Numerical modeling of geophysical granular flows: 2. Computer simulations of plinian clouds and pyroclastic flows and surges

Abstract

Geophysical granular flows display complex nonlinear, nonuniform, and unsteady rheologies, depending on the volumetric grain concentration within the flow: kinetic, kinetic‐collisional, and frictional. To account for the whole spectrum of granular rheologies (and hence concentrations), we have used and further developed for geophysical‐atmospheric applications a multiphase computer model initially developed by U.S. Department of Energy laboratories: (Geophysical) Multiphase Flow with Interphase Exchange. As demonstrated in this manuscript, (G)MFIX can successfully simulate a large span of pyroclastic phenomena and related processes: plinian clouds, pyroclastic flows and surges, flow transformations, and depositional processes. Plinian cloud simulations agree well with the classical plume theory and historical eruptions in the upper altitude of the cloud (HT) versus mass flux diagram. At high mass flux (>107 kg/s), plinian clouds pulsate periodically with time because of the vertical propagations of acoustic‐gravity waves within the clouds. The lowest undercooled temperature anomalies measured within the upper part of the column can be as low as −18 K, which agrees well with El Chichón and Mt. St. Helens eruptions. Vertical and horizontal speed profiles within the plinian cloud compare well with those inferred from simple plume models and from umbrella experiments. Pyroclastic flow and surge simulations show that both end‐members are closely tight together; e.g., an initially diluted flow may generate a denser basal underflow, which will eventually outrun the expanded head of the flow. We further illustrate evidence of vertical and lateral flow transformation processes between diluted and concentrated flows, particularly laterally from a turbulent “maintained over time fluidized zone” near source. Our comprehensive granular rheological model and our simulations demonstrate that the main depositional process is mainly a progressive vertical aggradation.