This paper concentrates on establishing the three-dimensional flow geometry associated with studies of shock wave reflection between two symmetrical wedges in supersonic flow. It considers the issue of hysteresis in such flows, and draws a distinction between three different aspects of hysteresis, associated with: ideal two-dimensional flow, flow with noise, and three-dimensional effects. The three-dimensional nature of the flow field is elucidated by the use of oblique shadowgraph photography where the optical axis of the shadowgraph system passes at an oblique angle, of as much as 55°, through the test section. The traces of the wave system reflecting off the tunnel window are identified and are used to assist in identification of wave profiles. The nature of the approach of the peripheral Mach reflections collapsing towards the centre of the flow becomes evident, as does the mechanism of transition from Mach reflection to regular reflection. Distinct evidence of the effects of flow perturbations at the mechanical equilibrium transition point are presented, as are changes in the rate of growth of the Mach stem near this point.

It is shown that three-dimensional effects can have a major effect on the wedge angle for transition. In the present tests, at Mach 3.1 and a wedge aspect ratio of 0.5, this occurs at a wedge angle of about 5° higher than the theoretical maximum for the corresponding two-dimensional flow, where the dual solution domain extends over only two degrees.