Flow control by traveling wave surface actuations

Traveling waves, employed by aquatic animals to propel themselves, have served as inspiration for the development of a novel, energy-efficient flow control technique, which utilizes surface actuations in the form of traveling waves. The utility and efficacy of this novel technique is demonstrated by complementary wind tunnel experiments and simulations. The traveling waves were generated on the suction side of the airfoil using multiple piezoelectric actuators. The measured lift force showed hysteresis loops, and the traveling wave actuations increased the stall angle relative to the unactuated airfoil in the upper, i.e., increasing angle of attack (AOA), branch of the hysteresis loop. The corresponding wall-resolved large eddy simulations (LES) with a sharp interface curvilinear immersed boundary (CurvIB) method, in which the airfoil's surface motion was prescribed from measurements, were validated against the lift measurements and captured their hysteresis loops. LES-CurvIB results confirmed that the higher and lower lift in the actuated and unactuated airfoils in the upper and lower branches of the hysteresis loop was due to attached and separated flow, respectively. Although the traveling waves keep the flow attached and delay stall to a higher AOA, the hysteresis loop indicates that the tested traveling waves might not be able to reattach an already separated flow. To reattach a fully separated flow and improve lift recovery, LES-CurvIB simulations initialized from a fully separated flow suggest these avenues for future design of traveling waves: (a) increasing the wave amplitude and (b) keeping the actuation frequency within the bounds of vortex shedding frequency from the leading edge of unactuated airfoil.