Direct Numerical Simulations of Wake Vortices in Intense Homogeneous Turbulence

Abstract

Three-dimensional direct numerical simulations of a vortex pair placed in an homogeneous turbulent ® eld are presented. The phenomenon of interaction between the vortices and the turbulence is analyzed in detail from two simulations calculated on a 135 3 nodes mesh grid (one case with axial core velocity, the other without). The results show that the role of large and small turbulent eddies can be separated into two fairly independent mechanisms. The large structures are stretched by velocity gradients induced by the vortex pair. That leads to the formation of both tubes of vorticity spinning azimuthally around each vortex and associated axial velocity sheets. These structures cause a strong decrease of the maximal velocity and the curvature of the vortices. These deformations induced by the turbulence initiate an antisymmetric long-wave instability of the pair, which will probably lead to the collapse of the vortices. Concurrently, the small structures can enhance the diffusion of the vorticity out of the vortex core. From this analysis 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 results to the random turbulent initial conditions.