Power Spectra of Velocity Fluctuations in Granular Heap Flow

This is the authors' abstract. We don't add key points for this paper.

This Letter used speckle visibility spectroscopy to probe velocity fluctuations in three-dimensional granular heap flow. The mean velocity profile comprises a fast-flowing surface layer over a creeping layer. In both layers, velocity fluctuations are scale invariant and self-similar, and the velocity spectra demonstrate power-law scaling, E(f)∝fα. In the surface layer, α is between -1.0 and -0.8, consistent with Hwa and Kardar's "running" sandpile model, indicative of a dynamically self-organized critical state. In the creep layer, α decreases with depth, reaching ≈-1.5 at a sufficiently deep layer; its theoretical basis remains unclear. Analysis of the fluctuation velocity distributions provides further insight into the microscopic origins of the spectrum. These findings may help resolve the long-standing puzzle of flow localization in gravity-driven granular flows, despite a uniform shear stress-to-pressure ratio.