A nonlinear mechanism for receptivity of free-stream disturbances

Abstract

Numerical experiments on the interaction of simple vortical free-stream disturbances with a laminar boundary layer are presented. Both spatial and temporal 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 turbulence and it performed equally well for disturbances elongated in the streamwise direction as for and oblique free-stream disturbances. The boundary layer response caused by the nonlinear mechanism was, depending on the initial 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 disturbances is more important for the investigated receptivity mechanisms than the streamwise component. The identified boundary layer receptivity mechanism, in which three-dimensional disturbances in the free-stream continuously force streaks inside the boundary layer, may explain differences between experimental results and previously suggested theories for the origin of streaks in boundary layers subjected to free-stream turbulence.