Search

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]

The dynamics of a fully-passive flapping-foil turbine, operating at a Reynolds number of 3.9 × 10 6 , is studied via two-dimensional fluid-structure numerical simulations. The foil is allowed to move freely, but only, in heave and in pitch by being simply attached with springs and dampers. These elastic supports eliminate the need for the more complex mechanisms that are traditionally used to prescribe specific foil motions. This study demonstrates that the optimal performance of fully-constrained flapping-foil turbines can be matched with this simpler concept when the structural parameters are adequately adjusted. An efficiency reaching 53.8% has been achieved. Also, the effects of varying the heaving mass and the heave stiffness can be effectively characterized by a single parameter, which is not the heave natural frequency. On the other hand, the pitch dynamics is appropriately characterized by the pitch natural frequency, which combines the moment of inertia and the pitch stiffness. An optimal efficiency can be maintained over large variations of the inertial and stiffness properties when the effective parameters are kept constant. It is also found that the presence of viscous friction in pitch is detrimental to the turbine performance, but its effect remains small with a realistic friction level. • A foil is elastically attached in heave and in pitch to extract energy from a flow. • An efficiency of 53.8% is reached with this simple flapping-foil turbine concept. • Effective inertial-stiffness parameters are proposed in heave and in pitch. • High efficiencies can be maintained over large variations of the parameters values. • A realistic viscous friction level in pitch only has a small effect on the turbine. [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]