Instabilities in Granular Flow
  We consider the stability of a system of equations which are a singular perturbation of the incompressible rigid-plastic flow equations used to model granular flow. A linear stability analysis shows that solutions of these equations are unstable, in general, and that these instabilities may lead to ill-posedness. However, the analysis suggests that these equations are more regular, in a certain sense, than the unperturbed incompressible rigid-plastic flow equations. Numerical solutions are presented which illustrate the growth of instabilities and the development of shear-bands in the flows. 
Shaun Hendy