Nonmodal traveling disturbances within the entropy layer of hypersonic boundary layers

Laminar-turbulent transition in boundary layers is a critical factor in the design of hypersonic vehicles, because of its impact on overall aerodynamic and heat transfer characteristics. Numerical and experimental studies have demonstrated that the modal growth of planar Mack modes is responsible for laminar-turbulent transition on sharp cones. However, the physical mechanisms that lead to transition onset within the swallowing distance of the entropy layer on sufficiently blunt cones are not well understood as yet. Because modal amplification is too weak to initiate transition at large bluntness values, transient growth has been investigated as the potential basis for a physics-based model of the transition reversal phenomenon. The present computations examine disturbance growth over both a variable bluntness, 7-degree cone that was tested in the AFRL Mach-6 high-Reynolds-number facility and a cone-eogive-cylinder configuration that was employed during experiments in the Boeing/AFOSR Mach-6 Quiet Tunnel at Purdue University. Nonmodal instability results are compared with the Purdue data to address the nature of the traveling disturbances measured outside the boundary layer. The implications of these disturbances for transition onset on these configurations are also discussed.