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, $E(f)\propto f^{\alpha }$. In the surface layer, $\alpha$ 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, $\alpha$ decreases with depth, reaching $\approx -1.5$ 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.

Close

PrevNext

View figure in article

• Granular flows
• Shear flows
• Granular materials