Large-scale separation-induced secondary turbulence amplification in shock wave/turbulent boundary layer interaction

Turbulence amplification in shock wave/turbulent boundary layer interactions critically affects flow unsteadiness and wall loading in supersonic aerodynamic configurations. Direct numerical simulations (DNS) are performed at shock generator angles of β=8°, 10°, and 12°. At the two smaller angles, turbulence amplification is confined to the vicinity of the separation point. As β increases to 12°, however, a pronounced secondary amplification emerges near the reattachment point, giving rise to a bimodal streamwise distribution of turbulent kinetic energy (TKE). Spectral analysis of the TKE production terms reveals that this secondary amplification is predominantly driven by shear production associated with large-scale vortical structures. Global stability analysis (GSA) of the mean flow further demonstrates that all unstable eigenmodes are intrinsically tied to the separation bubble, and the most unstable spanwise wavenumber predicted by GSA agrees closely with the dominant scale observed in the DNS. Reduced-order variational mode decomposition identifies an energetically dominant, low-frequency three-dimensional mode in the reattachment region for the β=12° case, characterized by coherent Görtler-like vortices whose spanwise wavelength is consistent with both the DNS and GSA results. These results suggest that the secondary turbulence amplification originates from the modal instability of the mean flow associated with the large-scale separation bubble and the coherent structures it induces.