Nose Bluntness Effects on the Amplification of External Disturbances in Hypersonic Flows

Experiments and computations have extensively studied hypersonic boundary layer transition over sharp and blunt cones. Transition on sharp and small bluntness cones is dominated by modal growth of planar waves. As bluntness increases, an entropy layer develops that stabilizes Mack modes and pushes the transition front downstream. Experiments show that beyond a critical nose bluntness, downstream movement of the transition front reverses and despite being modally stable the transition front moves up to the nose tip. Despite many experimental and numerical investigations, the transition reversal phenomenon has not been clearly linked to a physical mechanism. In this study, direct numerical simulation (DNS) and input-output (IO) are utilized to study the effects of nose bluntness on the amplification of external disturbances. DNS is performed on low dissipation baseflows that are forced stochastically with freestream noise and wall roughness. Spectral proper orthogonal decomposition is used on snapshots of the statistically steady state DNS solution to isolate globally dominant resolvent modes. An input-output framework is used to study the optimal flow response to forcing in the linear regime. Results from both the DNS and IO identify low frequency streak-like structures that grow significantly on the nose tip.These structures are shown to exist on both intermediate bluntness (1.524 mm radius) cones and large bluntness (15.24 mm radius) cones, but grow 2 orders of magnitude more in the large bluntness case. The structures are extremely receptive to roughness. The low frequency streak-like structures identified in this work behave in accordance with experimental observations of transition reversal, potentially providing a physical mechanism to explain the phenomenon.