Fourier transform rheology for dense granular flows

Granular rheology is of interest in many geophysical and industrial applications that require precise handling of particulate materials, including conveying, dosing, spreading, compaction, and other operations. The capability to handle dense flows is important for process throughput and efficiency. In this context, we explore oscillatory rheometry for dense granular flow using a chute-flow rheometer (CFR) having multimodal sensing capability. The CFR applies oscillatory shear in a Couette gap with measurements of torque, wall pressure, and axial flow. The resulting cyclic data were deconstructed into elastic and viscous components using Fourier transforms. Torque data were normalized to the active mass of granular material in the CFR, converted to specific torque and shear stress, and analyzed using Lissajous–Bowditch (LJB) plots. Viscoelastic crossovers were observed at relatively low shear rates consistent with quasistatic (i.e., friction-like) flows. At higher shear rates, secondary loops were observed in elastic (γ˙,τ) LJB plots, consistent with transition to dense inertial (i.e., fluid-like) flow. These rheological markers are discussed in terms of dense flow transitions. Further analysis considered inertial number scaling in the μ(I) granular rheology, comparing specific torque with the conventional definition using pressure and bulk density. Finally, a contour diagram summarizes the coupling of shear and normal forces in the CFR, where the wall pressure sensor detects the first normal force and the second normal force is in the direction of axial flow. The results suggest characterization criteria that are relevant to managing dense granular flows in quasistatic and dense inertial regimes.