Simulation of dense suspensions with discrete element method and a coupled lattice Boltzmann method
Najuch, Tim Peter
Suspensions, mixtures of a fluid and particles, are widespread in nature and industry. A better understanding of suspension flow physics could be gained by accurate simulations and hence, many different simulation techniques for fluid-solid flows have been proposed. The intention of this thesis is to establish an accurate, high-fidelity methodology to simulate sheared dense suspensions on microscale which allow for extraction of reliable data for macroscale models and better understanding of underlying physical processes. Therefore, two partially saturated coupled lattice Boltzmann discrete element methods (LBDEM) are analysed and evaluated with regard to the stresslet computation of suspension flow under simple shear at first. Simulation results for a single sphere, two spheres, and several hundred spheres, immersed in a sheared fluid show that a commonly used partially saturated method based on the non-equilibrium bounce-back lead to non-satisfactory results. But an alternative superposition method, which is mostly neglected in the literature, can result in accurate stresslet calculations. Furthermore, an ideal single relaxation parameter range for the fluid phase to reduce slip velocity effects is determined. Based on the stresslet study outcome, the superposition method is used to simulate two particle collisions in the second part of this work. Thereby, a careful assessment of the partially saturated LBDEM capabilities to resolve lubrication interactions between particles, which are important in suspension flows, is achieved. Moreover, a popular lubrication correction model, which is applied for very small gap distances between particles which cannot be resolved by the lattice size, is slightly modified and calibrated to be suitable when used with the partially saturated coupling method. In the third part of this thesis, a comparison between the expensive LBDEM coupling and a tremendously cheaper discrete element method (DEM) with a model for lubricated particle-particle interactions is carried out. It is shown that for low Reynolds number sheared suspensions, with intermediate to dense solid packing fractions, the differences between both aforementioned LBDEM and DEM approaches can be minor. Thus, it is demonstrated that a DEM with a lubrication model can be effectively used to simulate microscale processes of dense suspension under simple shear in a low Reynolds number regime for a significant lower fraction of the computational expenses necessary for LBDEM simulations. Hence, in the last part of this thesis, only a DEM with a lubrication model is employed to study particle-particle contact and lubrication interactions as well as the underlying dissipation mechanism in dense sheared suspensions on microscale. Thereby, providing evidence that the suspension bulk viscosity divergence is caused by the lubricated dissipative interactions while mechanical particle-particle contact is the stress dominating contribution for high solid fractions close to jamming.