Experiments on the Gas Dynamics of the Mt. St. Helens 1980 Lateral Blast

Abstract

Field evidence suggests that the lateral blast in the 1980 Mt. St. Helens eruption behaved like an underexpanded jet flow. We conduct two experiments to investigate this hypothesis. In our first experiment, we use the compressible flow--shallow-water analogy to measure the geometry of the shock structure around the underexpanded jet, which is comparable with the position of the interface between the direct and channelized blast zones described by Kieffer (1981). Also, Kieffer and Sturtevant (1988) identified furrows created by the blast which were possibly formed by scouring due to Goertler vortices induced by curvature in the terrain. In our second experiment, carried out in a compressible flow laboratory, we investigate an additional Goertler vortex generation mechanism due to the curvature of the shear layer adjacent to the intercepting shocks in the underexpanded jet. These experiments allow for a more-detailed scrutiny of the underexpanded jet--lateral blast analogy proposed by Kieffer (1981).

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