Near-wall characteristics of wall-normal jets generated by an annular dielectric-barrier-discharge plasma actuator

A dielectric-barrier-discharge plasma actuator with annular shape electrode geometry has been studied for evaluating the effectiveness in flow control applications. The flow field is investigated using time-resolved particle-image-velocimetry measurements and light-sheet flow visualization. The characterization of the actuator is carried out for both continuous- and burst-mode actuation. The Reynolds number, Re, of flow generated by the actuator is a function of actuation amplitude, Opp, and burst frequency, fb, of the actuation signal, which governs the acceleration, development, and vortical structures of the wall-normal jet generated by the actuator. The maximum velocity generated by the actuator in continuous-mode actuation is higher than that of the burst-mode actuation. The velocity of the wall-normal jet increases in the wall-normal direction to a maximum value, vmax, followed by a gradual decay for both modes of actuation. The decay rate of vmax is a function of actuation mode due to the difference in nature of the vortex structure evolution and interaction. The overall flow field generated by the actuator can be divided into four zones: (i) entrainment zone, (ii) recirculation zone, (iii) developing zone, and (iv) selfsimilar zone. Three different types of vortices dominate the induced flow, i.e., (i) starting vortex or periodic vortex generated due to impulsive action, (ii) shear layer vortex formed due to roll-up of the shear layer, and (iii) recirculation vortex ring created due to deflection of the approaching wall boundary layer. The self-similar velocity profile of the wall-normal jet generated by the actuator is akin to that of a free jet. The wall-normal jet generated by continuous-mode actuation shows low-frequency (f < 100 Hz) vortical structures. In the burst mode, fluctuations are locked in with burst frequency for fb 200 Hz. For a higher value of burst frequency ( fb 200 Hz), the effect is confined to the neighborhood of the exposed electrode, and flow field behavior approaches that of continuous-mode actuation.