Shear-Induced Pressure Anisotropy in Granular Flows of Nonspherical Particles

DEM study in a linear split-bottom shear cell (MercuryDPM)

Key insight

Particle shape fundamentally changes how stress is distributed in granular flow:

  • Spherical particles (AR = 1) → nearly uniform and isotropic pressure field
  • Elongated particles (AR = 5) → localized pressure increase inside the shear band

This pressure buildup is driven by:

  • Shear-induced alignment → particles orient with flow
  • Packing density increase → reduced void space
  • Normal stress anisotropy → σyy, σzz > σxx

The result is a localized pressure peak — a granular analogue of normal-stress-driven effects.

System (visual)

Linear split-bottom shear cell with nonspherical particles (DEM simulation).
See full simulations →

This work is available as a preprint and is under review at Physical Review E.
arXiv

What I did

  • Performed DEM simulations comparing spheres (AR = 1) and elongated particles (AR = 5)
  • Computed coarse-grained pressure and stress tensor fields in the y–z plane
  • Quantified the relationship between alignment, packing density, and stress anisotropy
  • Systematically varied friction (µ = 0.01–0.8) to study shear-band localization

Methods & tools

  • Simulation: MercuryDPM (DEM), Hertz–Mindlin viscoelastic contact model
  • Geometry: linear split-bottom shear cell (LSC), periodic in flow direction
  • Driving: two L-shaped walls moving at ±V₀/2 (V₀ = 0.038 m/s)
  • Particles:
    • spheres (AR = 1)
    • elongated multisphere particles (AR = 5)
  • System size: (Lx, Ly, Lz) = (25, 25, 20) dp, N ≈ 4500, ±20% polydispersity
  • Parameter range: friction coefficient µ = 0.01–0.8
  • Post-processing: coarse-graining via MercuryCG, averaged in x and steady state

Key results

1. Elongated particles generate a pressure peak

  • Pressure becomes higher inside the shear band than in the bulk
  • The pressure field localizes around the split region (y = 0)

2. Spheres remain nearly isotropic

  • Pressure stays uniform across the domain
  • Stress components remain close to isotropic

3. Mechanism: alignment → compaction → stress anisotropy

  • Particles align with the shear direction
  • Alignment reduces void space → higher packing density
  • Increased contact forces produce σyy, σzz > σxx
  • This leads to a localized pressure increase

4. Friction controls localization

  • Low friction (µ ≈ 0.01) → wide shear band, weak pressure variation
  • High friction (µ ≈ 0.8) → narrow shear band, strong pressure localization

Takeaway

Particle shape does not only affect structure —
it fundamentally changes how stress is transmitted in granular flow.

This work shows how alignment-driven packing and stress anisotropy generate localized pressure fields under shear.

Media

Publication

H. Rahim, S. Roy, and T. Pöschel
Shear-induced pressure anisotropy in granular materials of nonspherical particles
Under review, Physical Review E
arXiv

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