On the significance of absolute and convective instabilities in transitional hypersonic compression ramp flows

In real-life tunnel or flight environments, the laminar–turbulent transition of compression ramp flows can be initiated by absolute and/or convective instabilities. Currently, the relative significance of the two instabilities in such flows remains unclear. This paper addresses this issue by decoupling the effects of absolute and convective instabilities, which is an extension of our previous research (Huang et al. Phys. Fluids, vol. 37, 2025, p. 034110). Hypersonic flows over a compression ramp with a 15 Superscript ring ∘ $^\circ$ ramp angle at Mach number 8.0 and Reynolds number italic Re Subscript upper L Baseline equals 9.2 times 10 Superscript 5 Re L = 9.2 × 10 5 ${{\textit{Re}}_{L}}=9.2\times {{10}^{5}}$ based on the flat plate length are investigated using direct numerical simulation (DNS) and several stability analysis tools. Global stability analysis (GSA) reports that the flow is intrinsically unstable. Without external disturbances, DNS shows a good agreement with GSA regarding the growth rate and the disturbance shape. The flow in DNS is dominated by the most amplified stationary mode and the low-frequency oscillatory mode. However, transition in this case cannot be accomplished in the computational domain purely by the absolute instability. With external forcing, the coexistence of absolute and convective instabilities triggers the transition. For the forcing amplitudes examined in this study, which are representative of moderately to strongly noisy wind tunnel environments, the absolute instability is not significant in affecting the transition onset location. Nonetheless, the interplay between absolute and convective instabilities can profoundly alter the dominant spanwise wavelengths, the pronounced frequency band, the shape of coherent structures, etc. In summary, the interplay between the two types of instabilities recasts the transition scenario, albeit the transition onset location is dominated by the convective instability.