Direct simulations of dense suspensions of non-spherical particles

We describe a method for the direct simulation of high-solids-volume-fraction (up to 45%) suspensions of non-spherical rigid particles that are non-colloidal and slightly denser than the interstitial fluid. The lattice-Boltzmann method is used to solve for the flow of the interstitial Newtonian fluid, and the immersed boundary method is used to enforce a no-slip boundary condition at the surface of each particle. The surface points for the immersed boundary method are also employed for collision handling by applying repulsive forces between the surface points of nearby particles. We also discuss methods for integrating the equations of particle motion at low density ratios and propose a method with improved accuracy. The methods are used to simulate rigid particles shaped like red blood cells. We report on the effect of the solids volume fraction on the sedimentation rate using a Richardson-Zaki model, and we describe the orientation of the particles during sedimentation. The particles settle in a preferentially vertical orientation at terminal particle Reynolds numbers near one. We compare a simulation at a 35% solids volume fraction with typical erythrocyte sedimentation rates, a common blood test. We find an order of magnitude lower sedimentation rate than the value for healthy adults. The discrepancy is attributed to the omission of agglomeration-inducing inter-cellular forces and the treatment of the red blood cells as rigid particles in the simulations.