Turbulent jets induced by surface actuators

A round turbulent water jet is produced normal to, and at the center of a thin metal disk flush-mounted on a submerged flat plate and driven at resonance by a piezoelectric actuator. The jet is formed without mass injection across the actuator surface and thus is comprised entirely of axisymmetrically entrained surrounding fluid. The jet is formed concomitantly with the appearance of a small cluster of cavitation bubbles near the center of the disk. In the absence of the bubbles, the flow field induced by the disk is time-periodic with no net streamwise velocity. The bubbles appear and subsequently disappear during each excitation cycle whenever the excitation amplitude exceeds a given threshold. Thus it is speculated that the jet is synthesized by a train of turbulent puffs that are formed at the resonance frequency, and that each puffis produced by the coalescence of a cluster of vortex rings generated by the cavitation bubbles. Near the actuator, the characteristic width of the jet is of the same order as the width of the bubble cluster (1 to 2 mm). Even though the jet is synthesized by a strong time-periodic excitation that can be detected throughout the flow field, it is remarkable that its time-averaged far field characteristics are similar to those of a classical round turbulent jet in that the respective increase and decrease of the time-averaged width and centerline velocity are linear with streamwise distance. The fact that the jet can be manipulated on relatively short time scales, suggests that it can be effectively utilized for manipulation and control of wall-bounded shear flows.