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Issue 42, 2019
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Force fluctuations at the transition from quasi-static to inertial granular flow

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Abstract

We analyse the rheology of gravity-driven, dry granular flows in experiments where individual forces within the flow bulk are measured. We release photoelastic discs at the top of an incline to create a quasi-static erodible bed over which flows a steady 2D avalanche. The flowing layers we produce are dense (ϕ ≈ 0.8), thin (h ≈ 10d), and in the slow to intermediate flow regime (I = 0.1 to 1). Using particle tracking and photoelastic force measurements we report coarse-grained profiles for packing fraction, velocity, shear rate, inertial number, and stress tensor components. In addition, we define a quantitative measure for the rate of generation of new force chain networks and we observe that fluctuations extend below the boundary between dense flow and quasi-static layers. Finally, we evaluate several existing definitions for granular fluidity, and make comparisons among them and the behaviour of our experimentally-measured stress tensor components. Our measurements of the non-dimensional stress ratio μ show that our experiments lie within the local rheological regime, yet we observe rearrangements of the force network extending into the quasi-static layer where shear rates vanish. This elucidates why non-local rheological models rely on the notion of stress diffusion, and we thus propose non-local effects may in fact be dependent on the local force network fluctuation rate.

Graphical abstract: Force fluctuations at the transition from quasi-static to inertial granular flow

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Publication details

The article was received on 03 Jun 2019, accepted on 13 Oct 2019 and first published on 14 Oct 2019


Article type: Paper
DOI: 10.1039/C9SM01111K
Soft Matter, 2019,15, 8532-8542

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    Force fluctuations at the transition from quasi-static to inertial granular flow

    A. L. Thomas, Z. Tang, K. E. Daniels and N. M. Vriend, Soft Matter, 2019, 15, 8532
    DOI: 10.1039/C9SM01111K

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