Towards a quantitative understanding of pyroclastic flows: Effects of expansion on the dynamics of laboratory fluidized granular flows
Résumé
We conducted laboratory dam-break experiments on initially fluidized granular flows using two different fine-grained powders (mean grain sizes 47 and 67 μm) down a smooth, horizontal channel with an impermeable base. The powders were first fluidized and expanded to a known degree in the flume reservoir, then released down the channel by opening a sliding gate. The mixture formed rapidly moving flows that defluidized and deposited progressively as they propagated. The experiments were similar to those carried out previously using volcanic ash by Girolami et al. (2008, 2010) but explored a much larger range of initial aspect ratios (height-to-length ratio, a = 0.25 to 4). They were designed to investigate the effects of initial expansion (up to 50 vol.% above loose packing) and aspect ratio on the dynamics of flow propagation and deposition, and to explore different scalings in order to determine the physical parameters governing these processes. The flows exhibit a similar behaviour to other types of transient granular flows, including three well defined propagation phases (acceleration, constant velocity, and stopping phases) and the progressive aggradation of a basal static layer during emplacement. The deposit aggradation velocity depends only on the initial powder expansion and is similar to that of a collapsing bed of the same powder, expanded by the same amount, under quasi-static, non-shearing conditions. At a given initial expansion, the maximum runout distance scales with the initial bed height h0, the runout duration with (h0/g)1/2 and the maximum velocity with (gh0)1/2. However, runout distance and duration both increase with increasing initial expansion. This is attributed to the effect of hindered settling in delaying defluidization of the dense, but slightly expanded, suspension. The data enable us to identify an additive scaling law providing a smooth transition from non-expanded to expanded flows.