Flow-Induced Vibration and Energy Harvesting Using Fully-Passive Flapping Foils

When a fluid flows past an elastic body, the ensuing fluid-structure interaction can cause the body to vibrate as energy is transferred from the fluid to the elastic structure. This transfer is via work, and therefore this energy transfer is maximized when large oscillations of the structure occur with fluid forces in phase with the motion. A suitable structure is a plate or foil that can oscillate across the flow in heave, as well as rotate about an axis perpendicular to the flow in pitch. Here, we model the foil as an ellipse of aspect ratio 6, and the Reynolds number is fixed at 200. We consider the efficacy of this system to extract energy from a flowing fluid. Both the pitching and heaving motion are free so the motion is fully passive, but energy is only extracted from the heaving degree of freedom. We present results showing that the energy extracted is of similar magnitude to the net power of semi-active set-ups when the cost of the activation is accounted for. We also show that at this relatively low Reynolds number, the flow undergoes a spontaneous symmetry breaking, and the oscillating foil generates a mean lift force while still extracting energy. The optimal case for energy extraction is also compared to that from Veilleux and Dumas (J. Fluids Struct. 70:102-130, 2017) at a much higher Reynolds number and using a NACA0015 aerofoil. Interestingly it is found that the optimal heave parameters are reasonably similar, indicating that Reynolds number and the details of the body shape only play a minor role in the energy harvesting dynamics.