Evidence of physical turbulence cascade mechanism via reconnection cascade scenario

Direct numerical simulation of viscous anti-parallel vortex tube reconnection is performed up to vortex Reynolds number Re(=Γ/ν) = 40 000. To suppress the previously observed symmetry breaking through Kelvin-Helmholtz instability triggered by numerical round-off error, and also to explore reconnection phenomenon further, we impose the symmetric boundary condition. We report here, for the first time, the evidence of vortex reconnection cascade scenario first proposed by Melander and Hussain (1988, CTR Report), where the remaining threads continue to undergo successive reconnections. The secondary reconnections at Re=9000 lead to the creation of various small-scale coherent structures, including vortex rings and packets of hairpin-like vortices. As Re increases, higher (e.g. third) generation of reconnection is also observed, and the energy is more rapidly transferred to finer scales - forming a turbulent cloud consisting of tangle of slender vortices. In addition, we observe a -5/3 kinetic energy spectrum for large wavenumbers associated with the reconnected bridges and remaining threads. These observations clearly demonstrate and confirm our long-standing claim that vortex reconnection is an important physical mechanism of turbulent energy cascade, particularity at high Re’s.