Granular material in a cylindrical vessel undergoing rotational and rocking motions is modeled using a discrete element method. Rotational motion supplemented by rocking is compared to purely rotational motion via linear density profiles, velocity fields, and axial concentration profiles. The rocking motion, which imparts a time-dependent flow perturbation to purely rotational motion, dramatically enhanced mixing in laboratory studies. Simulation results appear to agree well with experimental results and observations.