Flow of supersonic jets across flat plates: Implications for ground‐level flow from volcanic blasts

Abstract

We report on laboratory experiments examining the interaction of a jet from an overpressurized reservoir with a canonical ground surface to simulate lateral blasts at volcanoes such as the 1980 blast at Mount St. Helens. These benchmark experiments test the application of supersonic jet models to simulate the flow of volcanic jets over a lateral topography. The internal shock structure of the free jet is modified such that the Mach disk shock is elevated above the surface. In elevation view, the width of the shock is reduced in comparison with a free jet, while in map view the dimensions are comparable. The distance of the Mach disk shock from the vent is in good agreement with free jet data and can be predicted with existing theory. The internal shock structures can interact with and penetrate the boundary layer. In the shock‐boundary layer interaction, an oblique shock foot is present in the schlieren images and a distinctive ground signature is evident in surface measurements. The location of the oblique shock foot and the surface demarcation are closely correlated with the Mach disk shock location during reservoir depletion, and therefore, estimates of a ground signature in a zone devastated by a blast can be based on the calculated shock location from free jet theory. These experiments, combined with scaling arguments, suggest that the imprint of the Mach disk shock on the ground should be within the range of 4–9 km at Mount St. Helens depending on assumed reservoir pressure and vent dimensions.

Researching the Earth—and a Few of Its Neighbors

“…Grad students Mara Orescanin and David Prisco performed studies that showed that for typical vulcanian eruptions and the Mount St. Helens blast, steady flow conditions would have had time to be established . Orescanin et al (2014) demonstrated that internal shock structures within a jet can interact with boundary layers to leave a footprint on the ground that mimics the free jet structure (Orescanin et al 2014). As of this writing the volcanic jet studies continue with Professor Austin's group, now at Caltech, and I am once again back at work on my Old Faithful data and high-speed flow processes.…”

“…When scaled up to the dimensions of Mount St. Helens, the model suggested that the supersonic zone extended roughly 11 km to the north (Kieffer 1981a,b). We later revised this distance to 4-9 km by expanding consideration of different possible vent shapes as suggested by Donnadieu & Merle (2001) and pressure ratios (Orescanin et al , 2014. This brought the simple model into reasonable agreement with a detailed supercomputer model that suggests a supersonic zone inside 6 km (Ongaro et al 2011(Ongaro et al , 2012.…”

“…Aside from the need to map out highly dangerous proximal zones near a volcano (e.g., Fisher 1990, Calder et al 2000, Rudolph et al 2011, Scarpati & Perrotta 2012, there are, by analogy to small-scale erosional processes, obvious implications for civil safety (e.g., Doronzo 2013). Although scaling from laboratory experiments is difficult, small-scale experiments can shed light on processes Orescanin et al , 2014. Taddeucci et al (2015) have, for example, used the simple gas experiments of Kieffer & Sturtevant (1984) to interpret gas-pyroclast behavior in unsteady flow in volcanic jets.…”

A model problem for a supersonic gas jet from a moon

“…Another examination of the phenomena exhibited by the Mt St Helens eruption was provided by Orescanin et al (2008). This was an experimental simulation of a supersonic jet impinging on a plane surface.…”

The contribution of experimental volcanology to the study of the physics of eruptive processes, and related scaling issues: A review

“…Notice that the lower part of the Mach disk shock is reduced compared to a free jet. FromOrescanin et al (2014).…”