DIRECT NUMERICAL SIMULATIONS OF WAKE VORTICES IN INTENSE HOMOGENEOUS TURBULENCE

Three-dimensional direct numerical simulations of a vortex pair placed in an homogeneous turbulent field are presented. The phenomenon of in teraction between the vortices and the turbulence is analyzed in detai l from two simulations calculated on a 135(3) nodes mesh grid (one cas e with axial core velocity, the other without). The results shove. tha t the role of large and small turbulent eddies can be separated into t wo fairly independent mechanisms. The large structures are stretched b y velocity gradients induced by the vortex pair. That lends to the for mation of both tubes of vorticity spinning azimuthally around each vor tex and associated axial velocity sheets. These structures cause a str ong decrease of the maximal velocity and the curvature of the vortices . These deformations induced by the turbulence initiate an antisymmetr ic long-wave instability of the pair, which will probably lead to the collapse of the vortices. Concurrently, the small structures can enhan ce the diffusion of the vorticity out of the vortex core. From this an alysis and scale considerations, the correct form for a model of decay of the vortices is derived. On the other hand, nine simulations on a 81(3) nodes mesh grid are used to determine the sensibility of the res ults to the random turbulent initial conditions.