Abstract
This paper shows how a conveyor belt setup can be used to study the dynamics of stationary granular flows. To visualise the flow within the granular bulk and, in particular, determine its composition and the velocity field, we used the refractive index matching (RIM) technique combined with particle tracking velocimetry and coarse-graining algorithms. Implementing RIM posed varied technical, design and construction difficulties. To test the experimental setup and go beyond a mere proof of concept, we carried out granular flow experiments involving monodisperse and bidisperse borosilicate glass beads. These flows resulted in stationary avalanches with distinct regions whose structures were classified as: (i) a convective-bulged front, (ii) a compact-layered tail and, between them, (iii) a breaking size-segregation wave structure. We found that the bulk strain rate, represented by its tensor invariants, varied significantly between the identified flow structures, and their values supported the observed avalanche characteristics. The flow velocity fields’ interpolated profiles adjusted well to a Bagnold-like profile, although a considerable basal velocity slip was measured. We calculated a segregation flux using recent developments in particle-size segregation theory. Along with vertical velocity changes and high expansion rates, segregation fluxes were markedly higher at the avalanche’s leading edge, suggesting a connection between flow rheology and grain segregation. The experimental conveyor belt’s results showed the potential for further theoretical developments in rheology and segregation-coupled models.Graphic
Highlights
Granular flows are often studied using steady-state experiments in long flumes with constant feed rates (e.g. MiDi 2004; Delannay et al 2017)
This paper presents an experimental three-dimensional setup based on a conveyor belt and refractive index matching (RIM) techniques that we used to study stationary granular avalanches
The conveyor belt was used to study the internal dynamics of granular flows made of monodisperse or bidisperse media
Summary
Granular flows are often studied using steady-state experiments in long flumes with constant feed rates (e.g. MiDi 2004; Delannay et al 2017). Immersing the whole setup in an indexmatched fluid usually solves these problems, but this solution has the disadvantage of requiring large volumes of fluids in long inclined flumes (van der Vaart et al 2018). Another difficulty common to RIM techniques is the limited number of fluid-grain combinations that come close to real-world magnitudes of fluid viscosity and density ratios (Wiederseiner et al 2011; Dijksman et al 2012). We provide a qualitative and quantitative picture of these avalanches by showing their bulk compositions, velocity profiles, strain-rate tensor invariants and segregation fluxes
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