The relative effect of ambient fluid and initial density in the mobility of fine granular flows.

Granular flows have been the focus of many recent studies in earth and astronomical sciences due to their extremely high mobility. Flow friction angles seem to be significantly reduced respect to the repose angle of same geological materials. Particle-fluid interactions appear as one of the most important mechanism accounting for friction reduction in granular flows. Recent dam-break experiments using fine particles at air ambient conditions show that initially fluidized beds run twice further than their non-fluidized counterparts. However, the increasing run-out of the flow respect to the initial degree of fluidization shows also to be strongly correlated with the initial mixture density. To highlight the relative effects of the ambient fluid and mixture density in the flow mobility, in this work we studied the flow run-out of loose and packed columns of particles at both local atmospheric pressure and vacuum conditions. Experiments were carried out in a lock-exchange channel using fine type-A particles ( 70 mm). Results indicate that flow initiation is strongly controlled by the initial mixture density: for loose mixtures the column drops almost vertically and for hard packed mixtures the column seems to slide into the channel while the rear part of the column always remain at rest. At local atmospheric pressure conditions this behavior seems to be correlated with the pore fluid pressure. While loose columns show an increase in pore fluid pressure when collapsing, packed columns show an initial decrease in pore fluid pressure due to an initial phase of volume expansion which delays flow initiation. These different behaviors results longer run-outs for columns lower densities in respect to more packed columns. Lowering the atmospheric pressure also reduced flow run-outs thus highlighting the effect of the interstitial fluid of enhancing the flow mobility of fine-particles flows.