Computational and Experimental Studies of Blockage Effects in a Blast Simulator

The confinement of a target by the walls of a blast simulator produces changes in the loading experienced by the target. The changes increase in magnitude with the blockage ratio, the ratio of target area to the simulator cross-sectional area. Computations were made using the HULL hydrocode in an axisymmetric configuration for non-decaying waves for different blockage ratios for a cylindrical target. It was found that the flow in the constricted region between the target and the shock tube wall was typically accelerated to velocities greater than the steady flow values behind the incident shock in an unobstructed shock tube. The net axial load on the target computed from the hydrocode results could be related directly to the dynamic pressure in the constricted region, which increased as the blockage was increased. This meant that non-decaying shocks which did not produce net loads sufficient to overturn a target in a shock tube at low blockage could be made to do so by increasing blockage while keeping shock overpressure constant. Thus, tests where the blockage is high can be misleading unless corrections are made for the effect of blockage. The step shock computations provided an extreme case for analysis; similar computations were performed for the other extreme case of a rapidly decaying wave. Although critical flow parameters in the constricted region (e.g. , particle velocity and dynamic pressure) were similarly increased for the decaying wave, the effect on the net force was found to be small. This was because of the overpressure gradient across the target and the reduced importance of drag loading compared to diffraction phase loading.