Premature secondary instabilities induced by freestream turbulence in swept flat plate boundary layer

Laminar-to-turbulent transition in the swept flat plate boundary layer is the result of the crossflow vortex collapse due to high-frequency secondary instabilities. The transition process is complicatedly influenced by disturbances induced by both wall roughness and freestream turbulence. We investigated the turbulent transition process caused by the interaction between unsteady fluctuations induced by free-stream turbulence and stationary vortices generated by roughness elements in a swept flat plate boundary layer, by means of direct numerical simulation. We found that short-wavelength freestream turbulence promotes turbulent transition without the involvement of CFV growth but rather by enhancing hairpin vortices as a premature high-frequency secondary instability. We determined the lower roughness height that induces early transition through hairpin vortices. Additionally, we discovered that roughness heights below this threshold lead to the occurrence of type-3 secondary instability.