Couette flow with a bidisperse particle mixture

We investigate the steady-state solutions of granular Couette flows with a bidisperse particle mixture. The Couette geometry allows us to study segregation in a simple nonuniform flow. Two continuum models using kinetic theory are examined, and their results are compared with particle dynamic (PD) simulations. A notable difference between the two models is their treatment of energy partition (equipartition versus nonequipartition). We consider mixtures of particles with different sizes, masses, and densities. For Couette flows, we find that energy equipartition breaks down with an increase in the system inelasticity and the mass ratio. The effect of the size ratio on nonequipartition of granular energy is very small if the two particle species have the same mass. Two forms of segregation are studied in the present work: total solids segregation in the system and solids species segregation. Total solids segregation is related to the distribution of the granular energy across the walls. Solids species segregation is due to a competition of three diffusion forces: the thermal diffusion force, the ordinary diffusion force, and the pressure diffusion force. For equal density but different size particles, we observe quantitative differences between the two continuum models, where one model predicts a much greater degree of both total solids and solids species segregation than the other model. For equal size but different mass particles, we observe qualitative differences between the models where one model and PD simulations predict a segregation transition based on the particle mass, which is not seen in the other model for the conditions we have examined.