Vortex dynamics and Reynolds number effects of an oscillating hydrofoil in energy harvesting mode

TL;DR

This study investigates vortex dynamics and energy harvesting efficiency of an oscillating hydrofoil at high Reynolds number using LES, revealing optimal kinematics, vortex behavior, and Reynolds number effects on power output.

Contribution

It provides detailed LES analysis of vortex shedding and energy extraction in oscillating hydrofoils at high Reynolds number, validated against experimental data, and explores the influence of kinematics and Reynolds number on performance.

Findings

High Reynolds number flows show increased power extraction (0.8-6.7%) compared to low Reynolds number.

Coherent LEV forms earlier and is stronger at high Reynolds number, enhancing efficiency.

LEV trajectory depends strongly on kinematics but is relatively Reynolds-independent after shedding.

Abstract

The energy extraction and vortex dynamics from the sinusoidal heaving and pitching motion of an elliptical hydrofoil is explored through large-eddy simulations (LES) at a Reynolds number of $50,000$. The LES is able to capture the time-dependent vortex shedding and dynamic stall properties of the foil as it undergoes high relative angles of attack. Results of the computations are validated against experimental flume data in terms of power extraction and leading edge vortex (LEV) position and trajectory. The kinematics for optimal efficiency are found in the range of heave amplitude $h_o/c=0.5-1$ and pitch amplitude $\theta_o=60^{\circ}-65^{\circ}$ for $fc/U_{\infty}=0.1$ and of $h_o/c=1-1.5$ and $\theta_o=75^{\circ}-85^{\circ}$ for $fc/U_{\infty}=0.15$. Direct comparison with low Reynolds number simulations and experiments demonstrate strong agreement in energy harvesting performance between Reynolds numbers of $1000$ to $50,000$, with the high Reynolds number flows demonstrating a moderate $0.8-6.7\%$ increase in power compared to the low Reynolds number flow. In the high Reynolds number flows, the coherent LEV, which is critical for high-efficiency energy conversion, forms earlier and is slightly stronger, resulting in more power extraction. After the LEV is shed from the foil, the LEV trajectory is demonstrated to be relatively independent of Reynolds number, but has a very strong nonlinear dependence with kinematics. It is shown that the LEV trajectories are highly influenced by the heave and pitch amplitudes as well as the oscillation frequency. This has strong implications for arrays of oscillating foils since the coherent LEVs can influence the energy extraction efficiency and performance of downstream foils.