A direct numerical simulation study of coherent oscillation effects of swirling flows

In this study, we carried out a direct numerical simulation of swirling jets to investigate the temporal characteristics of coherent structures related to the vortex breakdown. The type of coherent motion of oscillation, as a temporal characteristic of vortex breakdown, is observed through the analysis of correlation functions. It indicates that the vortex breakdown is accompanied with the resonance of fluctuations of flow velocity, pressure, vortices and etc. Moreover, it is found that the coherent oscillation occurs only for the swirling flows with a high level of swirl (larger than the critical value) when the bubble vortex breakdown takes place, and only at relatively moderate Reynolds numbers transiting from laminar to turbulence.