Influence of cavities on hypersonic boundary layer transition

Cavities such as installation gaps and high-temperature ablation stripping widely exist on the surfaces of aerospace vehicles. Their effects on the boundary-layer transition process are vital and thus need to be considered in the aircraft aerodynamic design. In order to explore the influence of cavities on the boundary-layer transition, this paper studies the evolutions of disturbance waves in both hypersonic (Ma = 5.92) and supersonic (Ma = 1.38) boundary layers passing through cavities of different sizes by using linear stability analysis and high-precision direct numerical simulations. The results show a strong recirculation zone inside the cavity, in which vortical structures become more complex with the increase of the cavity size. For hypersonic flows, when the cavity is located upstream of the synchronization points of the fast and slow modes, it promotes the development of disturbance waves. However, the opposite is true when the cavity is located near or downstream of the synchronization points. The suppression effect is the most significant for the cavity located at the synchronization point. Increasing the cavity size (width or depth) will strengthen its control effect on disturbance waves. For supersonic flows, the cavity always promotes disturbance growth in unstable regions, and the promotion effect increases with the increase of the cavity size. When the cavity size exceeds the critical value, the flow in the cavity can generate self-excited oscillations, which will introduce new disturbance sources to the downstream boundary layers and promote the transition. The results of this study are essential for aerodynamic design and thermal protection of aircraft surfaces.