Flow control with perpendicular acoustic forcing on NACA 2415 aerofoil at low Reynolds numbers

In this study, the effect of perpendicular acoustic excitation on laminar separation bubble, aerodynamic characteristics and stall characteristics over a NACA 2415 aerofoil was investigated experimentally at low Reynolds numbers at varying angles of attack. Extensive experiments such as force and pressure measurements, flow visualization via smoke-wire technique and velocity measurement via hot-wire system were conducted. The experimental results showed that when acoustic excitation of a certain frequency was applied, the length of the laminar separation bubble was shortened owing to the energy added to the flow by acoustic excitation. Because of the shortened laminar separation bubble, coefficient of lift was increased. Furthermore, at the stall angles the separated flow was forced to reattach to the surface of the aerofoil by acoustic forcing, so the stall angle and maximum coefficient of lift were increased, and drag coefficient was decreased.