Oblique shock reflection has been well investigated when the reflecting wedge is solid. In that case, the acoustic impedance is extremely high. However, the acoustic impedance of elastic materials is not so high that some inherent reflection characteristics can be expected. It is reported that spreading water over the floor of a gunpowder magazine greatly mitigates the pressure rise due to explosion. The cooling effect of water cannot affect the whole gas phase instantaneously, so the cause of pressure mitigation should be sought in the acoustic impedance of the reflecting surface.
In the present investigation, soft materials, such as rubber and silicone sealant, are used for the reflecting surface. We conducted experiments using the conventional shock tube in our institute. The working gas was air, and the driven section was set to room temperature and atmospheric pressure for each experiment run. The models are ordinary smooth wedges, on which a smooth plate of soft material with some thickness d is attached. The incident shock Mach number Mi is 1.30, and the reflecting wedge angle θw varies from 20 to 40 degrees.
A shadowgraphy method was employed to visualize the reflection configuration. The light source was a xenon flash lamp with a 180-ns pulse width. The flash lamp was triggered by the output of the pressure gauge nearest the test section via a digital delay circuit. The reflection configuration at any desired instant could be photographed by regulating the delay time.
The angle between the incident and reflected waves ωir and the coordinates of the triple point (x, y) are measured directly from photographic negatives enlarged by a factor of 50 using a profile projector. We compare these results with those for hard reflecting surfaces, to investigate the effect of acoustic impedance on oblique shock reflection.