{'fraction_non_alphanumeric': 0.0211146575006138, 'fraction_numerical': 0.0007365578197888534, 'mean_word_length': 5.528846153846154, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Numerical simulation of Newtonian/non-Newtonian multiphase flows : deformation and collision of droplets

The complex nature of multiphase flows, particularly in the presence of non-Newtonian rheologies in the phases, limits the applicability of theoretical analysis of physical equations as well as setting up laboratory experiments. As a result, Computational Fluid Dynamics (CFD) techniques are essential tools to study these problems. Despite the advances in numerical simulation techniques in this field in the past decade, the applicability of these approaches are limited by challenges appearing in specific applications, and particular consideration must be taken into account for each of these problems. The present thesis aims at three-dimensional numerical solution of Newtonian/non-Newtonian multiphase flow problems in the context of finite-volume discretization approach with applications in different natural and industrial processes. 
This thesis is organized in five chapters. The first chapter aims at providing an introduction to the motivation behind this work. We also present some application of the context of this thesis in industrial processes, followed by a small introductory on the CTTC research group, objectives and the outline of the thesis. The core of this thesis lays within chapters two, three and four. 
In chapter 2, using a conservative level-set method, three-dimensional direct numerical simulation of binary droplets collision is performed. A novel lamella stabilization approach is introduced to numerically resolve the thin lamella film appeared during a broad range of collision regimes. This approach demonstrates to be numerically efficient and accurate compared with experimental data, with a significant save-up on computational costs in three-dimensional cases. The numerical tools introduced are validated and verified against different experimental results for a wide range of collision regimes where very good agreement is seen. Besides, for all the cases studied in this chapter, a detailed study of the energy budgets are provided. 
In chapter 3, the physics of a single droplet subjected to shear flow is studied in details, with a primary focus on the effect of viscosity on walls critical confinement ratio. First, we highly validate the ability of the numerical tools on capturing the correct physics of droplet deformation. This chapter continues by three-dimensional DNS study of subcritical (steady-state) and supercritical (breakup) deformations of the droplet for a wide range of walls confinement in different viscosity ratios. The results indicate the existence of two steady-state regions in a viscosity ratio-walls confinement ratio graph, which are separated by a breakup region. Overall, these achievements indicate a promising potential of the current approach for simulating droplet deformation and breakup, in applications of dispersion science and mixing processes. 
In chapter 4, with the help of experience gained in the previous chapters, a finite-volume based conservative level-set method is used to numerically solve the non-Newtonian multiphase flow problems. One set of governing equations is written for the whole domain where different rheological properties may appear. Main challenging areas of numerical simulation of multiphase non-Newtonian fluids, including tracking of the interface, mass conservation of the phases, small timestep problems encountered by non-Newtonian fluids, numerical instabilities regarding the high Weissenberg Number Problem (HWNP), instabilities encouraged by low solvent to polymer viscosity ratio in viscoelastic fluids and instabilities encountered by surface tensions are discussed and proper numerical treatments are provided in the proposed method. The numerical method is validated for different types of non-Newtonian fluids, e.g. shear-thinning, shear-thickening and viscoelastic fluids using structured and unstructured meshes, where the extracted results are compared against analytical, numerical and experimental data available in the literature.