Regeneration mechanisms of oblique and streamwise vortices in oblique transition

Abstract

Oblique transition can be triggered by introducing componentwise optimal perturbations into a plane Couette flow. It is shown that in the middle stage of the transition, oblique vortices are generated again through transient spanwise velocity energy growth in the presence of developed streamwise streaks. In the following stage, streamwise independent kinetic energies of cross-streamwise velocities are intensified due to the nonlinear contribution caused by amplified oblique vortices together with the pressure transport, and the increased cross-stream velocity fluctuations can strengthen the lift-up effect. As a result, the streamwise vortices are formed again, and the streamwise streaks are even enhanced prior to their final breakdown to turbulence.