EXPERIMENTAL INVESTIGATION OF THE INTERACTION BETWEEN WEAK SHOCK-WAVES AND GRANULAR LAYERS

The paper describes new experimental results regarding the pressure fi elds in front of and inside granular layers of different materials dur ing their collision with weak shock waves. A variety of waves result f rom the shock wave-granular layer interaction. The pressure behind the reflected wave from the material interface approaches the equilibrium value, P-5, which would have been reached had the shock wave reflecte d from a solid end-wall. The wave succession inside the layer depends solely on two processes: the complex interaction of the compaction wav e with the granular material and the gas filtration, which affects the particles by the drag forces between the two phases. Inside a materia l with a permeability coefficient f> 0.001 mm(2) the transmitted wave moves with a constant velocity which is largely governed by the gas fi ltration. For low permeability materials (f< 0.0003 mm(2)) the transmi tted wave trajectory strongly depends on the compaction wave propagati on. In such cases the compaction wave was found to be unsteady and its acceleration was higher in material having low material densities. Th e maximum compressive stress values, P-5, reached at the shock tube en d-wall, covered by the materials under investigation, manifested as an unsteady pressure peak twice as large as the gas pressure P-5, measur ed ahead of the layer. Comparing the present data with those available in the literature showed that the amplitude of the unsteady pressure peak was higher in materials having low effective densities, gamma, an d small permeability coefficients f. Contrary to flexible foams where the available experimental data indicated that the compressive stress in the post peak period converges to P-5=P-g, the results obtained in the present study indicated that during the test time the compressive stress, P-5, was well preserved in the material and for most of the sa mple length its value was within the range P-s>P-5>P-g.