Volcanic ash aggregation in eruptive plumes

We present the result of a recent experimental and numerical investigation of ash aggregation in volcanic plumes. Volcanic models are currently based on the large scale dynamics of an eruption while this work is intended to improve on our understanding of how microphysical processes impact ash aggregation. Our approach is twofold, comparing the results of a laboratory experiment with those of a numerical simulation meant to mimic the experimental setup. The physical experiment was carried out in a contained tank designed to allow for the control of ‘atmospheric' conditions. The tank can be depressurized as needed, using the gas inlet and the attached vacuum pump, and the ambient humidity can be altered by adjusting the gas mixture at the inlet. Using image data collected by a high speed camera, the interactions between sub-millimeter size particles being expelled from a jet, representative of ash in an eruption, are recorded. The images are then analyzed using both the particle image velocimetry (PIV) technique and a probability distribution of aggregation potential that we have developed based on environmental conditions, which can be used to develop a subgrid model for mesoscale dynamics. We also validate an Eulerian- Lagrangian model through comparison with this experiment. We will present the results of aggregation probability and the effects of incorporating these results into a multiphase model, where the effects of ash aggregation are especially important in regions of high shear and high granular temperature.