Acoustic Control of Flow over NACA 2415 Airfoil at Low Reynolds Numbers

In this study, the effects of acoustic excitation frequency on flow over an NACA 2415 airfoil were determined, and all of the experiments were done both with and without the presence of the acoustic excitation. The acoustic excitation was applied for a range of angles of attack (0°–25°) and Reynolds numbers of 50,000, 75,000, 100,000, 150,000, and 200,000. To examine the effects of acoustic excitation on the flow, force measurements, pressure measurements, hot-wire anemometry, smoke-wire flow-visualization, and particle image velocimetry techniques were employed. The results indicated that for stall and some limited poststall angles of attack of the acoustic excitation having a frequency in a certain range forced the separated shear layer to reattach to the surface of the airfoil. As the Reynolds number increased, the effective excitation frequency increased, but the range of Zaman number [$\mathsf{St}/({\mathsf{R}}^{1/2})$] was the same. With the acoustic excitation, the stall angle was delayed from 12° to 16° at $\mathsf{R}=\mathrm{50,000}$, and there was a 30% and 50% increase on the maximum value of the lift coefficient and the ratio of the lift and drag forces, respectively. Moreover, the stall angle was delayed from 13° to 17° at $\mathsf{R}=\mathrm{75,000}$, from 15° to 18° at $\mathsf{R}=\mathrm{100,000}$, from 15° to 17° at $\mathsf{R}=\mathrm{150,000}$. Furthermore, it was concluded that acoustic excitation shrunk laminar separation bubble, and an effect of the acoustic control on the separation bubble decreased as the Reynolds number increased.