Air Circulation Within Beds of Hot Fragments and Mobility of Pyroclastic Flows

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Pyroclastic flows are among the deadliest of volcanic phenomena because of their high velocities, long run-out distances, high temperatures and large volumes. It has been suggested that their long run-out distances could be due to fluidisation. The fluidising gases are also probably responsible for the elutriation of fines and the generation of the giant co-ignimbrite plumes that override moving flows. These plumes produced the most historically significant injections of aerosol and ash into the stratosphere. Interestingly, only relatively small airflow velocities are necessary to fluidise volcanic ash. The goal of this project is to study with an experimental approach the fundamental processes and variables that affect airflow generation within beds of hot fragments that deform because of shear stresses and/or expand because of particle collisions. Of particular interest is whether ambient air is ingested by expanded beds of hot particles and whether this airflow can fluidise and/or elutriate the fines whose presence between larger rock fragments can affect the mobility of pyroclastic flows. The apparatus that will be used for the experiments produces scale models of pyroclastic flows whose temperature, interstitial fluid pressure and interstitial fluid velocity can be monitored using thermocouples, pressure transducers and flowmeters respectively. Scaling dimensionless parameters will be used to relate the processes in the lab to those in nature. This project will lead to a better understanding of pyroclastic flows dynamics and if thus of importance in the mitigation and forecast of volcanic hazards. This project will be carried out by one postdoc and will provide training for graduate and undergraduate students. In order to involve a larger number of persons and broaden the perspective of the research, a graduate level class based on the topics addressed in this proposal will be offered. This project will also lead to new interdisciplinary collaborations between the engineering and volcanology communities.