New developments in the analysis of column-collapse pyroclastic density currents through numerical simulations of multiphase flows

摘要

A granular multiphase model has been used to evaluate the action ofdifferently sized particles on the dynamics of fountains and associatedpyroclastic density currents. The model takes into account the overalldisequilibrium conditions between a gas phase and several solid phases, eachcharacterized by its own physical properties. The dynamics of the granularflows (fountains and pyroclastic density currents) has been simulated byadopting a Reynolds-averaged Navier-Stokes model for describing theturbulence effects. Numerical simulations have been carried out by usingdifferent values for the eruptive column temperature at the vent, solidparticle frictional concentration, turbulent kinetic energy, anddissipation. The results obtained provide evidence of the multiphase nature ofthe model and describe several disequilibrium effects. The low concentration(≤5 × 10−4) zones lie in the upper part of the granularflow, above the fountain, and above the tail and body of pyroclastic densitycurrent as thermal plumes. The high concentration zones, on the contrary,lie in the fountain and at the base of the current. Hence, pyroclasticdensity currents are assimilated to granular flows constituted by a lowconcentration suspension flowing above a high concentration basal layer(boundary layer), from the proximal regions to the distal ones. Interactionsamong the solid particles in the boundary layer of the granular flow arecontrolled by collisions between particles, whereas the dispersal ofparticles in the suspension is determined by the dragging of the gas phase.The simulations describe well the dynamics of a tractive boundary layerleading to the formation of stratified facies during Strombolian to Plinianeruptions.