Comment on "Flow of wet granular materials: A numerical study"

Area of Science:

• Granular physics
• Rheology
• Fluid dynamics

Background:

• The effective stress model by Khamseh et al. (Phys. Rev. E 92, 022201) approximates shear stress in wet granular flow.
• This model exhibits increasing bias at low normal pressures.
• The approximation of capillary stress as a spherical tensor is identified as a potential source of error.

Purpose of the Study:

• To identify the cause of prediction errors in the Khamseh et al. effective stress model at low normal pressures.
• To re-examine the model's predictions by addressing the capillary stress approximation.
• To validate an earlier effective stress expression for wet granular materials.

Main Methods:

• Analysis of the Khamseh et al. effective stress model.
• Re-evaluation of capillary stress approximation in wet granular flow.
• Comparison of model predictions with simulated steady-state flow data.

Main Results:

• The approximation of capillary stress by a spherical tensor is confirmed as the cause of prediction errors.
• An excellent agreement is found between the simulated data and a previously introduced effective stress expression.
• The study validates a previously unexplained empirical result in granular material flow.

Conclusions:

• The Khamseh et al. model's limitations at low normal pressures are attributed to its capillary stress approximation.
• An alternative effective stress expression provides a more accurate description of wet granular flow.
• This work reinforces the validity of an empirical finding in granular physics.