Experimental observations of fluid-inertial behaviour of dam-break, dense granular flows and their relevance for the propagation of pyroclastic flows
Seminar Room 1, Newton Institute
Pyroclastic flows are commonly generated during volcanic eruptions by the gravitational collapse of lava domes or explosive columns, and consist of dense, hot mixtures of gas and particles. In order to investigate the physics of these flows, we carried out laboratory dam-break experiments using both granular material and water, and the flow kinematics were studied quantitatively. Unsteady granular flows of glass beads of 60-90 Ęm in diameter generated in a horizontal channel from the release of initially fluidized, slightly expanded (2.5-4.5%) columns behave as their inertial water counterparts for about 65% and 80% of their flow duration and run-out, respectively. For a range of initial column height to length ratios of 0.5-3, both types of flows propagate in three stages, controlled by the timescale of column free fall ~(h0/g)1/2, where h0 is the column height and g the gravitational acceleration. Flows first accelerate as the column collapses. Transition to a second, constant velocity phase occurs at a normalised time t/(h0/g)1/2~1.5, when the flow height in the channel has a maximum value of about 0.2h0. The flow velocity is then U~ã2(gh0)1/2, larger than that for dry (non-initially fluidized) granular flows. The behaviour of the initially fluidized granular flows departs from that of the water flows when they enter a last, third phase at t/(h0/g)1/2~4, as the height of the collapsing column has dropped to about that of the resulting flow. The granular flows then steadily decelerate and their front position varies as t1/3, as in dry flows, and their motion ceases at t/(h0/g)1/2~6.5 and normalized run-out x/h0~5.5-6. The equivalent behaviour of water and highly concentrated granular flows up to the end of the second (constant velocity) phase indicates a similar overall bulk resistance, although mechanisms of energy dissipation in both cases would not be of the same nature. These results show that the interstitial air can damp particle-particle interactions. Further analysis suggests that air-particle viscous interactions can be dominant and can generate pore-fluid pressure sufficient to confer a fluid-inertial behaviour to granular flows propagating at almost maximum
concentration, before they enter a granular-frictional regime at late stages. This work suggests that analyses combining inertial-turbulent and granular-frictional form of resistance are the most appropriate to model the propagation of pyroclastic flows.