Fluidization in pyroclastic flows

Pyroclastic flows are mixtures of gas and particles generated by gravitational collapse of lava domes or fall-back of eruption columns during explosive eruptions. They are major natural hazards because they can propagate far away from the source at speeds that largely exceed those of most natural or anthropogenic granular flows. Hazard mitigation requires improved understanding of their complex dynamics to predict flow path, velocity, and run-out distance. The relevance of fluidization of granular solids in pyroclastic flows to their enhanced mobility has received substantive support. Nonetheless, broad uncertainties still characterize fundamental aspects like the prevailing nature and source of the fluidizing gas, the role of particle-phase stresses, turbulence and shear, the effect of solids polydispersity and fragmentation. Moreover, the dynamics of the frontal zone of the flow deserves deeper investigation due to its relevance to air-cushioning/entrainment and the establishment of self-sustained motion-induced fluidization.