DNS of Turbulent By-Pass Transition at the Entrance of a Plane Channel

Direct numerical simulations of boundary layers transition and their interaction in a very elongated plane channel are presented. Wall-normal velocity perturbations of small amplitude and with a spanwise wavelength of the order of the boundary layer thickness are introduced in one of the boundary layer. They generate steady elongated streaks by a transient growth mechanism. In the non-linear regime, a varicose instability mechanism pushes the low speed streaks away from the wall. Mushroom-shaped structures appear in the outer region of the laminar boundary layer. They are subjected to sinuous instabilities leading to transition. The turbulent boundary layer evolves further downstream and initiates the by-pass transition of the second boundary layer. An orthogonal decomposition of the velocity field is used to characterize the transition.