Effect of free-stream turbulence properties on different transition routes for a zero-pressure gradient boundary layer

The natural and bypass routes to boundary-layer transition to turbulence are traditionally investigated independently in fluid mechanics applications. Nevertheless, in certain flow regimes both mechanisms could coexist and interact. In this work, large-eddy simulations (LES) were performed for a zero-pressure gradient boundary layer developing over a flat plate to investigate the transition mechanism for variable free-stream turbulent properties. Four different combinations of turbulence intensity and integral length scale were analyzed, and two main transition mechanisms were observed. High free-stream turbulence intensity instigates pure bypass transition through the amplification of a continuous Orr-Sommerfeld (O-S) mode that breaks down after secondary instability. Instead, at low free-stream turbulence intensity, discrete and continuous O-S modes interact and are both involved in the transition process. Visual inspection of the LES snapshots provides a detailed insight in Tollmien-Schlichting (TS) waves-streaks mutual interaction and clearly identifies two main mechanisms involved in turbulence breakdown. On one hand, TS waves trigger varicose instability of streaky structures. On the other hand, streaks cause secondary instability of TS waves with emerging Lambda-structure formation. Then, dynamic mode decomposition (DMD) is applied to extract the main stability properties for both types of transition route and to highlight coherent structure dynamics, which is hardly observable in the literature. Specifically, for low-medium free-stream turbulence levels, DMD extracts unstable modes clearly related to streaks-TS waves interaction and leading to the formation of Lambda structures. Therefore, the streaks-TS waves interaction is proved to be destabilizing and to trigger secondary instability leading to turbulence breakdown. Published under an exclusive license by AIP Publishing