Granular flows generated by instantaneous release of initially gas-fluidised material 

O. Roche1*, M.A. Gilbertson2, J.C. Phillips1, R.S.J. Sparks1

University of Bristol, UK. 1 Department of Earth Sciences, 2 Department of Mechanical Engineering.

The emplacement of granular flows generated by instantaneous release of a bed 
of initially gas-fluidised material into a horizontal channel was studied 
experimentally. We used particles corresponding to groups A, B, and D of 
Geldart's classification. The flow regime depends (1) on the initial 
fluidisation and the flow motion which disrupt the inter-granular contacts 
network of the initial static bed and (2) on the time-scale needed to 
re-establish strong contacts, which decreases as the grain size increases. 
Initially non-fluidised and fluidised flows of coarse particles (groups B and 
D) and non-fluidised flows of fine particles (group A) propagate as a wedge 
at a decelerating velocity after the initial acceleration. In contrast, flows 
of fine particles generated from fluidised beds propagate at constant 
thickness and velocity, in a regime similar to the slumping phase of gravity 
currents of simple fluids driven by buoyancy. This implies that internal 
particles interactions can be neglected to describe these granular flows and 
we have determined a Froude number Fr~2.6. All types of flows are 
characterised by a final stopping phase whose time-scale gives an estimate of 
the kinetics required to convert the granular material back to the fully 
solid-like, static state.