Effects of modulation frequency on plasma-induced jet and vortex evolution

The evolution of jets and vortices induced by a pair of symmetrically placed plasma actuators is investigated under various modulation frequencies using two-dimensional, time-resolved particle image velocimetry. Despite similar time-averaged jet shapes, the peak jet velocity increases with the modulation frequency and eventually reaches a plateau. Due to the combined effects of the vortex induction and the modulation frequency, the high-velocity region concentrates near the actuator tip at higher frequencies and shifts downstream at lower frequencies. Compared to the continuous plasma jet, modulated jets demonstrate superior electro-hydrodynamic conversion efficiency and significantly enhanced lateral spreading, leading to increased momentum flux and entrainment coefficient. Despite frequency-induced variations, all jet velocity profiles collapse in the far field when properly non-dimensionalized. The modulation frequency governs the formation of three distinct plasma-induced vortical patterns, including vortex-free, leapfrogging, and multi-vortex, which in turn yield different turbulent kinetic energy distributions. The streamwise trajectories of vortices collapse when scaled by the convection time for three flow patterns. Estimated vortex-induced velocities account for approximately half of the total jet velocity at high modulation frequencies, confirming the crucial role of vortex dynamics in the plasma jet development. These findings highlight the importance of the modulation frequency in optimizing jet performance and actuator placement for effective flow control.