Abstract
When granular materials flow, the constituent particles segregate by size and align by shape. The impacts of these changes in fabric on the flow itself are not well understood, and thus novel non-invasive means are needed to observe the interior of the material. Here, we propose a new experimental technique using dynamic X-ray radiography to make such measurements possible. The technique is based on Fourier transformation to extract spatiotemporal fields of internal particle size and shape orientation distributions during flow, in addition to complementary measurements of velocity fields through image correlation. We show X-ray radiography captures the bulk flow properties, in contrast to optical methods which typically measure flow within boundary layers, as these are adjacent to any walls. Our results reveal the rich dynamic alignment of particles with respect to streamlines in the bulk during silo discharge, the understanding of which is critical to preventing destructive instabilities and undesirable clogging. The ideas developed in this paper are directly applicable to many other open questions in granular and soft matter systems, such as the evolution of size and shape distributions in foams and biological materials.
Highlights
After a building sinks into sand, one knows that the sand has moved, but not exactly where or how
Refractive Index-Matched Scanning (RIMS) has been applied to problems involving dynamic conditions, but this technique requires the use of a viscous interstitial fluid with refractive index matched to the particles under investigation, which significantly affects the nature of the granular flow[36,37,38]
While Particle Image Velocimetry (PIV) techniques have been applied to X-ray radiography for velocity measurements in fluids using tracers[53,54,55,56] or density fluctuations[57], similar techniques have seldom been used for granular flows[58]
Summary
After a building sinks into sand, one knows that the sand has moved, but not exactly where or how. RIMS has been applied to problems involving dynamic conditions, but this technique requires the use of a viscous interstitial fluid with refractive index matched to the particles under investigation, which significantly affects the nature of the granular flow[36,37,38]. Faced with these limitations it is more customary to study only part of the velocity field by acquiring images solely through transparent walls or along free surfaces. While PIV techniques have been applied to X-ray radiography for velocity measurements in fluids using tracers[53,54,55,56] or density fluctuations[57], similar techniques have seldom been used for granular flows[58]
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