Modeling structure-flow coupling in complex fluids

Complex fluids encompass a wide variety of natural and manufactured substances which are utilized in ever larger quantities as our technical understanding of them develops. Landslides, foams, micellar solutions, polymer melts and granular suspensions all display the characteristic non-Newtonian behavior typical of complex fluids, which implies that their resistance to imposed flow (viscosity) does not remain constant over time and/or the intensity of the flow. This non-Newtonian behavior can be largely attributed to the internal, structure forming constituents and their interactions. Under imposed flow, this structure responds to the shear, for instance, by breaking up or forming new aggregates, which can macroscopically manifest itself as variations in viscosity. Modeling such complex fluid structure and its coupling to flow dynamics is accomplished here both by a phenomenological coarse-grained kinetic model and a diffusion model coupled to flow mechanics via the Navier-Stokes equations in the continuum limit. These models are explored then in the context of shear banding, localized shear and dewatering events involving a complex fluid. Additionally, submerged granular flows are described employing multi-scale modeling involving interactions at the microscopic level using the Discrete Element Method (DEM) and the macroscopic multiphase Navier-Stokes equations in the continuum limit. The results concerning shear banding and shear localization indicate the importance of the finite fluid inertia, occasionally neglected in rheological modeling, that is present in e.g. start-up flows and Large Amplitude Oscillatory Shears (LAOS). Contrary to the assumptions utilized in the literature, this inertia is sufficient to trigger significant shear localization and shear banding without any elastic response considerations, which is distinguishable due to a viscosity contrast between congested and fluidized regions in the sample. Similar regions are also observed in dewatering events of fibrous suspensions and an optimized dewatering scheme is developed based on homogenizing the highly uneven viscosity profile by pressure pulsing. Finally, the issue of a surging flow rate reported in submerged granular hopper flows, lacking a theoretical explanation at the time this thesis is written, is also displayed to result from similar transitions, mediated by the force chains formed in the structure of the granular material.