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- \(400~\textrm{Hz}\) ), and boundary-layer momentum thickness Reynolds number ( \(170<Re_\theta <740\) ) 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.
