Water Mitigation of Explosion Effects. Part 1: The Dynamic Pressure from Partially Confined Spaces

Abstract : This work is a contribution to an ongoing co-operative research project between FOA in Sweden and TNO-PML in the Netherlands, aimed at investigating the physics of water mitigation and to formulate instructions and standards of how to use water barriers. FOA has focused on the scalemodeling laws and to investigate if the mitigation is affected when the charges are cased. The technique of damping blast from high explosives with "water barriers" of different types has recently been studied by some research institutes and consultants. Examples on practical applications are to reduce the most severe hazard area around an ammunition storage, on equipment for manufacturing or the destruction of ammunition etc. However, the results reported in this field indicate that the mitigation effect from water is not yet fully understood. Although it has been demonstrated to work well in many tests in small scale, unexpected results have occurred when tested in full scale. This paper contains a somewhat closer look on the blast mitigation effect from water for geometries similar to a duct attached to a confined space, e.g. an access tunnel in to an ammunition storage. Of special interest is then the dynamic pressure inside the access tunnel, caused by the shock and quasi-static pressure from an explosion in the storage chamber, as well as the jet formed outside the tunnel entrance. An effort is made to explain some results from scale model experiments in terms of elementary thermodynamics and shock wave theory. This is illustrated by a set of numerical simulations with the hydrocode AUTODYN using its two phase material model for water. The calculated results are compared with the experiments, and it is concluded that the numerical model to some extent describes the phenomena involved.