David G. Schaeffer John V. Matthews Pierre-Alain Gremaud Secondary Circulation in Granular Flow Through Nonaxisymmetric Hoppers. 583-600 2004 64 SIAM Journal of Applied Mathematics 2 http://dx.doi.org/10.1137/S0036139903415124 db/journals/siamam/siamam64.html#SchaefferMG04