Hypersonic Boundary-Layer Instabilities over Ogive-Cylinder Models

Computational investigations of a tangent ogive-cylinder geometry with varying forebodies at zero degrees angle of attack are presented. The model geometry and conditions are selected to match experiments conducted in the Air Force Research Laboratory (AFRL) Mach 6 Ludwieg Tube. Five forebodies of interest were selected for the computational studies herein: two sharp ogives, two blunt ogives, and one hemispherical forebody. Computations are performed at a freestream unit Reynolds number of 7.01x10^6 m-1. Each sharp and blunt tip ogive had a 14 and 28 degree version for the tip angles providing a 4 caliber and 2 caliber ogive forebody, respectively. A cylindrical section follows the forebody, resulting in a meter long model matching the experiments. The laminar flow solutions are analyzed. The boundary layer-edge properties and the velocity and temperature profiles are compared across streamwise locations aft of the ogive-cylinder junction. The blunt forebodies induce an entropy layer that envelopes the boundary-layer profiles. Modal stability analysis identifies most amplified frequencies corresponding to Mack’s second modes that agree with experimental results for the sharp forebodies. Similar to the experimental measurements based on wall-mounted pressure sensors, no unstable modes are found for the blunt models. Nonmodal analysis revealed a broadband set of disturbances present for the blunter forebodies, in agreement with experimental observations. Flow perturbation contours of most amplified planar and oblique disturbances are shown to qualitatively match wind tunnel schlieren images, with a switch from rope-like to elongated structures, i.e., from high frequency Mack’s second modes to low frequency Mack’s first modes, as the forebody angle is increased for the sharp tip, and from boundary-layer to entropy-layer disturbances as the bluntness is increased.