Hypersonic Boundary Layer Transition over Blunt Cones

Numerical and experimental studies have demonstrated laminar-turbulent transition in hypersonic boundary layers over sharp cones via the modal growth of planar Mack-mode instabilities. However, due to the strong reduction in Mack-mode growth at higher nose bluntness values, the mechanisms underlaying the observed onset of transition over the cone frustum upstream of the entropy-layer swallowing location are not fully understood yet. As the nose bluntness is further increased, and beyond the critical nose Reynolds number for transition reversal, the transition location rapidly moves upstream, and transition appears to depend on uncontrolled disturbances due to nose-tip roughness. For the "moderately-large bluntness" regime, linear nonmodal analysis shows that both planar and oblique traveling disturbances that peak within the entropy layer experience appreciable energy amplification. However, due to their weak signature within the boundary-layer region, the route to transition onset via nonmodal growth of traveling disturbances is investigated by nonlinear parabolized stability equations and direct numerical simulations. For modest amplitudes of the inflow disturbance, the oblique-mode breakdown is shown to lead to transition at the measured location of transition onset during the experiments at the AFRL Mach-6 high Reynolds number facility. On the other hand, the nonlinear development of the traveling waves along the entropy layer leads to the formation of inclined structures just above the boundary-layer edge and these structures are strongly reminiscent of the transitional events observed during blunt cone experiments by using schlieren flow visualizations.