Merging of Four Vortex Rings in a Round Jet Using a Sinusoidal Sound Wave

In the initial region of a round jet, there are periodic vortex rings, which merge to form a large-scale vortex ring that subsequently collapses and becomes turbulent. This study focuses on the merging of vortex rings in a round jet, and aims to clarify the conditions for the regular merging of vortex rings. Acoustic excitation using a sinusoidal wave was used to facilitate the regular merging of the vortex rings. To investigate the merging process of vortex rings, the streamwise cross-section of the excited jets with varying Reynolds numbers was visualized using a laser-light sheet. As a result, it was found that there is a specific range of Strouhal number within which four vortex rings merge. There is a suitable frequency for the merging by acoustic excitation. The reason is discussed for this phenomenon and it is found there is a suitable distance between vortex rings for successful merging.