Turbulent/non-turbulent interface in hypersonic boundary layers

The turbulent/non-turbulent interface (TNTI) plays a critical role in turbulent entrainment. While substantial progress has been achieved in understanding TNTI and entrainment in low Mach number flows, little attention is paid to TNTI and entrainment in hypersonic flows, especially in hypersonic boundary layers. To this end, direct numerical simulation of hypersonic boundary layers has been performed to investigate the impact of shock waves on TNTI and turbulent entrainment by comparing two cases with and without the shock wave/boundary layer interactions (SBLI). A novel approach, combining vorticity threshold and fuzzy clustering techniques with local normalization by boundary-layer thickness and friction velocity, is developed to improve TNTI detection by reducing shock-induced artifacts. Results show that SBLI significantly reduces TNTI thickness, intensifies enstrophy generation and dissipation, and alters the relative contributions of dominant transport terms. Analysis of entrainment mechanisms reveals that shock waves moderately enhance small-scale nibbling while weakening its dependence on interface curvature, but predominantly amplify large-scale engulfment, which accounts for over 90% of total entrainment mass flux under SBLI conditions. These findings provide new insights into TNTI structure and entrainment dynamics in hypersonic boundary layers subject to strong shock–turbulence interactions.