Dense granular flow around a penetrating object: Experiments and   hydrodynamic model
  We present in this Letter experimental results on the bidimensional flow field around a cylinder penetrating into dense granular matter together with drag force measurements. A hydrodynamic model based on extended kinetic theory for dense granular flow reproduces well the flow localization close to the cylinder and the corresponding scalings of the drag force, which is found to not depend on velocity, but linearly on the pressure and on the cylinder diameter and weakly on the grain size. Such a regime is found to be valid at a low enough "granular" Reynolds number. 
Antoine Seguin, Yann Bertho, Philippe Gondret and Jerome Crassous
cond-mat.soft physics.flu-dyn