Skip to content

Rheology of granular flows: implications for pyroclastic density currents

Natalia Lipiejko 1, Thomas J. Jones^1 ^

Abstract

Pyroclastic density currents (PDCs) are gravity-driven, hot, gas-particle mixtures that travel away from volcanic vents at speeds typically of the order of tens of metres per second. PDCs transport particles from micron-sized ash to clasts larger than a metre. High temperatures, large velocities and their ability to travel great distances on slopes of just a few degrees makes PDCs one of the most lethal geophysical flows. Such properties also make PDCs incredibly complex and, due to their destructive nature which limits direct observations, one of the least understood volcanic phenomena. Yet, understanding of the internal structure and the rheology of PDCs is crucial to accurately forecast the flow path and the run-out distance of these hazardous flows. This research investigates the rheology of fluidised granular columns through novel experiments where a granular mixture is simultaneously sheared and fluidised whilst the pressure, stress and apparent viscosity are measured. Firstly, we study the effects of the applied shear on the fluidisation behaviour of monodisperse granular mixtures comprising spherical glass beads. Secondly, we explore the rheological regimes of monodisperse and bimodal granular columns with variable particle volume fraction. Finally, to quantify the effects of the particle shape on the fluidisation and rheological behaviour of the granular mixtures, we perform the experiments using real pyroclastic material and compare the results with the glass beads data. The findings from this work can be used to inform large-scale simulations of fluidised granular flows comprising particles of various sizes and shapes, propagating over non-uniform topographies.