granularRheologyModel class
Dense granular-flow rheology model for particle phase stress.
References:
    GDRmidi (2004).
    On dense granular flows.
    The European Physical Journal E, 14, 341-365
    Forterre, Y. and Pouliquen, O. (2008).
    Flows of Dense Granular Media.
    Annual Review of Fluid Mechanics, 40, 1-24
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