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Power extraction performance of a fully-passive oscillating-plate hydrokinetic turbine prototype was investigated experimentally using measurements of the kinematics of the flat plate and the estimated power extraction. Two configurations were considered: a flat plate with a 6 ∘ sweep angle and an unswept plate (control configuration), which were undergoing fully-passive pitching and heaving motions in uniform inflow at Reynolds numbers ranging from 15,000 to 30,000. The resulting kinematic parameters and the power extraction performance were evaluated for both plates. The influence of the bottom endplate was also studied to investigate the extent of the spanwise flow and its effects at the tip of the plate. The swept plate experienced significant flow-induced forces acting in the direction of the heaving motion over a larger portion of the oscillation cycle, compared to the unswept plate. Consequently, the swept plate reached comparatively larger heaving amplitudes at high values of the inflow velocity, which resulted in higher power coefficient values but comparable efficiency values, relative to the unswept plate. Moreover, the energy-harvesting performance of the swept plate was consistent over a wider range of inflow velocities, compared to the unswept plate. The presence of the bottom endplate significantly impacted the kinematics of the swept plate, suggesting a dominant role of the spanwise convection of vorticity, which induced substantial tip losses. [ABSTRACT FROM AUTHOR]

Abstract Experiments were conducted to assess the performance of a fully-passive flapping-foil hydrokinetic turbine for which the self-induced and self-sustained blade motions are resulting from the interaction between the blade's elastic supports (springs and dampers) and the flow field. Previous numerical studies have shown that such a turbine can extract a substantial amount of energy from the flow while offering the possibility to simplify the complex mechanical apparatus generally needed to constrain and couple the blade pitching and heaving motions in the case of the conventional fully-constrained flapping-foil turbine. Based on these promising numerical investigations, a prototype was designed and tested in a water channel at a chord Reynolds number of 21000. Robust and periodic motions of large amplitudes were observed leading to an energy harvesting efficiency reaching 31% and a power coefficient of 0.86. The sensitivity of the turbine dynamics to seven different structural and inflow parameters was evaluated experimentally around a baseline case achieving a high level of performance. It was found that the turbine maintains a good performance over a large range of parameters. [ABSTRACT FROM AUTHOR]

This paper deals with an aeroelastic problem that consists into self-sustained, pitch-heave oscillations of an elastically-mounted airfoil. Such oscillations of an airfoil could be used in order to develop a novel, fully-passive hydrokinetic energy flow harvester that is relatively simple from a mechanical point of view. Indeed, the motion of such an airfoil emerges as a result of the fluid-structure interaction between the flow, the airfoil and its elastic supports, and is sustained through a net transfer of energy from the flow to the structure. In this numerical study, the OpenFOAM-2.1.x CFD toolbox is used for solving the aeroelastic problem. Through unsteady two-dimensional viscous simulations at a Reynolds number of 500,000, the fully-passive turbine is optimized and investigated to develop a better understanding of the physics at play. Following a gradient-like optimization of the turbine, two-dimensional efficiencies as high as 34% have been obtained, and two fundamental mechanisms have been found to be very beneficial for enhancing the performances of the turbine: the adequate synchronization between both degrees-of-freedom, and the nonsinusoidal shape of the pitching motion. [ABSTRACT FROM AUTHOR]

A self-propelled flexible flapping wing 2D numerical model undergoing a combined pitching and heaving motion is presented. Since such freely moving foil experiences zero net thrust, a definition of efficiency for this kind of problem is proposed and discussed against other formulations found in the literature. It is also shown that the deviation motion of wings such as that found in natural flyers is likely a consequence of the fluid–structure dynamics of the wings. The passive deviation motion observed in numerical simulations is either a consequence of a feathering mechanism referred to as rigid feathering or of the inertial displacement caused by the wing deformation. The effects of flexibility on the performance of the wing are also presented. It is found that flexibility may significantly enhance the efficiency in pressure-driven deformation cases. The rigid feathering mechanism is found to have an effect similar to that of the feathering caused by wing flexibility on the performances of pressure-driven deformation cases. [ABSTRACT FROM AUTHOR]

This paper presents a numerical investigation of the effects of chordwise flexibility on flapping wings at low Reynolds number. The numerical simulations are performed with a partitioned fluid–structure interaction algorithm using artificial compressibility stabilization. The choice of the structural dimensionless parameters is based on scaling arguments and is compared against parameters used by other authors. The different regimes, namely inertia-driven and pressure-driven wing deformations, are presented along with their effects on the topology of the flow and on the performance of a heaving and pitching flapping wing in propulsion regime. It is found that pressure-driven deformations can significantly increase the thrust efficiency if a suitable amount of flexibility is used. Significant thrust increases are also observed in zero pitching amplitude cases. The effects of the second and third deformation modes on the performances of pressure-driven deformation cases are discussed. On the other hand, inertia-driven deformations generally deteriorate aerodynamic performances of flapping wings unless the behavior of the wing deformation is modified by the presence of sustainable superharmonics in a way that produces slight improvements. It is also shown that wing flexibility can act as an efficient passive pitching mechanism that allows fair thrust and better efficiency to be achieved when compared to a rigid pitching–heaving wing. [ABSTRACT FROM AUTHOR]

This work investigates the dynamics of a semi-passive flapping-foil with a prescribed sinusoidal heave motion and a passive pitch motion with the objective of extracting energy from an oncoming fluid flow. This implies that the heave motion is mechanically driven while the foil is elastically supported in pitch. The pitch motion therefore results from the interaction of the foil with the flow and its elastic supports, namely springs and dampers. Numerical simulations have been conducted at a Reynolds number of 3. 9 × 1 0 6 based on the chord length. Positive efficiencies and periodic pitch motions of large amplitude are obtained when the frequency of the pitch motion synchronizes itself to the frequency of the prescribed heave motion. The conditions under which it happens are explored. The results of this study demonstrate that an optimal power-generation performance can be maintained over large variations of the moment of inertia and pitch stiffness, provided that they are properly scaled. This is achieved by combining these two structural parameters into a single effective parameter: the effective pitch stiffness coefficient. Moreover, four different positions of the pitch axis are considered, ranging from the leading edge to the three-quarter-chord point. By adjusting the governing structural parameters adequately, efficiencies exceeding 40% can be achieved with all four positions of the pitch axis, with a maximum of 46.0% obtained when the pitch axis is located at the quarter-chord point. It is found that a phase lag near 90 ° between the heave and the pitch motions is only optimal with this specific position of the pitch axis. It needs to be larger than 90 ° when the pitch axis is located upstream of the quarter-chord point and smaller than 90 ° when it is located downstream of this position. [ABSTRACT FROM AUTHOR]

An experimental study was conducted to assess fidelity of model-scale representations of full-scale oscillating-foil turbines operating at high Reynolds numbers. The experiments were performed using a NACA 0015 foil with an aspect ratio of 7.5 undergoing controlled oscillations in pitch and heave in a water tunnel. We considered Reynolds numbers ranging from 20,000 to 50,000 to investigate the effects of dynamic stall, channel confinement and forced transition to turbulence using distributed surface roughness. Direct force measurements were used to quantify the performance of the turbine, and quantitative flow imaging was performed using particle image velocimetry. Oscillations of the foil resulted in formation of turbulent boundary layers, particularly in the cases involving the roughened foils. This accelerated transition led to turbine performance characteristics (i.e. efficiency and extracted power coefficient) comparable to those reported for a numerical benchmark case of R e = 500 , 000. [ABSTRACT FROM AUTHOR]

Abstract Due to the inherent complexity of the mechanisms needed to prescribe the heave and the pitch motions of optimal flapping-foil turbines, several research groups are now investigating the potential of using unconstrained motions. The amplitude, the phase and the frequency of such passive motions are resulting from the interaction of the foil with the flow and its elastic supports, namely springs and dampers. More specifically, this work proposes an innovative semi-passive flapping-foil turbine concept with a prescribed sinusoidal heave motion and a passive pitch motion. Two-dimensional numerical simulations have been carried out at a Reynolds number of 3. 9 × 1 0 6 based on the chord length with a foil having its pitch axis located at the quarter-chord point. A parametric study has been conducted by varying the value of the static moment, which involves the distance between the center of mass and the pitch axis, and the frequency of the prescribed heave motion. Different responses of the foil have been observed and one of them corresponds to an energy-extraction regime that is characterized by periodic limit-cycle oscillations of large amplitudes with a phase lag between the heave and the pitch motions ranging between 90 ° and 105 °. A maximum efficiency of 45.4% has been reached, hence confirming the great potential of this turbine concept. This works shows that such good performance is achieved when the center of mass of the foil is located downstream of the pitch axis and when no leading-edge vortices are formed. [ABSTRACT FROM AUTHOR]

A self-sustained fully-passive flapping foil hydrokinetic turbine prototype is investigated experimentally. The foil was undergoing elastically constrained oscillations in pitch and heave in water at Re = 21000. The prototype was subjected to inflow disturbances in the form of symmetric vortices from a wake of an actuated oscillating foil positioned upstream of the test foil. The effects of the periodic inflow disturbances on the fully-passive turbine performance were characterized using measurements of its kinematics, power coefficient and the flow structures in the near-wake region. Stable operation of the turbine was observed below a threshold reduced frequency of the incoming vortices. Above the threshold frequency, intermittent resonance with the upstream flow disturbance resulted in unstable oscillations of the turbine. The range of the frequencies of the upstream disturbance that were beneficial to the power performance of the fully-passive turbine was not consistent with the frequencies of the turbine oscillations in a uniform stream with equivalent momentum flux. Peak power extraction could be obtained in a narrow range of kinematics of the upstream oscillating-foil. [ABSTRACT FROM AUTHOR]