The Rankine–Hugoniot relation for shock waves in very porous media

Abstract

AN entropy rise accompanies shock wave propagation in any medium. The entropy jump at a shock front is especially high when a medium contains a large number of pores—small empty holes randomly distributed. Shock compression of a porous medium leads to intense heating of the material surrounding the pores because of its nonelastic deformation. Intense heating of a porous material at a shock front yields an ‘anomalous’ shape for the Rankine–Hugoniot curve on the p–V plane^1,2. This effect of porosity was clearly seen in our experiments with cast foamed polystyrene of various initial densities, ρ₀. The solid lines in Fig. 1 represent Rankine–Hugoniot curves obtained experimentally. It can be seen that the specific volume of compressed material is much greater than the volume of polystyrene in the condensed (solid or liquid) state. It is clear that such a great increase in volume is possible only when initially condensed porous material turns into the gaseous state, and cannot be explained in terms of thermal expansion.