Numerical Investigation of Boundary-Layer Height and Actuation-Parameter Effects of a Circular Synthetic Jet Actuator in Crossflow

Abstract

Three-dimensional unsteady numerical simulations are performed to investigate the effects of blowing ratio $C_B$ ($0.85<{\overline{U}}_j/U_{\infty }<1.7$), stroke ratio $L^{+}$ ($10.6<{\overline{U}}_j/(fd)<21.3$), and boundary-layer height ratio $D^{+}$ ($2.1<\hat{I}´/d<8.0$) on circular synthetic jet actuator (SJA) performance in crossflow. Nine cases are examined at constant free-stream velocity $U_{\infty }$, with systematic independent variation of averaged jet velocity ${\overline{U}}_j$, actuation frequency $f$ ($200$-$400\mathrm{Hz}$), and boundary-layer momentum thickness Reynolds number ($170<R{e}_{\hat{I}}¸<740$) to isolate the individual effects of these parameters on a circular-nozzle SJA with fixed nozzle diameter $d$ in crossflow. Instantaneous vortical structures exhibited tilted vortex rings with a trailing vortex pair at low actuation frequency; closely packed expelled vortical structures for higher frequency SJAs, and the largest boundary-layer height ratio induced hairpin-like vortices. Near-wall tertiary vortices, which promote downwash and increase wall shear stress, remain coherent longer and have extended spanwise coverage for low $D^{+}$. Time-averaged boundary-layer profiles and skin-friction distributions reveal that SJAs with low to moderate $D^{+}$ have the greatest potential for separation control, maintaining increased near-wall momentum over extended streamwise distances.

25 pages, 16 figures, 3 tables