Actuation parameters and boundary layer height effects on a circular synthetic jet in crossflow

Three-dimensional unsteady numerical simulations are performed to investigate the effects of blowing ratio CB ( 0.85< Ūⱼ/U_∞ < 1.7 ), stroke ratio L⁺ ( 10.6< Ūⱼ /(fd) < 21.3 ), and boundary-layer height ratio D⁺ ( 2.1<δ /d<8.0 ) on circular synthetic jet actuator (SJA) performance in crossflow. Nine cases are examined at constant free-stream velocity U_∞ , with systematic independent variation of averaged jet velocity Ūⱼ , actuation frequency f (200- 400~Hz ), and boundary-layer momentum thickness Reynolds number ( 170<Re_θ <740 ) to examine the influence of these parameters across varying boundary-layer conditions 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.