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New developments in the analysis of volcanic pyroclastic density currents through numerical simulations of multiphase flows

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Author(s): S. Lepore | C. Scarpati

Journal: Solid Earth Discussions
ISSN 1869-9537

Volume: 4;
Issue: 1;
Start page: 173;
Date: 2012;
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ABSTRACT
A granular multiphase model has been used to evaluate the action of differently sized particles on the dynamics of fountains and associated pyroclastic density currents. The model takes into account the overall disequilibrium conditions between a gas phase and several solid phases, each characterized by its own physical properties. The dynamics of the granular flows has been simulated by adopting a Reynolds Average Navier-Stokes model for describing the turbulence effects. Numerical simulations have been carried out by using different values for the eruptive column temperature at the vent, solid particles frictional concentration, turbulent kinetic energy, and dissipation. The results obtained underline the importance of the multiphase nature of the model and characterize several disequilibrium effects. The low concentration (≤ 5 · 10–4) sectors lie in the upper part of the granular flow, above the fountain, and above the pyroclastic current tail and body as thermal plumes. The high concentration sectors, on the contrary, form the fountain and remain along the ground of the granular flow. Hence, pyroclastic density currents are assimilated to granular flows constituted by a low concentration suspension flowing above a high concentration basal layer (boundary layer), from the proximal regions to the distal ones. Interactions among solid, differently sized particles in the boundary layer of the granular flow are controlled by collisions between particles, whereas particles dispersal in the suspension is determined by the dragging of the gas phase. The simulations describe well the dynamics of a tractive boundary layer leading to the formation of stratified facies during eruptions having a different magnitude.
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