Streak transient growth (STG) and internal shear layers (ISL) in the outer regions of TBL: some new perspectives

The STG mechanism in a TBL, previously studied only near the wall (y⁺<60), is now extended to the outer region (y⁺>500) via DNS of a turbulent channel flow (Re_t=2000). This is found to be also the underlying mechanism of large-scale motions (LSM) in the outer layer of TBLs. LSM – typically about 3h to 6h long, where 2h is the channel height -- are characterized by uniform momentum zones, vortex clusters, hairpin head vortices, etc. Our study shows that these features are indeed captured in the outer layer STG. We show that the LSM appears due to the Kelvin-Helmholtz instability of the outer-layer ISL – inherently formed during the outer STG – and evolves independently of the near wall vortices. The outer-layer STG is distinguished by a length scale, topology, and dynamics different from those of the near-wall STG. The outer-layer ISL -- inherent to the outer layer STG -- rolls up, forming either localized vortex clusters or a series of approximately equispaced spanwise structures (hairpin heads). The ISL, along with these spanwise sub-structures, then undergoes a rapid transition (avalanche) to turbulence. We will discuss the more realistic scenario where the STG perturbation is concurrently applied to the near-wall and outer streaks and detail the dynamical interaction of these two evolving structures.