PIV measurement of the merging process of four consecutive vortex rings using acoustic excitation

Abstract

Round jets roll up the shear layer near the nozzle exit and form vortex ring street. The formed vortex ring street then merge to form large vortex rings. The large-scale vortex rings eventually break down and become disordered due to the azimuthal instability of the vortex rings. Since the formation of vortex rings is a periodic phenomenon based on the instability of the shear layer, acoustic excitation, which is one of the active control methods using periodic sound waves, is highly effective for controlling vortex rings. Acoustic excitation can cause the regular merging of vortex rings, which occurs irregularly in the natural transition of jets. We have previously shown through experiments by flow visualization that four consecutive vortex rings can be merged by acoustic excitation with a single sinusoidal wave of a single frequency, and have shown the condition of Re and St of jets for the merging of four vortex rings. The streamwise distance between the vortex rings when the four vortex rings merge has been presented. Here, PIV measurements were conducted to investigate the advective velocity of the vortex rings in the above merging process. As a result, we found that there was a significant difference in both the distance between the merged vortex rings and the advection velocity when the four vortex rings merged compared to when they did not.