Luca Brandt
Department of Mechanics
KTH
Stockholm
We perform numerical simulations of suspensions of rigid spheres. An Immersed Boundary Method is employed to account for the dispersed solid phase on a structured Eulerian mesh. Particle-particle and particle-wall interactions are modelled with lubrication corrections and collision forces.
Analysis of the stress of the mixture of neutrally buoyant particles flowing in a plane channel enable us to identify 3 different regimes, each dominated by one of the three contributions to the total momentum transfer. A laminar regime at low particle volume fractions and low Reynolds number (low inertia), where viscous stresses are the main responsible for the total stress, a turbulent regime at higher Reynolds numbers and moderate volume fractions, dominated by the turbulent Reynolds stresses, and a regime denoted as inertial shear-thickening associated to intense particle stresses at high volume fractions. To further disentangle the role of particle and fluid inertia, we perform additional simulations varying the particle volume fraction and the fluid to particle density ratio while keeping the total mass fraction constant and neglecting settling. The results indicate that changes of the density ratio between 1 and 10 at constant volume fraction do not alter the turbulent statistics significantly. On the contrary, simulations at constant mass fraction and different volume fraction display significant modifications, which indicates that, in the parameter range investigated, the excluded volume effect is the main responsible of the modifications of the flow in the presence of the particles.