Convective Instabilities in Hypersonic Compression Ramp Flows

This study investigates the Mach-6 compression ramp flows at a unit Reynolds number of Formula: see text in a Ludwieg tube tunnel, with ramp angles ranging from 4 to 10 deg. Global stability analysis indicates that global instability is absent in all cases. Resolvent analysis identifies three convective instability modes: Mack’s first mode, second mode, and a streak mode, each exhibiting distinct features across different cases. A combination of schlieren visualization, Rayleigh scattering photography, surface pressure measurements, and infrared thermography is employed to reveal the convective instability characteristics and the associated laminar-to-turbulent boundary-layer transition mechanisms. It is found that the transition point significantly shifts upstream when flow separation occurs at the ramp corner for ramp angles exceeding 4 deg. The shear-layer mode and Mack’s second mode are measured by pressure sensors. The spatial evolution of detailed flow structures within the reattached boundary layer, such as staggered wavelike structures, Formula: see text vortices, and streamwise streaks, is visualized. Analysis of the evolution and breakdown of these structures reveals that the nonlinear evolution of the first mode predominantly drives boundary-layer transition in the 4-deg-ramp case, referred to as the oblique transition. Görtler instability is significantly amplified near the separation and reattachment regions, with its strength increasing with the ramp angle. Consequently, it plays a more critical role in the transition process at higher ramp angles and dominates in the 10-deg-ramp case. For moderate ramp angles, both the first mode and Görtler instability are pronounced in the transition process.