Attachment-line turbulent boundary layer over a swept leading edge at Mach 6: Simulation and flow physics

Direct numerical simulations (DNS) are performed to investigate the spatial evolution of a three-dimensional (3-D) turbulent boundary layer over a swept blunt leading edge at freestream Mach number upper M Subscript normal infinity Baseline equals 6.0 M ∞ = 6.0 $M_\infty =6.0$ and unit Reynolds number 1.8 times 10 Superscript 7 Baseline m Superscript negative 1 1.8 × 10 7 m − 1 $1.8 \times 10^7\,\text{m}^{-1}$ . Unlike canonical configurations, this flow evolves continuously from a quasi-two-dimensional equilibrium state at the attachment line to a highly 3-D, non-equilibrium state downstream under strong geometry-induced pressure gradients. The near-wall turbulent streaks exhibit a pronounced structural lag, characterised by an ordered angular response of the streak orientation relative to the external streamline and the wall shear direction. This lag is accompanied by a progressive stress depletion: the Townsend structure parameter collapses below its equilibrium value, and the turbulent kinetic energy budget reveals a global production deficit, indicating a progressive chordwise tendency towards relaminarisation. Spectral analysis further shows that this relaminarisation tendency is scale-selective – low-wavenumber motions are strongly suppressed within the resolved DNS domain, whereas highly organised, skewed near-wall streaks survive. The temperature field remains structurally locked to these residual momentum streaks throughout the process, with the strong Reynolds analogy holding robustly, while wall-normal and lateral thermal transport are effectively extinguished. The dataset exposes fundamental limitations of linear eddy-viscosity closures in this regime and provides a benchmark for developing advanced models capable of capturing such strongly non-equilibrium 3-D boundary layers.