Unsteady aerodynamic flow control at low Reynolds numbers via internal acoustic excitation

Aerodynamic flow control using internal acoustic excitation holds promise as it combines the simplicity of passive flow control techniques (in terms of added weight and operational complexity) with the control authority of active flow control methods. While previous studies have analyzed the effects of acoustic excitation on steady-wing aerodynamics, the effect of excitation on the unsteady aerodynamics is not known, which is the aim of the current effort. Internally mounted speakers on a symmetric National Advisory Committee for Aeronautics (NACA) 0012 wing are used to excite the unsteady boundary layer at the wing's leading edge as it executes linear pitch motions ranging from quasi-steady (trailing-edge driven stall) to vortex-dominated (mixed leading- and trailing-edge driven stall) motions at freestream Reynolds numbers (Re) of 120 000 and 180 000. Experimental results show that, although acoustic excitation delays stall for quasi-steady motions, it enhances lift in the linear region and increases leading-edge vortex strength for vortex-dominated motions. The degree of change was observed to be a function of the excitation frequency, with lower frequencies (≤ 250 Hz) leading to an increase in aerodynamic efficiency and higher frequencies having a negligible effect. The current work establishes the effects of acoustic flow excitation in unsteady, low-Re wing aerodynamics and provides insights on the path forward to effectively implement the method for active flow control.