Actuation parameters and boundary layer height effects on a circular synthetic jet 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- $$400~\textrm{Hz}$$ ), 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.