Experiments on the diffraction of strong blast waves

Abstract

This paper considers the diffraction of strong shocks over rigid concave comers, and complements an earlier paper (Henderson & Siegenthaler 1980), which dealt with weak shocks. It is shown that the von Neumann theory of strong Mach reflexion does not agree with experiment once the comer signal everywhere overtakes the reflected shock. We show that the difficulty is due to the assumption that the flow is self-similar along the trajectory path passing through the comer and that the theory may be reconstructed by choosing a new path that does not necessarily pass through the comer. The flow is assumed to be self-similar with respect to the new path. The reconstructed theory is in good agreement with experiment. One obtains from it a new model of strong Mach reflexion beyond the catch-up condition that features a length scale apparently introduced into the flow by viscous effects at the comer. The reflected shock is weaker according to the new theory which implies that the blast loading on sloping surfaces will be less after catch-up than predicted by the classical theory. Experimental evidence is also presented on transition between regular and Mach reflexions, and it supports the normal shock criterion first proposed by von Neumann but largely ignored by the textbooks.