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

The effective stress model of Khamseh et al., Phys. Rev. E 92, 022201 (2015)PLEEE81539-375510.1103/PhysRevE.92.022201 is in semiquantitative agreement with the shear stress in simulated steady state flow of a wet granular material. Nonetheless, the predictions are increasingly biased at low normal pressure. The approximation of the capillary stress by a spherical tensor in this model is a sufficient cause of the prediction errors, as shown in this Comment. The re-examination reveals an excellent agreement between the data and the effective stress expression formerly introduced for similar systems, further validating a yet unexplained empirical result.