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Experimental study of the formation of dilute or concentrated pyroclastic currents in the impact zone of eruptive fountains

Baptiste Penlou1, Olivier Roche1, Siet van den Wildenberg1,2

  • Affiliations: 1Laboratoire Magmas et Volcans, Université Clermont Auvergne-CNRS-IRD, Clermont-Ferrand, France 2Laboratoire de Physique de Clermont, Université Clermont Auvergne-CNRS, Clermont-Ferrand, France 

  • Presentation type: Poster

  • Presentation time: Friday 16:30 - 18:00, Room Poster Hall

  • Poster Board Number: 172

  • Programme No: 3.9.10

  • Theme 3 > Session 9

Abstract

The conditions that control the nature of pyroclastic density currents (i.e., fully dilute and turbulent or with a concentrated base) that form in the impact zone of a collapsing fountain remain poorly understood. We address this question through laboratory experiments to better understand the degree of air-particle coupling during the impact of a dilute mixture on a solid surface. We released quasi monodisperse glass beads (mean sizes: 29 - 269 µm) from a hopper placed at a height of 1.63 or 3.27 m above a 5 m-long horizontal channel. Grids were installed in the hopper to create highly dilute air-particle mixtures. The granular mixtures accelerated and diluted further as they dropped from the hopper. We made precise measurements of the particle concentrations and fall velocities of the mixtures by acoustic measurements and high-speed imaging, respectively. We found that the particle concentration in the impact zone of the channel varied from 15 to 0.3 vol.%, depending on the particle size, the grid, and the fall height. We show that, depending on the particle size, concentration, and fall velocity, the mixtures either remained dilute, with particles coupled to the air, or transformed into concentrated flows. The Stokes number (St) is a key parameter in characterizing air-particle coupling in the impact zone. We determine a scaling law between St and the particle concentration of the mixtures that characterizes the transition between dilute or concentrated regimes. Finally, we discuss our results in the framework of recent numerical simulations and their applicability to natural systems.