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 $D^+$ ($2.1<\delta/d<8.0$) 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 ($170<Re_\theta<740$) 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.