THE 3-DIMENSIONAL INTERACTION OF A VORTEX PAIR WITH A WALL

The interaction of vortices passing near a solid surface has been exam ined using direct numerical simulation. The configuration studied is a counter-rotating vortex pair approaching a wall in an otherwise quies cent fluid. The focus of these simulations is on the three-dimensional effects, of which little is known. To the authors' knowledge, this is the first three-dimensional simulation that lends support to the shor t-wavelength instability of the secondary vortex. It has been shown ho w this Crow-type instability leads to three dimensionality after the r ebound of a vortex pair. The growth of the instability of the secondar y vortex in the presence of the stronger primary vortex leads to the t urning and intense stretching of the secondary vortex. As the instabil ity grows the secondary vortex is bent, stretched, and wrapped around the stronger primary. During this process reconnection was observed be tween the two secondary vortices. Reconnection also begins between the primary and secondary vortices but the weaker secondary vortex dissip ates before the primary, leaving reconnection incomplete. Evidence is presented for a new type of energy cascade based on the short-waveleng th instability and the formation of continual smaller vortices at the wall. Ultimately the secondary vortex is destroyed by stretching and d issipation leaving the primary vortex with a permanently distorted sha pe but relatively unaffected strength compared to an isolated vortex. (C) 1997 American Institute of Physics.