Multiparticle simulation of collapsing volcanic columns and pyroclastic flow

Abstract

[1] A multiparticle thermofluid dynamic model was developed to assess the effect of a range of particle size on the transient two-dimensional behavior of collapsing columns and associated pyroclastic flows. The model accounts for full mechanical and thermal nonequilibrium conditions between a continuous gas phase and N solid particulate phases, each characterized by specific physical parameters and properties. The dynamics of the process were simulated by adopting a large eddy simulation approach able to resolve the large-scale features of the flow and by parametrizing the subgrid gas turbulence. Viscous and interphase effects were expressed in terms of Newtonian stress tensors and gasparticle and particle-particle coefficients, respectively. Numerical simulations were carried out by using different grain-size distributions of the mixture at the vent, constitutive equations, and numerical resolutions. Dispersal dynamics describe the formation of the vertical jet, the column collapse and the building of the pyroclastic fountain, the generation of radially spreading pyroclastic flows, and the development of thermal convective instabilities from the fountain and the flow. The results highlight the importance of the multiparticle formulation of the model and describe several mechanical and thermal nonequilibrium effects. Finer particles tend to follow the hot ascending gas, mainly in the phoenix column and, secondarily, in the convective plume above the fountain. Coarser particles tend to segregate mainly along the ground both in the proximal area close to the crater rim because of the recycling of material from the fountain and in the distal area, because of the loss of radial momentum. As a result, pyroclastic flows were described as formed by a dilute fine-rich suspension current overlying a dense underflow rich in coarse particles from the proximal region of the flow. Nonequilibrium effects between particles of different sizes appear to be controlled by particle-particle collisions in the basal layer of the flow, whereas particle dispersal in the suspension current and ascending plumes is determined by the gas-particle drag. Simulations performed with a different grain-size distribution at the vent indicate that a fine-grained mixture produces a thicker and more mobile current, a larger runout distance, and a greater elutriated mass than the coarse-grained mixture.

A three‐dimensional numerical simulation of spreading umbrella clouds

“…[5] Over the past 20 years, the development of multidimensional (2-D and/or 3-D), time-dependent fluid dynamics models enables us to calculate the dynamics of eruption clouds for given eruption conditions at the vent without empirical constants like those in the simplified models [e.g., Wohletz et al, 1984;Valentine and Wohletz, 1989;Dobran et al, 1993;Neri and Dobran, 1994;Neri and Macedonio, 1996;Neri et al, 1998Neri et al, , 2002Neri et al, , 2003aNeri et al, , 2003bNeri et al, , 2007Oberhuber et al, 1998;Clarke et al, 2002;Dartevelle et al, 2004;Ishimine, 2005;Suzuki et al, 2005;Textor et al, 2005Textor et al, , 2006. These multidimensional fluid dynamics models, however, also have difficulties to quantitatively reproduce the dynamics of eruption clouds.…”

Great challenges in volcanology: how does the volcano factory work?

“…composition and rheology and the underlying topography, or to define the modalities of development and propagation of pyroclastic density currents, or even the dispersion and fall of tephra (e.g., Neri et al, 2003;Webley et al, 2012;Roche et al, 2013). Modeling has been playing a key role in unraveling these processes, even though the need to introduce more realistic constraints is clear, especially for the most complicated emplacement mechanisms.…”

Epistemic uncertainties and natural hazard risk assessment. 1. A review of different natural hazard areas

“…The model solves the fundamental multiphase flow transport equation of Neri et al (2003) and provides a means to describe the complexities of the column collapse and the temporal and spatial evolution of 15 flows over the whole 3D topography of the volcano. However, the full ranges of plausible volcanic and other physical input parameter variations are not amenable to comprehensive exploration in a restricted number of scenario runs, which are limited by computing power and cost.…”