Shock wave interactions with aqueous foams

Weaver, Paul Michael (1991) Shock wave interactions with aqueous foams. University of Southampton, Doctoral Thesis.

Abstract

Aqueous foams are noted for their ability to attenuate and absorb energetic disturbances caused by processes such as explosions or rocket and gun firing which generate hazardous high temperature high velocity flows with shock waves. A critical review of the literature is presented which revealed that while the potential of aqueous foams in this respect has been noted little is known of the mechanisms of the interactions of shock waves with aqueous foams. The objectives of the present study were therefore to acquire a data base for the study of wave interactions with aqueous foams and to explore the physical mechanisms governing the observed phenomena. Experiments are reported that examine the interactions of shock waves initially in air (incident shock Mach numbers 1.3-2.4) with aqueous foams (density 5-90kh m^-3) using pressure measurements and a novel optical technique for tracking the downstream foam/air boundary. The study concentrated on the following areas:1. Experimental data were obtained, for the first time, on the effects of shock refraction at air/foam and foam/air boundaries. Results showed that a compression wave was reflected at the upstream air/foam boundary causing an increase of pressure within the foam by a factor of 1.5-3. A rarefaction wave was reflected at the foam/air boundary resulting in a shock propagating into the downstream air with up to 30% reduction in pressure over the foam-free case. This was accompanied by a typically 4-fold increase in kinetic energy flux in the downstream region compared to the foam-free case. These effects could be explained by modelling the foam as an equivalent ideal gas and reasonable quantitative agreement was obtained.2. Wave propagation and structure in aqueous foams. Double fronted waves were observed for wave pressure ratios > 3 and foam densities > 20kg m^-3. Data are presented on wave and sound speeds and the state of the compressed foam. Equivalent ideal and Clausius gas models provide no explanation for double fronted waves. Phase change of the water in the foam was considered to be unlikely to have a significant effect over the range of conditions studied. An anomalous Hugoniot function and staged structural response of the foam are considered as possible explanations. The results also showed evidence of strong non-1-dimensionality in the flow caused by foam drainage and significant unsteadiness. Strong (up to 100bar) pressure transients were observed near the bottom of the shock tube, thought to be caused by bubble oscillation.3. Reflection at an end wall. Reflected wave pressures were more than double those attained in air, but not as high as predicted by equivalent gas models. It is suggested that this is because subsequent wave interactions and long pressure rise times in the foam prevent the attainment of equilibrium. Novel data are presented on the 3-dimensional structure of the reflecting wave. The wave front appears to be non-planar with a significant inclination (typically 50^o) to the vertical caused by drainage of the foam under gravity.

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