A complex thermo-fluid dynamic model was employed to model collapsing volcanic columns. The two-phase flow model accounts for the mechanical and thermal nonequilibrium between the gas and solid particles. The gas phase involves water vapor and air, and the solids phase involves only one particle size class. The particle collisions, which produce particle viscosity and pressure, were modeled by a kinetic theory model in terms of the granular temperature, whereas the gas phase turbulence was modeled by a turbulent subgrid scale model. The partial differential equations of conservation of mass, linear momentum, energy, and granular temperature were numerically solved for an axisymmetric flow configuration with different vent diameters and two-phase flow conditions. The numerical solutions involved different grid sizes and computational domains in order to assess the adequacy of the model and computational procedure. The results from simulations of collapsing volcanic columns show how after an initial period of fountain building the columns collapse and build radially spreading pyroclastic flows and inward moving column material which is recycled by the columns. For a low-height collapsing column it was found that the fountain reaches a steady state height, whereas for columns with collapsing heights of several kilometers and fine computational grids the fountain heights vary cyclically with periods which are influenced by the dynamics of material recirculation into the columns. The radially spreading pyroclastic flows of the collapsed columns were found to develop convective instabilities whereby rising clouds of gas and particles are developed on the top of the flows. In very large scale volcanic eruptions the numerical results predicted multiple rising clouds on very thick pyroclastic flows. The results from simulations were shown to be consistent with simple column modeling approaches, laboratory experiments, and field observations.

No abstract

A complex two‐phase flow model was employed to study the influence of eruption parameters on the thermofluid dynamics of collapsing volcanic columns. The physical model accounts for thermal and mechanical nonequilibrium between the gas and solid phases and for mixing of water vapor leaving the vent with the pressure‐ and temperature‐stratified atmosphere. The effects of particle collisions and pressure were modeled by a kinetic theory of granular flow. Depending on the eruption, the simulations were performed from several to tens of minutes after the beginning of the eruptions. Initial conditions for simulations involved different vent diameters, exit velocities, and temperatures of the two phases, particle diameters, and water vapor contents of the flow mixture. After an initial period of fountain building, characterized by column collapse and formation of pyroclastic flows and material recycling from the collapsed column into the fountain, the simulated columns developed different structures, producing stationary or oscillatory behaviors which depend on the geometry and physical parameters of the gas and pyroclasts at the vent. Such behaviors of the collapsed columns may strongly influence the dynamics of pyroclastic flows, causing variations of the flow properties and strong pulsations of their mass flow rates. The application of spectral analysis to timewise distributions of some properties at different points in the fountains and pyroclastic flows permitted identification of the main frequencies associated with the nonstationary column phenomena. The columns at the transition between the collapsing and plinian behaviors developed characteristic suspended flows that spread radially at the fountain height level for several kilometers from the vent and their heads intermittently collapsed due to the changing atmospheric dynamics above and below the suspended flows. The results from computer simulations of collapsing volcanic columns are consistent with previous simple and complex modeling approaches, laboratory experiments, and field observations. The results also show that the thermofluid dynamics of collapsing volcanic columns may produce very complex depositional structures in pyroclastic flows and should therefore provide important clues in the interpretations of field deposits associated with explosive volcanic eruptions.

No abstract