Abstract
A laminar boundary layer separates in a region of adverse pressure gradient on a flat plate and undergoes transition. The detached shear layer rolls up into spanwise vortices that rapidly break down into small-scale turbulence. Finally, the turbulent boundary layer reattaches, forming a laminar separation bubble. Development and role of three-dimensional disturbances for transition in such a separation bubble are studied by means of direct numerical simulation with controlled disturbance input. In the present case, the level of incoming three-dimensional perturbations is not relevant due to an absolute secondary instability of these disturbances in the region of convective two-dimensional shear layer rollup. In particular, this is true for steady perturbations up to moderate amplitudes. Following their generation by nonlinear interaction of disturbance waves in the region of favorable pressure gradient, these steady disturbances develop as streaks. Their downstream evolution can be first attributed to transient behavior, depending on initial excitation, followed by a universal state with characteristics of a modal instability. Numerical results are confirmed by a comparison with experimental data.