Lagrangian modeling of large volcanic particles: Application to Vulcanian explosions

Abstract

[1] A new 2D/3D Lagrangian particle model (named LPAC) for the dynamics of clasts ejected during explosive eruptions is presented. The novelty of the model lies in the one-way coupling of the carrier flow field, given by a Eulerian multiphase flow code, and the particles. The model is based on a simplification of the Basset-Boussinesq-Oseen equation, expressing the Lagrangian equation of a particle as the sum of the forces exerted on it along its trajectory. It is assumed that particles are non-interacting and do not affect the background carrier flow and that the drag coefficient is constant. The model was applied to large clasts produced by Vulcanian explosions, in particular those occurring in August 1997 at Soufrière Hills Volcano, Montserrat (West Indies, UK). Simulation results allowed parametric studies as well as semi-quantitative comparisons between modeling results and field evidence. Major results include (1) the carrier flow was found to play a fundamental role even for meter-sized particles-a 1 m diameter block is predicted to reach a distance that is about 70% greater than that predicted without the effect of the carrier flow (assuming the same initial velocity), (2) assumption of the initial velocity of the particle was dropped thanks to the description of both the acceleration and deceleration phases along the particle trajectory, (3) by adopting experimentally based drag coefficients, large particles were able to reach greater distances with respect to smaller particles consistently with field observations and (4) the initial depth of the particle in the conduit was found to mainly influence the ejection velocity while the initial radial position with respect to the conduit axis was found to play a major role on the distance reached by the particle.

Effect of particle volume fraction on the settling velocity of volcanic ash particles: insights from joint experimental and numerical simulations

“…Most numerical models of ash dispersal adopt a one-way coupling approach1314151617181920, an assumption justified in the dilute regions of the plume where ϕ p is less than 10 −6  31, in agreement with our above statements. However, depending on plume height, ϕ p may be 10 −5 –10 −6 in regions within a few hundreds of kilometres and up to 10 −3 more proximally293031.…”

“…This approach considers the settling of an individual particle in a still fluid, ignoring the presence of neighbouring particles. Similarly, the effect of particle volume fraction ( ϕ p ) on the settling velocity of individual (i.e., not aggregated) particles is not included in most numerical models of ash dispersal1314151617181920 which are mostly concerned with the fate of volcanic ash in the dilute, medium-distal regions of the plume. These models assume particles are only affected by the drag due to the local velocity of the carrier flow (one-way coupling), ignoring the effect of particle motion on the flow itself (two-way coupling) and inter-particle collisions (four-way coupling).…”

Phreatic eruptions at crater lakes: occurrence statistics and probabilistic hazard forecast

“…More sophisticated fluid dynamic models of the conduit acceleration phase take into account the effect of the carrier flow field on the eruptive mixture more accurately (e.g. de'Michieli Vitturi et al 2010), but this is beyond the scope of this study.…”

Maximum horizontal range of volcanic ballistic projectiles ejected during explosive eruptions at Santorini caldera

“…One such limitation concerns values corresponding to the extent of the reduced drag zone, that have been chosen mostly based on empirical criteria (namely observations from other volcanoes and the vent diameters found on the Kameni islands). More accurate estimates could be obtained by performing modeling of the multiphase carrier flow using a methodology similar to that of de' Michieli Vitturi et al (2010). Another limitation has to do with VBP densities as well as drag coefficients that could be estimated directly from VBP samples from Santorini rather than assuming an average density and drag coefficients for ideal geometrical shapes like cubes.…”