Impact of gas injection on instability and transition in hypersonic boundary layers

To model pyrolysis gas injection induced by surface ablation, this study adopts surface gas injection with various species and elucidates its effects on hypersonic boundary-layer instability and transition using a combined numerical and asymptotic framework. Results demonstrate that the influence of injection on Mack-mode instability is strongly dependent on the injection molar mass ratio, particularly for the second mode. This effect exhibits a distinct frequency dependence: gas injection destabilizes the second mode at frequencies below a critical threshold, whereas it exerts a stabilizing influence above this threshold. This behavior persists across all injected species; however, the destabilizing influence is markedly weaker for lighter gases, such as helium. Further asymptotic analysis reveals that for low injection rates, the critical frequency remains invariant across different injection species and closely aligns with the most unstable frequency of the second mode. Using the e–N transition prediction method, we observe that the N-factor curve undergoes a sharp increase when the dominant perturbation switches from the first mode to the second mode. Injection of lighter species delays this modal switching, thereby postponing the onset of transition to turbulence. Collectively, these findings underscore a fundamental physics advance: by tuning the injection molar mass, one can selectively amplify or suppress the second mode, thereby offering a physically grounded strategy to control hypersonic transition.