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It has recently been established that the porosity of surfaces can have a significant effect on the reflection geometry of shock waves, and thus on the loads that are generated. This paper describes a comprehensive series of tests on the patterns of shock wave reflection from a surface covered with a series of slits, over the full range of angles of incidence from glancing to normal. The use of double-pulse interferometry is shown to be ideally suited to the study of complex compressible flow fields of this type, not only because of the high resolution but also because the tracking of fringes gives a very clear indication of both the general flow field as well as the fine structure, and thus helps clarify the mechanisms whereby the interaction process is modified from the case of reflection off a plane impervious wall. These features of the method allow a number of effects to be established which have not previously been evident, or are in conflict with the assumptions of previous studies. Specifically it is shown that the inflow angle relative to the plate is almost constant (at about 17 degrees) for shock incidence angles from zero to about 50 degrees and that the flow leaves the plate almost normal to the surface; although there is a slight drift in a direction opposite to that of the shock induced flow. Furthermore it is shown that many of the flow variables, and specifically the inflow velocity, exhibit a maximum in the vicinity of transition from regular to Mach reflection. An analysis of the motion of the acoustic waves generated from the lips of the perforations allows estimates to be made of the constancy of the inflow along the surface. These waves are convected with the flow and as they do not meet the wall at a right angle show that the convection is towards the wall. Using this method it is found that the inflow is constant behind the reflected wave in regular reflection but variable for the case of Mach reflection. Photographs of the region under the plate clearly show how the wavelets emerging from the perforations coalesce to form a transmitted wave. Measurements of this wave enable the pressure below the plate to be assessed and thus to obtain an estimate of the pressure drop across the plate. A striking feature on the underside of the plate is that the contact surface separating flow that was initially above the plate from that engulfed by the transmitted shock is made up of a string of vortices arising from the wave diffraction into the perforations.
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