Bluntness Effects on Perturbation Growth in Hypersonic Ogive-Cylinder Boundary Layers

Accurate prediction of laminar-to-turbulent flow transition is challenging because of its complex nature. In this study, we use a combination of linear stability theory (LST) and direct numerical simulations (DNSs) to examine perturbation growth on ogive-cylinder forebodies, relevant to modern hypersonic vehicles, focusing on the effect of nose bluntness. The spatial evolution of disturbances is first examined through the solution of a linear eigenvalue problem. As the ogive nose bluntness is increased, the first mode, which was unstable downstream for the sharp nose, moves upstream, and its oblique component displays higher instability. The LST predictions are used to inform DNS to examine the effect of amplitude on perturbation growth and modal characteristics. For small 2D perturbations, second-mode growth with DNS yields similar results as LST for all bluntness cases examined, suggesting that the streamwise gradients inherent to ogive forebodies have relatively little effect on this behavior. When the forcing is azimuthally localized, the oblique nature associated with low-frequency perturbations becomes apparent, analogous to the first mode. These are, however, inhibited when the forcing is azimuthally coherent. Furthermore, at higher amplitudes, nonlinear interactions appear among unstable modes, providing insights into the different interactions governing transition in such nose geometries.