Bypass Transition in Three-dimensional Time-dependent Boundary Layers

The influence of spanwise wall oscillation on transition in boundary layers exposed to free-stream turbulence is examined. Results from direct numerical simulations show that for moderate amplitudes of the wall motion, transition to turbulence can be consider- ably delayed. On the other hand, high-amplitude forcing leads to breakdown to turbulence upstream of the unforced reference flow configuration. Flow visualizations and statistical results show that the stabilization of the flow at the optimal forcing parameters is the result of a substantial weakening of the low-frequency boundary layer streaks. On the other hand, the early breakdown at high forcing amplitudes is associated with the amplification of new high-frequency instabilities in the pre-transitional flow.