The reflection of weak shock waves from absorbent surfaces

Previous models of the transient behavior of air within the cavities of a layer of porous material on a solid backplate dealt only with the influence of viscous resistance to motion within the porosities. The short timescales implicit in the reflection of a weak blast wave from such a layer require the inclusion of inertia effects together with those of viscous resistance. The first part of the present study is devoted to a calculation of the velocity of inflow to the porous layer, from which all other quantities, such as reflected shock strength, can be calculated. The model assumes that the porous material consists of an aggregate of long thin tubes, of different and irregular lengths; the porous layer is attached to a solid backplate. The results of this analysis are compared with experimental observations that were made specifically to test the theory, by using a small shock tube that had its low-pressure end closed by a solid-backed porous plug of polyurethane foam.