On Flow Measurements and Rheology of Time-Dependent Phenomena in non-Newtonian Fluid Flows

QC260416

Abstract

Non-Newtonian fluids are omnipresent in all natural and synthetic processes surrounding us. Due to our literal submergence in water and air, Newtonian fluids as such have a prominent vitality in many aspects of the industrial advancements, yet from a technical perspective, Newtonian fluids form a sub-set of the whole picture - the physics of flowing matter - and the expanded, general description, without any assumptions on the (i) constant viscosity, and (ii) single-source stress (i.e., viscosity), belongs to the non-Newtonian fluids. Under this taxonomy, an ever increasing level of complexity can be found, which deviates from the norm and intuitively familiar behavior of Newtonian fluids, and - with its many open problems at present - requires in depth, fundamental level investigations. Non-Newtonian fluids of different classes can have shear dependent varying viscosities, store elastic energy and release it out-of-phase with the local flow field and at multi-mode time-scales, undergo continuous and bi-directional micro-structural break-down and recovery, exhibit memory effects, age, and be made of living constituents. 

In this thesis, a variety of phenomena relevant to time-dependent flows and state transitions of non-Newtonian fluids are investigated experimentally using a wide range of flow measurement and rheological techniques. Fluids of viscoelastic (VE), elastoviscoplastic (EVP), and thixo-elastoviscoplastic (TEVP) natures were studied in regards to wall-slippage, pressure-driven duct flows, thixotropy and memory effects, elastic- and shear-banding driven instabilities, inertia-dominated instabilities and transition to turbulence, sub-yielding dynamics, and flow around obstacles and ducts with varying cross-sections. To that end, flow velocimetry techniques such as Particle Image Velocimetry (PIV), Laser Doppler Velocimetry (LDV), Doppler-Optical Coherence Tomography (D-OCT), medium structure measurements with intensity recordings of Optical Coherence Tomography (OCT) and Polarized Light Imaging (PLI), and rheometric assessments with steady- and oscillatory shear-, as well as parallel superposition rheology, have been used. The results regarding the development and usage of the measurement techniques, as well as physical interpretations of the studied instationary flow phenomena, are reported in the appended papers, and summarized in the upcoming chapters. 

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