Gas dynamic and physical behaviour of compressible porous foams struck by a weak shock wave

A detailed set of experiments has been conducted on the interaction of a Mach 1.4 shock wave with porous compressible foams of densities 32.5 and 35 kg/m3. It is shown that the velocity of the front face of the foam is approximately the same as the gas velocity behind the first reflected wave (55 m/s). A well defined wave propagating through the skeleton material at some 90 m/s is detected, whereas the leading edge of the gas disturbance moves at approximately 200 m/s. It is concluded that the frequently made assumption that the foam/air combination behaves as a homogeneous material is innapropriate, in that the existence of two distinct waves is established. The wave in the gas within the foam becomes less steep with time, as noted by other workers, and is consistent with a pressure drop resulting from the gas been forced through the foam skeleton. Furthermore it is shown that a contact surface emerges from the foam as it approaches the position of maximum compression, in contrast to the usual assumption that there is no flow across the interface. The motion of this surface follows that of the gas in front of the foam. A wave diagram is presented consistent with the above results.