Blast wave attenuation using liquid sheets

Abstract

Strong blast waves, such as those associated with gun fire or rocket exhausts, can cause serious physiological and/or structural damage. It is necessary, therefore, to develop ways in which to minimise this damage whilst still allowing the system that produces the blast waves to function normally. In an effort to develop such a system this study has examined, both theoretically and experimentally, the interaction that occurs when a shock wave is passed along a free unconstrained liquid tube before emerging into the surrounding atmosphere. In the theoretical analysis the problem was confined to the two-dimensional case and involved dividing the liquid sheet up into infinitesimal sections that were then regarded as small piston-cylinder systems, which were driven by the high pressure shocked gas behind the shock wave. The results from these one-dimensional systems were then used as input into a two-dimensional solution of the wave equation which predicted the gross changes in the remainder of the space. The experimental investigation involved laboratory experiments that examined, visually and with pressure transducers, the result of the shock/liquid interactions in two-dimensional and axi-symmetric cases, both between and external to the liquid sheets. The experimental investigation also included field trials that examined the pressure profiles of blast waves that were produced when a shock wave, which resulted from the ignition of a rocket motor, was passed along a liquid tube. From this work it was found that the high pressure gas, behind the shock wave, caused the liquid sheets to move perpendicularly from the line of travel of the shock wave which in turn caused expansion waves and compression waves to propagate out from the face of the water sheets, into the shocked gas and the surrounding atmosphere, respectively. These compression waves were then found to interact with the weak blast (produced when the shock, which had been weakened by the expansion waves, emerged into the atmosphere) in such a way that they produced a weaker blast field than would have been the case had the shock wave emerged directly into the atmosphere; the maximum observed reduction in the strength of the blast wave was 16.4 dB. Experiments were also performed that examined the effect of using rigid sheets in place of liquid sheets. From these experiments it was found that the differences between the liquid and rigid sheet cases was a function of the size of the inertial barrier (i.e. the mass of the water sheet) that the water presented to the shocked gas. Consequently, it was noted that, in terms of attenuating the blast wave, the rigid sheets proved to be inferior to the thicker water sheets and superior to the thinner water sheets. However, when the spectra of the pressure disturbances were examined it was found that, with regard to the attenuation of the 2-4 kHz region of the spectra, all the liquid sheet results showed an improvement in relation to the rigid sheet results.

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