Propagation and hindered settling of laboratory ash flows

Abstract

[1] The fluidal behavior of pyroclastic flows is commonly attributed to high gas pore pressures and associated fluidization effects. We carried out experiments on flows of fluidized volcanic ash at 170°C, which is hot enough to reduce cohesive effects of moisture. The flows were generated in a 3-m-long, horizontal lock-exchange flume. The ash was fluidized and expanded uniformly in the flume reservoir by up to 43% above loose packing and was then released. Each flow defluidized progressively down the flume until motion ceased. Initial expansion E and initial height h 0 were varied independently of one another. The flows traveled in a laminar manner. Flow fronts exhibited three main phases of transport: (1) a brief initial phase of gravitational slumping, (2) a dominant, approximately constant velocity phase, and (3) a brief stopping phase. Phase 2 frontal velocities scaled with ffiffiffiffiffiffiffi gh 0 p , like other types of dam-break flow. Deposition from initially expanded flows took place by progressive sediment aggradation at a rate that was independent of distance and varied only with E. Despite rates of shear up to 80 s À1 , aggradation rates were identical to those determined independently, at the same value of E, in quasi-static collapse tests. Sedimentation caused the flows to thin progressively during transit until they ran out of volume. The dynamics were governed to a first order by two dimensionless parameters: (1) the initial aspect ratio h 0 /x 0 in the lock reservoir and (2) the ratio t sett /t grav of two timescales: a particle settling time t sett and a gravitational acceleration time t grav .Citation: Girolami, L., T. H. Druitt, O. Roche, and Z. Khrabrykh (2008), Propagation and hindered settling of laboratory ash flows,

Particle velocity fields and depositional processes in laboratory ash flows, with implications for the sedimentation of dense pyroclastic flows

“…Similar experiments were carried out by Girolami et al (2008) with hot volcanic ash. Hot ash differs from glass beads in that it can be expanded considerably (up to ∼40%) above loose packing when fluidized above U mf , allowing the effect of initial expansion on flow kinematics to be investigated.…”

“…1; see Girolami et al 2008 for a detailed description). The high temperature was provided by external heating tapes regulated by thermostats and covering both the reservoir and an underlying windbox, and all experiments were carried out with the incoming gas and reservoir contents at the same temperature.…”

“…The aim of the present study is to investigate in more detail the internal kinematics and deposition behavior of experimental flows of volcanic ash similar to those described by Girolami et al (2008). For this, we made high-speed videos of the experiments, which were subsequently treated with sophisticated techniques of motion field estimation.…”

“…Development of quantitative models of pyroclastic flows is a priority for modern volcanology, and this requires better understanding of their physical properties. While the physics of gravitational dry granular flows (in which it is assumed that particle interactions dominate and that interstitial gas plays a negligible role), has been explored extensively (for reviews, see GDR MiDi 2004 andForterre andPouliquen 2008), research on dense gas-particle flows dominated by fluidparticle interactions is much less advanced (Roche et al 2004(Roche et al , 2010Girolami et al 2008). Up to now the dynamics of pyroclastic flows has been largely based on field observations of active flows (e.g., Hoblitt 1986;Levine and Kieffer 1991), and in particular inferred from textural studies of flow deposits (Sparks 1976;Branney and Kokelaar 2002 and references therein).…”

“…Laboratory studies of dense gravitational gas-particle flows include those of both continuously fluidized flows (Eames and Gilbertson 2000;Takahashi and Tsujimoto 2000;Gilbertson et al 2008), and dam-break (i.e., transient) flows of initially fluidized particles released from a reservoir and that defluidize progressively during propagation (Roche et al 2004(Roche et al , 2010Girolami et al 2008). Roche et al (2008) showed that dam-break flows of small (<100 µm) group-A glass beads initially fluidized at U mb (with a corresponding bed expansion of 2-4%) propagate on a horizontal substrate in three distinct phases based on observations of the flow front: (1) a short initial acceleration phase as the reservoir empties, (2) a dominant phase in which the front has an approximately constant velocity U∼√(2gh 0 ), where g is gravitational acceleration and h 0 is the height of the initial material in the reservoir, and (3) a short stopping phase.…”