A domino effect in delayed subharmonic transition in wall-cooled high-subsonic boundary layer

We examine our direct numerical simulation data of a subharmonic transition in high-subsonic boundary layers on pseudo-adiabatic, heated, and cooled walls. The employed wall heating and cooling increase and decrease the local growth rate of the unstable Mack's first mode by about 10%, respectively. As a result, the wall cooling significantly delays the transition. A chain-like nonlinear mechanism (as we call the domino effect) is clearly identified in the cooled case, which causes the delayed transition. First, the decay of the first mode by wall cooling alleviates the secondary subharmonic instability. The relatively weak but unstable subharmonic modes create a laminar streak through their nonlinear interactions, and this laminar streak also suppresses the first mode's growth as in streak control. Consequently, the first mode becomes subdominant in the nonlinear stage, leading to different dominant nonlinear mode interactions compared to other temperature cases and delaying the turbulent breakdown. In this way, the turbulent transition is delayed in the cooled case in a domino-like fashion. This domino effect also suggests a new potential approach to transition control that balances effectiveness and energy efficiency by devising appropriate wall cooling spots.