Fluid Dynamics of Pyroclastic Density Currents
Georgia Institute of Technology
Pyroclastic density currents (PDCs) are the most dangerous local hazard during volcanic eruptions. PDCs are ground-hugging gravity currents composed of hot gases, pyroclasts (fragments of magmatic material), and rocks entrained from the substrate. The danger and opaqueness of PDCs make it impossible to have in situ thermal or concentration measurements. In addition, laboratory experiments are not always able to scale to the complexity of the currents found in nature. The combination of high-resolution multiphase numerical models in concert with detailed field measurements of PDC deposits allows us to better understand the evolving concentration profiles and temperatures of PDCs. This ultimately leads to a better understanding of current dynamics and potential hazards. We employ a multiphase Eulerian-Eulerian-Lagrangian (EEL) model, high-resolution topography, and detailed field data to understand the 2006 eruption of the Tungurahua volcano that generated multiple PDCs. With this model, the flow transformation from a dense, granular flow regime to a dilute, turbulent regime is examined. The high-resolution model quantifies that a significant amount of air is entrained during the more turbulent phase of the flow through Kelvin-Helmholtz instabilities and that secondary ash plumes form above the current. The granular instabilities and the entrainment during the turbulent flow regime cause unsteady behavior that results in deposits with cross-stratification. Using the EEL model results, we subsequently model the development of low vesicular rinds on pyroclasts carried by the flow. This allows us to use pyroclasts deposited in the field as natural thermometers that help constrain the evolving current temperature. The model reveals that as the current temperature decreases due to air entrainment, the rinds on pyroclasts progressively increase in thickness. Through these models, we are able to look inside these currents that are otherwise inaccessible and interpret the deposits found in the field, leading to a better understanding of the eruptive process.