Numerical simulations of dense granular suspensions in laminar flow under constant and varying shear rates

Using an immersed boundary-lattice Boltzmann method, we investigated the response of dense granular suspensions to time-varying shear rates and flow reversals. The evolution of the relative apparent viscosity and particle structures was analysed. The concentration of solids (ϕ_v) and particle Reynolds numbers (Re_p) were varied over the ranges 6%≤ϕ_v≤47% and 0.105≤Re_p≤0.529. The simulations included sub-grid scale corrections for unresolved lubrication forces and torques (normal and tangential). When ϕ_v surpasses 30%, the contribution of the tangential lubrication corrections to the shear stress is dominant. While for intermediate solids fractions we find weak shear-thinning, we see weak shear-thickening for ϕ_v>40%. We show how the structure and apparent viscosity of a suspension evolves after a reversal of the shear direction. For 47% solids, simulations with step changes in the shear rate show the effects of the previous shear history on the viscosity of the suspension.