A nonlinear mechanism for receptivity of free-stream disturbances

Numerical experiments on the interaction of simple vortical free-stream dis turbances with a laminar boundary layer are presented. Both spatial and tem poral direct numerical simulations (DNS) have been performed for two types of free-stream disturbances. A linear and a nonlinear receptivity mechanism were identified. The nonlinear mechanism was found to force streaks inside the boundary layer similar to those found in experiments on free-stream tu rbulence and it performed equally well for disturbances elongated in the st reamwise direction as for and oblique free-stream disturbances. The boundar y layer response caused by the nonlinear mechanism was, depending on the in itial disturbance energy, comparable to that of the linear mechanism, which was only efficient for free-stream streamwise vortices. A parameter study revealed that the wall normal velocity component of the free-stream disturb ances is more important for the investigated receptivity mechanisms than th e streamwise component. The identified boundary layer receptivity mechanism , in which three-dimensional disturbances in the free-stream continuously f orce streaks inside the boundary layer, may explain differences between exp erimental results and previously suggested theories for the origin of strea ks in boundary layers subjected to free-stream turbulence. (C) 1999 America n Institute of Physics. [S1070-6631(99)01012-0].