Numerical simulation of collapsing volcanic columns with particles of two sizes

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Abstract
A three‐phase thermofluid‐dynamic model was employed to simulate the behavior of collapsing volcanic columns and related pyroclastic flows. The model accounts for the mechanical and thermal nonequilibrium between a gas phase and two solid phases representative of particles of two different sizes. The gas phase has two components: hot water vapor leaving the vent and atmospheric air. Collisions between particles of the same size were accounted by a solids elasticity modulus, whereas a semiempirical correlation was employed to account for particle‐particle interactions between particles of different sizes. The gas phase turbulence was modeled by a turbulent subgrid scale model. The partial differential equations of conservation of mass, momentum, and energy were solved numerically, by a finite difference scheme, on an axisymmetric physical domain for different granulometric compositions at the vent. Simulations were limited to particles of few hundreds microns, and therefore to dilute flows, in order to mantain a reasonable computational load. Results show the formation of the initial vertical jet, column collapse, building of a pyroclastic fountain followed by the generation of a radially spreading pyroclastic flow, and the development of convective instabilities from the upper layer of the flow which lead to the formation of coignimbritic or phoenix clouds. The analysis of the spatial and temporal distributions of the two solid phases in the different parts of the domain shows nonequilibrium effects between them and allow us to quantify important emplacement processes as pyroclast sedimentation and ash dispersion. In particular, the importance of coupling effects between the two solid phases leads to relevant differences between the behavior of columns with one or two solid phases. A significant influence of the granulometric composition was observed on the pyroclastic flow runout, flow thickness, and particle distribution in the flow and phoenix cloud. The results from simulations appear to be qualitatively in agreement with simple laboratory experiments and field observations.