Open cavities with different door-opening angles are investigated using high-speed schlieren visualization and dynamic pressure measurements in hypersonic flow with a freestream Mach number of 6. With the help of numerical simulations, the shear layer deformation and pressure increase in the cavities due to the impingement of the door-leading-edge shocks are identified via comparison with those in the cavity without doors. As the door-opening angle decreases from 90°, the shear layer above the forepart of the cavity is gradually raised by the high pressure in the cavity. When the door-opening angle decreases to 30° and 15°, the boundary layer on the upstream flat plate of the cavity separates, and separation shock is observed. The doors enhance the instability of the cavity flow and increase the pressure fluctuations in the cavities. A new oscillation pattern, referred to as coupled oscillation, is observed in the cases with separation on the upstream flat plate, in which the separation shock oscillates at the same dominant frequency as the flow inside the cavity. Compared with the cavity without doors, this coupled oscillation causes a lower oscillation frequency and a larger overall sound pressure level. Cross-correlation analyses between pressure signals indicate that the disturbances generated at the trailing edge of the cavity can propagate to the separation on the upstream flat plate and cause coupled oscillation of the separation shock. The fundamental frequencies of the coupled oscillations can be normalized to approximately the same Strouhal number as that of the cavity without doors. These findings support that the oscillation mechanisms of hypersonic cavities without and with doors are primarily dominated by acoustic feedback.

The Mach reflection in axisymmetric flows, which is compressed by an internal cone, is investigated by using a combination of numerical simulations and theoretical analyses at a free-stream Mach number of 6. The effects of the leading-edge angle and cone length on the convergence behaviour of incident shocks and the flow downstream of Mach disk are examined. The results show that the leading-edge angle determines the point of von Neumann strength of the convergent incident shock, and also the location of the Mach disk. In addition to the shock convergence, the position of Mach disk is influenced by the downstream flow as well. According to whether the sonic throat of the stream tube downstream of the Mach disk is affected by the expansion waves generated from the trailing edge of the wall or not, two types of flow patterns downstream of the Mach disk can be classified depending on the leading-edge angle and the cone length. When the sonic throat is independent of the expansion waves, the pressure upstream of the sonic throat is balanced by the post-shock pressure of the reflected shock, and the position of Mach disk will not shift even if the cone length changes. On the other hand, when the sonic throat depends on the expansion waves, the stream tube upstream of the sonic throat needs to match the change of the pressure resulting from expansion waves. The position of Mach disk can be theoretically predicted based on the matching relations between the flow downstream of reflected shocks and the flow downstream of Mach disk.