NUMERICAL-SIMULATION OF COLLAPSING VOLCANIC COLUMNS WITH PARTICLES OF2 SIZES

A three-phase thermofluid-dynamic model was employed to simulate the b ehavior of collapsing volcanic columns and related pyroclastic flows. The model accounts for the mechanical and thermal nonequilibrium betwe en a gas phase and two solid phases representative of particles of two different sizes. The gas phase has two components: hot water vapor le aving the vent and atmospheric air. Collisions between particles of th e same size were accounted by a solids elasticity modulus, whereas a s emiempirical correlation was employed to account for particle-particle interactions between particles of different sizes. The gas phase turb ulence was modeled by a turbulent subgrid scale model. The partial dif ferential equations of conservation of mass, momentum, and energy were solved numerically, by a finite difference scheme, on ar; axisymmetri c physical domain for different granulometric compositions at the vent . Simulations were limited to particles of few hundreds microns, and t herefore to dilute flows, in order to mantain a reasonable computation al load. Results show the formation of the initial vertical jet, colum n collapse, building of a pyroclastic fountain followed by the generat ion of a radially spreading pyroclastic flow, and the development of c onvective instabilities from the upper layer of the how 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 diff erent parts of the domain shows nonequilibrium effects between them an d allow us to quantify important emplacement processes as pyroclast se dimentation and ash dispersion. In particular, the importance of coupl ing effects between the two solid phases leads to relevant differences between the behavior of columns with one or two solid phases. A signi ficant influence of the granulometric composition was observed on the pyroclastic flow runout, flow thickness, and particle distribution in the how and phoenix cloud. The results from simulations appear to be q ualitatively in agreement with simple laboratory experiments and field observations.