Settling of heavy particles in concentrated suspensions of neutrally buoyant particles under uniform shear

Numerical simulations are used to study the settling of heavy particles in a background suspension of equal size, neutrally buoyant particles while the suspension as a whole is subject to a uniform shear flow. The heavy particles are settling in the direction of shear and the system is nominally homogeneous. The total volume fraction of the particles is in the range 20%-40% and the fraction of heavy particles varies between 0.05 and 0.3. While the shear flow is a dominant factor compared to the settling motion, the suspension remains in a mixed homogeneous state but as the Stokes settling velocity increases there is a transition with the heavy particles separating to form a band of rapidly settling particles. Segregation of a bidisperse suspension of particles sedimenting under gravity is a known phenomenon but here the shear flow stabilizes the system. The dependence of the mean particle velocities on the suspension parameters is evaluated both for a mixed state and with segregation. Additional results are given for the velocity fluctuations and the particle stresses of each phase.