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Dense granular flows exhibit unique diffusion properties driven by granular vortices. These vortex clusters significantly influence particle movement and diffusion scaling in granular materials.

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Area of Science:

  • Physics
  • Granular Mechanics
  • Fluid Dynamics

Background:

  • Dense granular flows are complex systems with poorly understood transport properties.
  • Existing models often fail to capture the emergent behaviors observed in these flows.

Purpose of the Study:

  • To investigate the unexpected diffusion properties in dense granular flows.
  • To identify the underlying mechanisms responsible for granular diffusion.

Main Methods:

  • Systematic measurement of transverse diffusion coefficients and average vortex sizes in simulated plane shear flows.
  • Analysis of varying inertial numbers (I) and their effect on flow properties.
  • Examination of grain trajectories to understand diffusion mechanisms.

Main Results:

  • Observed strong deviations from expected scaling laws for diffusion coefficients (D) with grain size (d) and shear rate (γ̇).
  • Found that average vortex size (ℓ) increases at low inertial numbers, following ℓ∝dI^{-1/2}.
  • Introduced a general scaling D∝ℓdγ̇ that accurately captures measurements, highlighting the role of vortex size.

Conclusions:

  • Diffusion in dense granular flows is primarily driven by vortex-driven random walks.
  • Established a new scaling for diffusivity D∝d^{2}sqrt[γ̇/tᵢ], incorporating shear and inertial time scales.
  • Partially jammed clusters, or granular vortices, are key to understanding granular flow diffusion.