  The attachment-line boundary layer is critical in hypersonic flows because of
its significant impact on heat transfer and aerodynamic performance. In this
study, high-fidelity numerical simulations are conducted to analyze the
subcritical roughness-induced laminar-turbulent transition at the leading-edge
attachment-line boundary layer of a blunt swept body under hypersonic
conditions. This simulation represents a significant advancement by
successfully reproducing the complete leading-edge contamination process
induced by surface roughness elements in a realistic configuration, thereby
providing previously unattainable insights. Two roughness elements of different
heights are examined. For the lower-height roughness element, additional
unsteady perturbations are required to trigger a transition in the wake,
suggesting that the flow field around the roughness element acts as a
disturbance amplifier for upstream perturbations. Conversely, a higher
roughness element can independently induce the transition. A low-frequency
absolute instability is detected behind the roughness, leading to the formation
of streaks. The secondary instabilities of these streaks are identified as the
direct cause of the final transition.
