Numerical Investigations of Hypersonic Boundary Layer Stability for a Blunt Flat Delta Wing

Stability calculations were carried out to investigate second-mode and crossflow instabilities for a flat plate with a delta shape planform (“Delta Wing”) at several Mach numbers (6.5, 7.3, and 8). The sweep angle of the (blunt) leading edge of the Delta Wing is 75 degrees and the angle of attack considered is minus seven degrees. Results from primary instability investigations using Linear Stability Theory (LST) indicate the existence of both second-mode and crossflow-mode instabilities for the investigated geometry and flow conditions. The LST calculations were performed with respect to both the second-mode and crossflow instabilities in an attempt to assess which instability mechanisms may be dominant for the conditions of planned wind tunnel experiments. Both the baseflow and the LST stability characteristics of the Delta Wing were compared in detail with the so-called Flat-BOLT geometry (Wernz [1]). The Flat-BOLT was found to be suitable surrogate model of the original BOLT geometry in the sense that both baseflow and the stability characteristics (and thus the resulting transition predictions) were very well captured by the Flat-BOLT geometry. Comparison of the baseflow and the stability characteristics of the Delta Wing with the analogous Flat-BOLT results have shown very good agreement except for the region near the symmetry plane of the respective geometries. These differences are likely caused by the low sweep angle leading edge of the BOLT and Flat-BOLT which leads to a lower edge-Mach number development along the centerline compared to the Delta Wing. Therefore, the Delta Wing can also be considered as a direct surrogate geometry for investigating stability and transition in the outboard regions of the original BOLT geometry, albeit at somewhat lower fidelity (constant sweep angle) than the Flat-BOLT. The Delta Wing surrogate was subsequently used to assess if transition delay using “roughness bumps” for attenuating second-mode disturbances (Duan et al. [2]) is feasible for the Flat-BOLT geometry. Therefore, the synchronization locations of the fast and slow acoustic modes were also calculated for the conditions of the planned CUBRC wind tunnel experiments. For this transition delay strategy, the roughness bumps would have to be placed downstream of the synchronization locations.