Dynamical compressibility of dense granular shear flows
  It has been conjectured by Bagnold [1] that an assembly of hard non-deformable spheres could behave as a compressible medium when slowly sheared, as the average density of such a system effectively depends on the confining pressure. Here we use discrete element simulations to show the existence of transverse and sagittal waves associated to this dynamical compressibility. For this purpose, we study the resonance of these waves in a linear Couette cell and compare the results with those predicted from a continuum local constitutive relation. 
Martin Trulsson, Mehdi Bouzid, Philippe Claudin, and Bruno Andreotti
cond-mat.stat-mech cond-mat.soft