HR: 17:00h
AN: V24B-05    [Abstracts]
TI: An Experimental Investigation of the Role of Solid Particles on the Collapse of Explosive Volcanic Plumes
AU: * Carazzo, G
EM: carazzo@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, 4 place Jussieu, Paris, 75005, France
AU: Kaminski, E
EM: kaminski@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, 4 place Jussieu, Paris, 75005, France
AU: Tait, S
EM: tait@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, 4 place Jussieu, Paris, 75005, France
AB: Pyroclastic density currents generated by the collapse of an explosive volcanic plume represent the most dangerous flows associated with such eruptions. The study of the mechanical processes leading to column collapse is therefore at the heart of current investigations. Fluid dynamic models show that the behavior of a volcanic jet is mainly controlled by the efficiency with which it entrains and heats atmospheric air. The volcanic mixture initially denser than the atmosphere can thus become buoyant if both processes are effective. The complex role of the particle load and heat exchange makes it difficult to study their effect on the jet dynamics other than by sophisticated numerical simulations. Nevertheless to develop an alternative approach, we present an experimental study in which a turbulent 2-phase jet of hot gas and hot particles is propelled into a large chamber of cold air. The jet is initially driven by momentum and naturally collapses, but if the mixing with the surrounding environment is sufficient the buoyancy can reverse to drive a convective plume. We focus on the influence of source particle concentration and source gas velocity on the threshold between the convective and the collapsing regimes. In the range of the source conditions investigated the jet mostly separated into a po sitively buoyant part and a denser collapsing part. We quantify the fraction of the jet collapsed by collecting the particles and we show that the degree of jet collapse is mainly controlled by the initial amount of particles. A 1D model of turbulent jets accounting for the effect of the reversing buoyancy on the turbulent entrainment, the aggregation, the sedimentation and the recycling of particles is presented. The model is found in good agreement with the data. Further work is necessary to understand the fundamental physics behind the semi-empirical parametrization of re-entrainment and aggregation processes.
DE: 8428 Explosive volcanism
DE: 8445 Experimental volcanism
DE: 8488 Volcanic hazards and risks
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting