Small-amplitude Oscillations in Hypersonic Double-cone Flow

Abstract

Unsteady compressible flows typically pose problems with rich dynamics. The broad concern of this thesis is the shock wave unsteadiness that arises in external high-speed flow over a double-cone body. This unsteadiness is driven by complex interaction between the shock wave and the region of high shear and separation in the external flow. It is well known that the canonical double-cone problem exhibits two different classes of unsteadiness, labeled “pulsations” and “oscillations.” The former is characterized by unsteady shock wave motion over large spatial scales, whereas in the latter the nature of unsteadiness is distinct and occurs at a relatively smaller scale. The detailed mechanisms that sustain pulsations and oscillations are yet to be completely understood. In the present work, experiments were performed in the Roddam Narasimha Hypersonic Wind Tunnel (RNHWT) at Mach 6 to carefully investigate the phenomena of oscillations. Time-resolved schlieren and wall pressure data were obtained for various double-cone models with the second cone angle fixed at 90◦, while the first cone angle and ratio of the slant lengths were varied as parameters. Schlieren data revealed two dissimilar types, or modes, of flow oscillations. Spectral proper orthogonal decomposition (SPOD) analysis performed on experimental data showed the existence of a dominant time scale for the oscillations, and also provided the associated low-rank dynamics. The two different oscillation modes are found to exhibit distinct Strouhal number scaling. Given the direct dependence of shock strength on the flow Mach number (M ), the boundaries of unsteady flow states are expected to show slight changes with M. However, qualitative flow features and the underlying mechanisms that drive unsteadiness are expected to remain the same. Overall, this work reveals new flow behavior and furthers our understanding of the double-cone flow.