Numerical Modeling of Gravitational Instabilities at the Base of Volcanic Plumes
Abstract
Many field observations have shown how volcanic ash typically sediments in the form of particle clusters and/or gravitational instabilities. Both processes increase particle settling velocity and promote ash fall closer to the volcano than expected. In particular, gravitational instabilities during particle sedimentation cause the boundary layer between volcanic clouds and atmosphere to become unstable and trigger the formation of downward-moving fingers. We simulate this process using a 3D hybrid Lattice Boltzmann-Finite Difference (LB-FD) model where particles are coupled with the fluid. The first step is a single-phase model where particles are modelled as a continuum field which evolves according to an advection-diffusion (AD) equation coupled with the fluid Navier-Stokes momentum equation through a buoyant body force term (Boussinesq approximation). Additionally, assuming that the particle drag force is in equilibrium with the gravitational force, the velocity field in the AD equation represents the fluid velocity corrected by the particle settling velocity. Within this two-way coupling scheme, we solve the AD equation using an upwind finite difference method and the Navier-Stokes equation with a LB model. The initial configuration consists of a buoyant ash layer overlaying a denser ambient. As the ash settles, the boundary layer between the ash layer and the ambient migrates downwards during the simulation. Our 3D LB-FD model, tested against dedicated laboratory experiments, is used to explore various initial conditions including particle size distribution, particle concentration and scale.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V23G0293L
- Keywords:
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- 4314 Mathematical and computer modeling;
- NATURAL HAZARDS;
- 8414 Eruption mechanisms and flow emplacement;
- VOLCANOLOGY;
- 8428 Explosive volcanism;
- VOLCANOLOGY;
- 8445 Experimental volcanism;
- VOLCANOLOGY