Effects of Nose Bluntness on Hypersonic Transition over Ogive-Cylinder Forebodies

Abstract

Ogive cylinders are representative designs of hypersonic forebodies and harbor several flow complexities. This work evaluates the effects of nose bluntness of ogive-cylinder forebodies on their laminar, transitional, and turbulent boundary layers (BLs). Laminar simulations indicate that the thickness of BL and entropy layer (EL) increases with nose bluntness, with the latter displaying a stronger sensitivity to nose radius due to the increased strength and curvature of the leading-edge shock system. Linear perturbation analysis identifies the presence of the second mode, the first mode, and EL modes with increasing bluntness. While the former two are prevalent over a significant streamwise extent, EL modes are limited to the upstream region, well within the swallowing length. Using direct numerical simulations, it is found that transition over the sharpest nose design occurs through the modal fundamental resonance route, driven by second-mode instabilities. Due to weakened BL modes, the blunter geometries undergo transition further downstream in the low-amplitude freestream perturbation environment tested in this study. The transition mechanism in the blunter geometries is intermittent in nature and appears to be driven by turbulent spots generated from streamwise vortices reminiscent of Klebanoff modes.