HR: 0800h
AN: V31E-0706    [Abstracts]
TI: Dynamics of Experimental Rapid Shear Flows
AU: * Girolami, L
EM: L.Girolami@opgc.univ-bpclermont.fr
AF: Laboratoire Magmas et Volcans, Université Blaise Pascal, CNRS, IRD, 5, rue Kessler, Clermont-Ferrand, 63038, France
AU: Druitt, T H
EM: T.Druitt@opgc.univ-bpclermont.fr
AF: Laboratoire Magmas et Volcans, Université Blaise Pascal, CNRS, IRD, 5, rue Kessler, Clermont-Ferrand, 63038, France
AU: Corpetti, T
EM: thomas.corpetti@uhb.fr
AF: Equipe COSTEL, Université Rennes 2, CNRS, 6, avenue Gaston Berger, Rennes, 35043, France
AU: Roche, O
EM: O.Roche@opgc.univ-bpclermont.fr
AF: Laboratoire Magmas et Volcans, Université Blaise Pascal, CNRS, IRD, 5, rue Kessler, Clermont-Ferrand, 63038, France
AB: Pyroclastic flows are dense mass flows of hot (< ~ 700° C) particles and gas generated by volcanic eruptions. Their fluidal behavior is attributed to high interstitial gas pore pressures and associated fluidization effects. We carried out experiments on the dynamics of laboratory-scale flows of fluidized volcanic ash at 170° C, which is hot enough to render negligible effects of humidity-derived interparticle cohesion. The flows were generated in a 3.5-m-long, horizontal lock-exchange flume built of aluminum and pyrex. The ash was expanded by fluidization to 0 to 42 vol % above loose packing, then released down the flume as thin (< 10 cm), but fast-moving (1.5-2.3 m/s) shear flows. Since the floor of the flume was impermeable, the ash defluidized progressively until motion ceased. A key feature of the flows was that deposition occurred by progressive sediment aggradation at the flow base. Particle hindered settling velocities in the flows were indistinguishable within error from those measured in (static) 1-D bed-collapse tests at given values of expansion. In order to investigate the flow dynamics in more detail, we filmed a number of experiments using a high-speed (1000 frames per second) video camera. By repeating each experiment several times, we were able to observe the sedimentation behavior both as function of time and with distance down the flume in the short-lived and highly unsteady flows. This was possible because the experiments were reproducible. Using specially designed image analysis software, we measured the sediment aggradation rates and the temporal and spatial evolution of velocity fields and velocity gradients in the flows. Video analysis shows a distinct deposition behavior between expanded flows and non expanded flows. Flow duration was governed by the ratio of two times: t_sett, a characteristic time of hindered settling and t_grav, a characteristic time of gravitational acceleration. Analyses confirme that this ratio increases with increasing initial expansion.
DE: 8414 Eruption mechanisms and flow emplacement
DE: 8428 Explosive volcanism
DE: 8445 Experimental volcanism
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting