China Electronics Technology Group Corporation No.38 Research Institute, Hefei 230088, P.R. China
Instability and fl w transition of a vortex ring impinging on a wall were investig ated by means of large-eddy simulation for two vortex core thicknesses corresponding to thin and thick vortex rings. Various fundamental mechanisms dictating the fl w behaviours, such as evolution of vortical structures, instability and breakdown of vortex rings, development of modal energies, and transition from laminar to turbulent state, have been studied systematically . Analysis of the enstrophy of wrapping vortices and turbulent kinetic energy (TKE) in fl w fiel indicates that the formation and evolution of wrapping vortices are closely associated with the fl w transition to turbulent state. It is found that the temporal development of wrapping vortices and the growth rate of axial fl w generated around the circumference of the core region for the thin ring are faster than those for the thick ring. The azimuthal instabilities of primary and secondary vortex rings are analysed and the development of modal energies reveals the fl w transition to turbulent state. The law of energy decay follows a characteristic k 5=3 law, indicating that the vortical fl w has become turbulent. The results obtained in this study provide physical insight into the understanding of the instability mechanisms relevant to the vortical fl w evolution.