http://personal.strath.ac.uk/qing.xiao/pdf/a review on flow energy harvesters based on flapping foils.pdf

A review on flow energy harvesters based on flapping foils

A review of progress and challenges in flapping foil power generation

Unsteady aerofoil flows are often characterized by leading-edge vortex (LEV) shedding. While experiments and high-order computations have contributed to our understanding of these flows, fast low-order methods are needed for engineering tasks. Classical unsteady aerofoil theories are limited to small amplitudes and attached leading-edge flows. Discrete-vortex methods that model vortex shedding from leading edges assume continuous shedding, valid only for sharp leading edges, or shedding governed by ad-hoc criteria such as a critical angle of attack, valid only for a restricted set of kinematics. We present a criterion for intermittent vortex shedding from rounded leading edges that is governed by a maximum allowable leading-edge suction. We show that, when using unsteady thin aerofoil theory, this leading-edge suction parameter (LESP) is related to the term in the Fourier series representing the chordwise variation of bound vorticity. Furthermore, for any aerofoil and Reynolds number, there is a critical value of the LESP, which is independent of the motion kinematics. When the instantaneous LESP value exceeds the critical value, vortex shedding occurs at the leading edge. We have augmented a discrete-time, arbitrary-motion, unsteady thin aerofoil theory with discrete-vortex shedding from the leading edge governed by the instantaneous LESP. Thus, the use of a single empirical parameter, the critical-LESP value, allows us to determine the onset, growth, and termination of LEVs. We show, by comparison with experimental and computational results for several aerofoils, motions and Reynolds numbers, that this computationally inexpensive method is successful in predicting the complex flows and forces resulting from intermittent LEV shedding, thus validating the LESP concept.

Discrete-vortex method with novel shedding criterion for unsteady aerofoil flows with intermittent leading-edge vortex shedding

Prototype testing of a hydrokinetic turbine based on oscillating hydrofoils

Efficiency investigation on energy harvesting from airflows in HVAC system based on galloping of isosceles triangle sectioned bluff bodies

A wing that is heaving and pitching simultaneously may extract energy from an oncoming flow, thus acting as a turbine. The theoretical performance of such a concept is investigated here through unsteady two-dimensional laminar-flow simulations using the commercial finite volume computational fluid dynamics code FLUENT. Computations are performed in the heaving reference frame of the airfoil, thus leaving only the pitching motion of the airfoil to be dealt with through a rigid-body mesh rotation and a circular nonconformal sliding interface. Unsteady aerodynamics basics of the oscillating airfoil are first exposed, with a description of the operating regimes. Effects of unsteadiness are stressed and the inadequacy of a quasi-steady approach to take them into account is exposed. We present a mapping of power-extraction efficiency for a single oscillating airfoil in the frequency and pitching-amplitude domain: 0 55deg in which efficiencies are higher than 20%. Results from a parametric study are then provided and discussed. It is found that motion-related parameters such as heaving amplitude and frequency have the strongest effects on airfoil performances, whereas geometry and viscous parameters turn out to play a secondary role.

Parametric Study of an Oscillating Airfoil in a Power-Extraction Regime

The performance of a new concept of hydrokinetic turbine using oscillating hydrofoils to extract energy from water currents (tidal or gravitational) is investigated using URANS numerical simulations. The numerical predictions are compared with experimental data from a 2 kW prototype, composed of two rectangular oscillating hydrofoils of aspect ratio 7 in a tandem spatial configuration. 3D computational fluid dynamics (CFD) predictions are found to compare favorably with experimental data especially for the case of a single-hydrofoil turbine. The validity of approximating the actual arc-circle trajectory of each hydrofoil by an idealized vertical plunging motion is also addressed by numerical simulations. Furthermore, a sensitivity study of the turbine’s performance in relation to fluctuating operating conditions is performed by feeding the simulations with the actual time-varying experimentally recorded conditions. It is found that cycle-averaged values, as the power-extraction efficiency, are little sensitive to perturbations in the foil kinematics and upstream velocity.

Computational Fluid Dynamics Analysis of a Hydrokinetic Turbine Based on Oscillating Hydrofoils

Optimal Tandem Configuration for Oscillating-Foils Hydrokinetic Turbine

Three-dimensional hydrodynamic losses are assessed in this investigation for a foil oscillating sinusoidally in a combined heave and pitch motion with large amplitudes. Simulations are performed using a unsteady Reynolds-Averaged-Navier-Stokes (URANS) solver on an oscillating foil in a power-extraction mode; thus acting as a hydrokinetic turbine at high Reynolds number. Foils of various aspect ratios (span to chord length ratio) are considered, both with and without endplates for one representative operation point. Hydrodynamic forces and extracted power are compared with results from the equivalent two-dimensional (2D) computations. It is found that the relative drop of performance (cycle-averaged power extracted) due to 3D hydrodynamic losses can be limited to 10% of the 2D prediction when endplates are used on a foil of aspect ratio greater than ten. The practical consideration of an oscillating-foil hydrokinetic turbine operating in an imperfectly-aligned upstream water flow is also addressed with simulations considering an upstream flow at a yaw angle up to 30° with respect to the foil chord line. Effects on performance are found to be proportional to the projected kinetic energy flux.

Thomas Kinsey

Papers

Parametric Study of an Oscillating Airfoil in a Power-Extraction Regime

Prototype testing of a hydrokinetic turbine based on oscillating hydrofoils

Computational Fluid Dynamics Analysis of a Hydrokinetic Turbine Based on Oscillating Hydrofoils

Optimal Tandem Configuration for Oscillating-Foils Hydrokinetic Turbine

Three-Dimensional Effects on an Oscillating-Foil Hydrokinetic Turbine