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Center for Nonlinear Studies |
Pyroclastic density currents (PDCs) are hazardous multiphase gravity currents that form during volcanic eruptions when erupted mixtures of gas and particles are denser than the ambient atmosphere. Their flow characteristics span a variety of regimes, from dense granular flows to dilute turbulent suspensions, and can occur over a wide range of length and time scales. Direct measurement of active PDCs is difficult so our best methods of studying their dynamics are numerical modeling and experiments. In this talk, I will discuss a type of PDC formed when falling gas-particle mixtures impact the ground, analogous to an impinging jet, and our efforts to use multifield numerical modeling to study them. The transition from vertical to lateral flow is complex due to the range of coupling between particles of different sizes and densities and the carrier gas, and due to the potential for compressible flow phenomena. We characterize the behavior of the multiphase currents using a new formulation of the Stokes number, the “impact” Stokes number, which considers the deceleration length scale of the impacting mixture, which is itself sensitive to the mixture sound speed. The dominant characteristics that control the type of downstream flow are whether the impacting jet is mono- or polydisperse and whether the jet is sub- or supersonic. I will discuss some potential geologic implications of the results and address how the modeling might be further validated by future experiments.
Host: Jeffrey Hyman