A Study of spatiotemporal features of sweeping jets acting on afterbody vortices using low-operation-rate stereo PIV

Abstract

Although low-operation-rate particle image velocimetry (PIV) provides a good spatial accuracy in measurements at relatively affordable costs, it faces some challenges in capturing unsteady features of oscillatory flow. In this paper, a single sweeping jet actuated to control afterbody vortices from a 30^◦ slanted-base cylinder is investigated at a Reynolds number of 87,000. Phase-locked stereo PIV measurements combining triggering reference obtaining and real-time processing via field programmable gate array (FPGA) are leveraged to reveal the unsteady characteristics of the sweeping jet. The examined cases show that the phase-locked method can well identify jet’s spatiotemporal development process in each oscillation cycle. A sinusoidal-like interaction along phases between the jet and the afterbody vortex can be reasonably detected. At each moment, coherent small vortical structures form at the upper and bottom jet/ambient interfaces, which are caused by Kelvin-Helmholtz instability. Since the induced vortex has the same rotation direction as the afterbody vortex on each side, they merge with each other as the jet approaches the vortex, causing an increase in vorticity. Meanwhile, the sweeping jet’s intrusion into the vortex region induces a rise in turbulent kinetic energy in that area, causing turbulence ingestion of the vortex which weakens the velocity gradient. The sweeping behavior of the jet dominates the afterbody vortex to meander as the jet pushes its way underneath the vortex. The findings of this study provide encouraging evidence for future applications of sweeping jets in control of afterbody vortices.