Dense granular flows: interpolating between grain inertia and fluid   viscosity based constitutive laws
  A scalar constitutive law was recently obtained for dense granular flows from a two-grain argument, both in the inertial regime (grain inertia) and in the viscous regime. As the resulting law is not exactly the same in both regimes, we here provide an expression for the crossover between both regimes. 
Pierre Rognon (MSC, CRPP), Cyprien Gay (MSC)