Distinguished EPJ Referees

EPJE Colloquium: Dense granular flows - fluidity revisited

Local shear rate ˙Γ for a system of frictional soft grains
Bouzid et al.

In this new EPJ E Colloquium, a group of authors from ESPCI, Univ. Paris-Diderot and Univ. P.M. Curie use the interpretive frame-work of non-locality to describe the rheology and fluidity in dense granular flows. Is a “good fitting” of the velocity profiles sufficient to demonstrate the validity of a particular model for non-locality?

The article reviews and compares various approaches based on different constitutive relations and choices for the fluidity parameter, focusing on the kinetic elasto-plastic model introduced by Bocquet et al. [Phys. Rev. Lett 103, 036001 (2009)] for soft matter and adapted for granular matter by Kamrin et al. [Phys. Rev. Lett. 108, 178301 (2012)], as well as the gradient expansion of the local rheology μ(I) that the authors presented in Phys. Rev. Lett. 111, 238301 (2013).

The authors point out that, in order to discriminate between these approaches, one must go beyond the predictions derived from linearisation around a uniform stress profile, such as those obtained in a simple shear cell. Finally the authors discuss future tests, to better understand the mechanisms at the origin of non-local effects, that should be based on the nature of the chosen fluidity parameter and the related boundary conditions, as well as the hypothesis made to derive the models and the mechanisms underlying their dynamics.

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