COHERENCE OF INTENSE LOCALIZED VORTICITY IN DECAYING 2-DIMENSIONAL NAVIER-STOKES TURBULENCE

The coupled evolution of an intense localized vortex and decaying Navier-Stokes turbulence in two dimensions is examined in order to identify and describe the physical process allowing regions of intense vorticity to decouple from the eddy cascade and survive as coherent structures. It is found that the shear straining of turbulent eddies by the differential rotation of the intense vortex gives rise to vortex coherence. Dimensional analysis and analytic solution of a closure representation in the asymptotic limit of large shear are used to determine a nonuniform vortex diffusivity produced by turbulent straining, and the spatial structure of turbulence in the vortex. When the shear straining rate exceeds the appropriate measure of the ambient turbulent straining rate, turbulent fluctuations in the vortex interior localize to a narrow layer in the vortex edge. Likewise, vortex diffusion becomes weak, the vortex lifetime becomes large, and greatly exceeds the eddy lifetimes of ambient turbulence.