Shock wave propagation through wet particulate foam

Abstract

Among the materials, which are capable of dissipating or absorbing the blast energy, aqueous foams is a good solution, when the place and the time of the blast are known in advance. If the height and the density of foam barrier are sufficient, the blast energy transmitted to nearby objects and personnel could be reduced to harmless levels. Unfortunately, a relationship between these two parameters (height and density) and the optimal protection effect is not specified yet. The reason is that the mechanisms responsible for blast attenuation in foam are interrelated and the contribution of each mechanism is not clear. Actually blast waves can destroy the foam barrier, which in turn is unstable and sensitive to the pre-blast decay. Little has been done even to evaluate the conditions when these mechanisms become important or/and to estimate their consequences. Fortunately, since the foam shattering occurs within milliseconds, while the pre-blast decay occurs in the order of minutes and even hours, these two processes could be treated independently, assuming that the protective action depends on: (a) The shock wave conditions: intensity, profile and duration of the pressure pulse. (b) The foam conditions: formulation, preparation method and the aging time, Δ t, before the blast exposure. Following our strategy to simplify the problem we recently studied the mitigation of weak shock waves having a blast shaped pressure profile [5]. Low intensity and short duration of the impact as well as the freshly prepared foam samples that were used in this study precluded the bubble shattering and/or foam non-homogeneous caused by the decay. In the present study, which is a continuation of previous tests [3–5] we have focused on particles-related phenomena, which modify the foam stability and thus may cause indirect effect on the propagation history of the shock wave. To separate these factors, we initially have put our concern on the free decay of particulate foams and thereafter the shock wave mitigation. In both cases, as the main parameters controlling the problem, we changed the aging time before the impact, Δ t i , and the particles mass fraction or loading factor, n 0.