Session Ci - Vortex Dynamics.
ORAL session, Sunday, November 23
309, Moscone Center
Using DNS, we study 3D instability and transition of a vortex column with nonuniform core size in a uniform plane shear normal to its axis. Internal (core) dynamics leads to propagation of vorticity wavepackets along the column which can extract energy from shear, and amplifies oscillations of core size and vortex line twisting. The exponential 3D energy growth rate increases with increasing shear strength. Collision of oppositely propagating wavepackets forms a low-enstrophy pocket in the core surrounded by a thin sheath of intense vorticity which rolls up by a local Kelvin-Helmholtz type instability into an array of fine-scale vortices along the periphery. Reconnection of folded vortex lines within the core leads to concurrent fine-scale generation. These constitute physical-space scenarios of cascade mechanisms, quite likely to be prototypical in turbulent flows.