Nonsinusoidal motion effects on energy extraction performance of a flapping foil

To seek better energy extraction performance of a flapping hydrofoil, various nonsinusoidal motion profiles are employed instead of conventional sinusoidal flapping motions. The effects of nonsinusoidal motions are investigated for four kinds of nonsinusoidal flapping motions, i.e. varying effective angle of attack profile, nonsinusoidal pitching motion combined with sinusoidal plunging, nonsinusoidal plunging motion combined with sinusoidal pitching, and combined nonsinusoidal pitching and nonsinusoidal plunging motion. An adjustable parameters K is used to realize various nonsinusoidal profile varying from a sawtooth wave to a square wave profile. Numerical results show that by imposing a square-like effective angle of attack profile, extraction power and efficiency can be significantly increased compared with sinusoidal flapping motion. By accelerating the formation time and development of leading edge vortex (LEV), the square-like effective angle of attack profile leads to a better synchronization between the vertical force and plunging velocity. Similar effect on the flapping foil energy extraction performance is also found by imposing nonsinusoidal pitching profile. While the output power enhancement is quite limited by using the nonsinusoidal plunging profile. Moreover, the energy extraction performance can be significantly improved with an appropriate combination of nonsinusoidal pitching and nonsinusoidal plunging motion. Of all the nonsinusoidal motions studied, a toothed-like plunging profile together with square-like pitching profile should be selected for the best energy extraction performance.