces for a wide range of tip speed ratios, and proved to be suitable for exploratory simulations. Harmonic pitch control was thus implemented for a tip speed ratio of 5, close to an operational value for a crossflow turbine. First, second and third harmonics pitch function were tested. It was shown that an improvement of more than 50% could be achieved with the second harmonics, with a large reduction in thrust. The flow inside the turbine and close to the blade was examined so that the case of performance improvement due to pitch control could be clearly understood. It was observed that turbine efficiency improvement requires a very slight recirculation and an angle of attack decrease on the upstream part of the turbine, and an angle of attack increase on the downstream part. The flow deceleration through the turbine was found to be a primary factor in pitch function as well. Moreover the hydrodynamic torque and thus the energy required to control the pitch were found to be insignificant.Nonlinear disturbance evolution in a two-dimensional boundary layer along an elastic plate and induced radiated sound
http://hdl.handle.net/10985/8934
Nonlinear disturbance evolution in a two-dimensional boundary layer along an elastic plate and induced radiated sound
GOBERT, Marie-Laure; EHRENSTEIN, Uwe; ASTOLFI, Jacques Andre; BOT, Patrick
The interaction between a boundary-layer flow and an elastic plate is addressed by direct numerical simulation, taking into account the full coupling between the fluid flow and the flexible wall. The convectively unstable flow state is harmonically forced and two-dimensional nonlinearly saturated wavelike disturbances are computed along archetype-plates with respect to stiffness and natural frequencies. In the aim of determining the low-Mach-number radiated sound for the system, the simulation data are used to solve the Lighthill’s equation in terms of a Green’s function in the wavenumber-frequency space. Different degrees of fluid-structure coupling are implemented in the radiated sound model and the resulting acoustic pressure levels are compared. The sound radiation levels are shown to be increased in the presence of flexible walls with however significant differences in the radiated pressure levels for different coupling assumptions
Fri, 01 Jan 2010 00:00:00 GMThttp://hdl.handle.net/10985/89342010-01-01T00:00:00ZGOBERT, Marie-LaureEHRENSTEIN, UweASTOLFI, Jacques AndreBOT, PatrickThe interaction between a boundary-layer flow and an elastic plate is addressed by direct numerical simulation, taking into account the full coupling between the fluid flow and the flexible wall. The convectively unstable flow state is harmonically forced and two-dimensional nonlinearly saturated wavelike disturbances are computed along archetype-plates with respect to stiffness and natural frequencies. In the aim of determining the low-Mach-number radiated sound for the system, the simulation data are used to solve the Lighthill’s equation in terms of a Green’s function in the wavenumber-frequency space. Different degrees of fluid-structure coupling are implemented in the radiated sound model and the resulting acoustic pressure levels are compared. The sound radiation levels are shown to be increased in the presence of flexible walls with however significant differences in the radiated pressure levels for different coupling assumptionsEffect of the laminar separation bubble induced transition on the hydrodynamic performance of a hydrofoil
http://hdl.handle.net/10985/8996
Effect of the laminar separation bubble induced transition on the hydrodynamic performance of a hydrofoil
DELAFIN, Pierre-Luc; DENISET, François; ASTOLFI, Jacques Andre
The present study deals with the effect of the laminar separation bubble (LSB) induced transition on the lift, drag and moment coefficients of a hydrofoil. A 2D numerical study, based on the SST γ –Reθ transition model of ANSYS-CFX⃝R , is conducted on a NACA66 hydrofoil. Angles of attack range from −4° to 14° and the chord-based Reynolds number is Re = 7.5 × 105. An experimental investigation is carried out in the French naval academy research institute’s hydrodynamic tunnel based on the measurements of lift, drag and moment. Experiments on a smooth, mirror finished, hydrofoil enable comparison with RANS calculations using the transition model. Experiments with a roughness added on the leading edge enable comparison with RANS calculations using the SST fully turbulent model. For angles of attack below 6°, the LSB triggered laminar to turbulent transition of the boundary layers of the suction and pressure sides is located near the trailing edge of the smooth NACA66. As the angle of attack reaches 6°, the LSB suddenly moves to the leading edge on the suction side while transition is located at the trailing edge on the pressure side. The smooth hydrofoil shows higher CL and CM and lower CD than the rough leading edge one from −4° to 6°. Both experiments lead to the same coefficients from 6° to 14°. The calculations show that both models are in good agreement with their corresponding experiments. Velocity profiles in the vicinity of the LSB at an angle of attack of 2° and pressure coefficients of the calculations using the transition model are compared with published experimental studies and show very good agreement. The SST γ –Reθ transition model proves to be a relevant, even essential, prediction tool for lifting bodies operating at a moderate Reynolds number.
The authors thank the technical staff of IRENav for their contribution to the experimental set up.
Wed, 01 Jan 2014 00:00:00 GMThttp://hdl.handle.net/10985/89962014-01-01T00:00:00ZDELAFIN, Pierre-LucDENISET, FrançoisASTOLFI, Jacques AndreThe present study deals with the effect of the laminar separation bubble (LSB) induced transition on the lift, drag and moment coefficients of a hydrofoil. A 2D numerical study, based on the SST γ –Reθ transition model of ANSYS-CFX⃝R , is conducted on a NACA66 hydrofoil. Angles of attack range from −4° to 14° and the chord-based Reynolds number is Re = 7.5 × 105. An experimental investigation is carried out in the French naval academy research institute’s hydrodynamic tunnel based on the measurements of lift, drag and moment. Experiments on a smooth, mirror finished, hydrofoil enable comparison with RANS calculations using the transition model. Experiments with a roughness added on the leading edge enable comparison with RANS calculations using the SST fully turbulent model. For angles of attack below 6°, the LSB triggered laminar to turbulent transition of the boundary layers of the suction and pressure sides is located near the trailing edge of the smooth NACA66. As the angle of attack reaches 6°, the LSB suddenly moves to the leading edge on the suction side while transition is located at the trailing edge on the pressure side. The smooth hydrofoil shows higher CL and CM and lower CD than the rough leading edge one from −4° to 6°. Both experiments lead to the same coefficients from 6° to 14°. The calculations show that both models are in good agreement with their corresponding experiments. Velocity profiles in the vicinity of the LSB at an angle of attack of 2° and pressure coefficients of the calculations using the transition model are compared with published experimental studies and show very good agreement. The SST γ –Reθ transition model proves to be a relevant, even essential, prediction tool for lifting bodies operating at a moderate Reynolds number.An experimental study of boundary-layer transition induced vibrations on a hydrofoil
http://hdl.handle.net/10985/8905
An experimental study of boundary-layer transition induced vibrations on a hydrofoil
DUCOIN, Antoine; ASTOLFI, Jacques Andre; GOBERT, Marie-Laure
This paper aims at characterizing experimentally laminar to turbulent transition induced vibrations. Here, the transition is known to be triggered by a Laminar Separation Bubble that results from a laminar separation of the boundary-layer flow on a hydrofoil. In this study we consider two NACA66312 (Mod) laminar hydrofoils at low angles of incidence (mostly 2° and 4°) and Reynolds numbers ranging from Re=450 000 to 1 200 000, in order to get transitional regimes. The first hydrofoil, made of steel (E=2.1×1011 Pa), is referred to as the rigid hydrofoil, although it is seen to vibrate under the action of the LSB. To better understand the possible interaction between the flow and the foil vibrations, vibration measurements are repeated using a flexible hydrofoil (E=3×109 Pa) of same geometry (under zero loading) and in close configurations. The experiments are carried out at the French Naval Academy Research Institute (IRENav, France). Wall pressure and flow velocity measurements enable a characterization of the laminar separation bubble and the identification of a vortex shedding at a given frequency. It is hence shown that the boundary-layer transition induces important foil vibrations, whose characteristics in terms of frequency and amplitude depend on the vortex shedding frequency, and can be coupled with natural frequencies of the hydrofoils.
Sun, 01 Jan 2012 00:00:00 GMThttp://hdl.handle.net/10985/89052012-01-01T00:00:00ZDUCOIN, AntoineASTOLFI, Jacques AndreGOBERT, Marie-LaureThis paper aims at characterizing experimentally laminar to turbulent transition induced vibrations. Here, the transition is known to be triggered by a Laminar Separation Bubble that results from a laminar separation of the boundary-layer flow on a hydrofoil. In this study we consider two NACA66312 (Mod) laminar hydrofoils at low angles of incidence (mostly 2° and 4°) and Reynolds numbers ranging from Re=450 000 to 1 200 000, in order to get transitional regimes. The first hydrofoil, made of steel (E=2.1×1011 Pa), is referred to as the rigid hydrofoil, although it is seen to vibrate under the action of the LSB. To better understand the possible interaction between the flow and the foil vibrations, vibration measurements are repeated using a flexible hydrofoil (E=3×109 Pa) of same geometry (under zero loading) and in close configurations. The experiments are carried out at the French Naval Academy Research Institute (IRENav, France). Wall pressure and flow velocity measurements enable a characterization of the laminar separation bubble and the identification of a vortex shedding at a given frequency. It is hence shown that the boundary-layer transition induces important foil vibrations, whose characteristics in terms of frequency and amplitude depend on the vortex shedding frequency, and can be coupled with natural frequencies of the hydrofoils.Simulating variable pitch crossflow water turbines: A coupled unsteady ONERA-EDLIN model and streamtube model
http://hdl.handle.net/10985/8837
Simulating variable pitch crossflow water turbines: A coupled unsteady ONERA-EDLIN model and streamtube model
PAILLARD, Benoît; HAUVILLE, Frédéric; ASTOLFI, Jacques Andre
This article describes a new method for simulating unsteady hydrodynamics forces and moments on the blades of a crossflow ‘Darrieus’ turbine with active pitch variation. This method is based on the ONERAEDLIN dynamic stall model, coupled with a momentum streamtube model to take into account the turbine interference on the flow. Both models are presented, and compared separately with experimental results for a pitching airfoil for the ONERA-EDLIN model; and for Darrieus turbine for the momentum theory. The model coupling is then detailed and compared with experimental data taken from the open literature [1] The turbine has 2 straight blades with a NACA 0012 section operating in water at a mean chord Reynolds number of 4 104 for tip speed ratio l ¼ 2.5, 5 and 7.5. Good agreement was found for average l ¼ 5, and qualitative agreement could be obtained at low and high l, where dynamic stall effects and interference effects respectively are predominant. This is positive because l ¼ 5 is the closest value from the optimal power production point. Variable pitch is finally introduced in the model and several functions are tested in order to increase efficiency. A maximum increase of 53% on the power coefficient was found to occur with a sinusoidal law. 2012 Elsevier Ltd. All rights reserved. 1. Introduction Tidal turbines are currently the power source that shows the most advantages [2]. No land occupation like a dam, steady predictable power input and output unlike wind turbines, no waste or side effects like fossil or nuclear power plants. These devices can consist of a classic horizontal axis screw-like systems, or crossflow turbines which have many advantages in water [3], such as being independent of the tide direction. Variable pitch crossflow turbines enable a Darrieus system to improve its performance and decrease parasitic forces,mainly responsible for fatigue and systemfailure [4]. They have been studied at IRENAV since 2007 as the SHIVA project. This project of novel tidal turbines deals with three topics,which will be introduced here. Darrieus turbines have been studied extensively during the 70s and 80s, especially by SANDIA organization [5e8]. A reference publication on this topic can be found in [9]. Though almost no Darrieus turbine produced electrical power from wind since early 90s, a renewed interest arose from water turbines because most drawbacks which prevented this system from becoming
Tue, 01 Jan 2013 00:00:00 GMThttp://hdl.handle.net/10985/88372013-01-01T00:00:00ZPAILLARD, BenoîtHAUVILLE, FrédéricASTOLFI, Jacques AndreThis article describes a new method for simulating unsteady hydrodynamics forces and moments on the blades of a crossflow ‘Darrieus’ turbine with active pitch variation. This method is based on the ONERAEDLIN dynamic stall model, coupled with a momentum streamtube model to take into account the turbine interference on the flow. Both models are presented, and compared separately with experimental results for a pitching airfoil for the ONERA-EDLIN model; and for Darrieus turbine for the momentum theory. The model coupling is then detailed and compared with experimental data taken from the open literature [1] The turbine has 2 straight blades with a NACA 0012 section operating in water at a mean chord Reynolds number of 4 104 for tip speed ratio l ¼ 2.5, 5 and 7.5. Good agreement was found for average l ¼ 5, and qualitative agreement could be obtained at low and high l, where dynamic stall effects and interference effects respectively are predominant. This is positive because l ¼ 5 is the closest value from the optimal power production point. Variable pitch is finally introduced in the model and several functions are tested in order to increase efficiency. A maximum increase of 53% on the power coefficient was found to occur with a sinusoidal law. 2012 Elsevier Ltd. All rights reserved. 1. Introduction Tidal turbines are currently the power source that shows the most advantages [2]. No land occupation like a dam, steady predictable power input and output unlike wind turbines, no waste or side effects like fossil or nuclear power plants. These devices can consist of a classic horizontal axis screw-like systems, or crossflow turbines which have many advantages in water [3], such as being independent of the tide direction. Variable pitch crossflow turbines enable a Darrieus system to improve its performance and decrease parasitic forces,mainly responsible for fatigue and systemfailure [4]. They have been studied at IRENAV since 2007 as the SHIVA project. This project of novel tidal turbines deals with three topics,which will be introduced here. Darrieus turbines have been studied extensively during the 70s and 80s, especially by SANDIA organization [5e8]. A reference publication on this topic can be found in [9]. Though almost no Darrieus turbine produced electrical power from wind since early 90s, a renewed interest arose from water turbines because most drawbacks which prevented this system from becoming