Flow through a perforated surface due to shock-wave impact

Abstract

The factor which is of prime importance in influencing the shock reflection geometry, and resulting pressures, following impingement of a shock wave on a porous surface is the velocity of the flow into the surface. A set of experiments has been conducted, using holographic inferometry in a shock tube, on the impingement of a shock wave on a surface covered with slits, over the full range of shock incidence angles from 0 to 90°. Inverse shock pressure ratios of 0.4, 0.5 and 0.7 were used, and detailed characterization of the flow fields determined. A number of methods are used to infer the inflow into the surface, and measurements are also conducted on the downstream side of the slit plate in order to establish the pressure ratio across the plate. The tests include choking of the flow through the slits. Shock reflection angles are found to be depressed compared to reflection from an impervious wall for cases of regular reflection, but are similar in the case of Mach reflection with the incident wave near glancing incidence. Contrary to assumptions made in previous work it is shown that for wall angles from zero up to approximately 60° the inflow to the plate is inclined to the surface at about 17° and then tends to straighten out until, for normal shock reflection, the flow is also normal to the plate. It appears that this behaviour is linked to the separation of the flow at the inlet to the pores of the model, due to shock wave diffraction. The maximum value of the absolute inflow velocity occurs in the region of transition from regular to Mach reflection. A series of starting vortices is shed on the underside of the slit and is found to follow a path nearly normal to the plate. These vortices lie along a contact surface whose motion is compatible with the strength of the shock wave transmitted through the plate.