Superstable Granular Heap in a Thin Channel

Abstract

We observed experimentally a new regime for granular flows in an inclined channel with a flow-rate-controlled system. For high flow rates, the flow occurs atop a static granular heap whose angle is considerably higher than those usually exhibited by granular heaps. The properties of such superstable heaps (SSH) are drastically affected by a change in the channel width W. This indicates that the unusual stability of these heaps can be accounted for by the flowing layer and its friction on the sidewalls. A simple depth-averaged model, assuming Coulomb friction, shows that the SSH angle scales as h/W (W being the channel width), and that grain size plays no part.

Sensitivity to solid volume fraction of gravitational instability in a granular medium

“…Nearly the same characteristic length of the exponential tail is observed in [10] with, above this transition region of the velocity profile, a linear part that progressively extends in depth up to a maximal flow thickness. It has been shown that the size of this flowing layer is governed by the channel width and the friction on the sidewalls [13,30,32]. In [11], it appears that the avalanche duration is too small for the linear part to be observed in the velocity profile.…”

“…[10,11,13,[28][29][30]. To compare and discuss more in details those velocity profiles, we will first refer to the work carried out by Courrech du Pont et al [14] who have defined three flow regimes for a granular avalanche in a fluid based on the fall under gravity of one grain in fluid: a free fall regime for which there is no fluid influence (i.e.…”

“…In our set-up we do not know the value of friction at the sidewalls W µ , but using the value obtained in [30], one gets for the flowing thickness at inclination 60°: tan tan 2…”

“…Those predictions are no more valid for surface flows on a granular heap. In this case, several studies in heap and rotating drum configurations have indeed revealed both in free fall regime [13,28,30,32] and inertial limit regime [33] that the steady velocity profiles remain located in the vicinity of the surface and are composed of an upper linear part in the flowing layer followed by a lower exponential tail that vanishes with depth in the granular bulk. A phenomenological model succeeds in accounting for the linear profile [29].…”

Temporally Heterogeneous Dynamics in Granular Flows

“…Granular materials persist at the forefront of contemporary research due to the extreme richness in their dynamics, either under shear [1], flowing out of a hopper [2], or driven by gravity [3]. Because thermodynamic temperature plays little role in determining these features, granular materials are widely recognized as a macroscopic analogue of athermal systems far from equilibrium [4].…”