Numerical studies on elastoviscoplastic fluid flows
Time: Mon 2025-10-27 10.00
Location: Kollegiesalen, Brinellvägen 8, Stockholm
Language: English
Subject area: Engineering Mechanics
Doctoral student: Kazi Tassawar Iqbal , Strömningsmekanik, Complex Fluids Group (FLOW)
Opponent: Professor Ian Frigaard, University of British Columbia, Department of Mechanical Engineering, Vancouver, BC Canada
Supervisor: Professor Outi Tammisola, Strömningsmekanik; Luca Brandt,
QC251009
Abstract
Yield stress fluids are ubiquitous in our environment and industrial processes - from food, cosmetics and hygiene products to paper pulp processing and geophysical flows of magma, mudslides and avalanches. The flow behaviour of these fluids is qualitatively different from water and other simple fluids, for instance because they behave as soft solid materials before starting to flow. Therefore it is important for applications to be able to understand and model yield stress fluid flow.
Bingham formulated his seminal model for yield stress fluids already in the beginning of 20\textsuperscript{th} century, but in the recent years experiments have shown that many flow phenomena cannot be explained by the Bingham model. A new central insight is that the elasticity of the material plays a role, and results in behaviours similar to viscoelastic fluids. To capture such effects in computer simulations, one has to use an elastoviscoplastic model that combines yield stress and elasticity of the fluid.
This thesis consists of computer simulations of flows of elastoviscoplastic fluids in fundamental canonical flow configurations of relevance for industries. Also multiphase flows are studied – suspensions of particles, droplets and bubbles – with specialised numerical methods. In suspensions of bubbles or droplets in planar shear flow, we show how rheology and droplet dynamics is affected by yield stress and elasticity of the surrounding material. Fluid elasticity makes the droplets migrate towards channel walls, while yield stress increases the effective viscosity of the fluid, slowing down this migration. When two bubbles come close to each other in shear flow, they either attract or repel each other in the vorticity direction depending on their relative separation.
Particle suspensions are analysed in a three-dimensional channel with a quadratic cross-section. We observe, similarly to experiments, that spherical particles move towards the four corners of the channel, and we explain this phenomenon by analysing the stress fields obtained from our simulations. Non-spherical particles on the other hand can obtain several distinct stable orientations, and precessing or tumbling motions, when interacting with the solid part of the material, the ``central plug region".
Building on the studies of multiphase flows comprising elastoviscoplastic carrier fluids under laminar flow conditions, attention was turned to the behaviour of viscous droplets suspended in such fluids under turbulent flow conditions. The elasticity and yield stress of the carrier fluid are found to play a central role in droplet break-up and coalescence. As elasticity and yield stress increase, droplet break-up is suppressed, leading to the formation of large droplets and an accompanying skewness in the size distribution of droplets. The elasticity of the carrier fluid generates lift forces across the channel that drive droplet migration towards the centre. This migration depletes droplet layers near the walls, hence no significant variations in drag are observed.
Next, the first study of heat transfer by natural convection in elastoviscoplastic fluids is performed. The findings show that material elasticity can account for experimentally observed behaviour in Carbopol, such as the immediate onset of motion when a temperature gradient is applied in fluids with sufficiently low yield stress. In fluids with higher (super-critical) yield stress, the solid-like material is found to undergo elasto-inertial vibrations, resembling the behaviour of a damped spring-mass system.
Finally, three-dimensional computer simulations are conducted to successfully reproduce for the first time a classical wake instability in the flow viscoelastic fluids past a confined, circular cylinder in both steady and time-dependent regimes. The influence of different fluid behaviours is examined - in particular, the impact of shear-thinning in viscoelastic liquids and the role of yield stress in elastoviscoplastic materials. The results show that a small degree of shear-thinning in the first normal stress coefficient of viscoelastic liquids can rapidly suppress the instability. In elastoviscoplastic fluids, the characteristic instability is found to persist, with velocity fluctuations remarkably extending into the surrounding un-yielded regions of the material - which behave like soft solids - through elastic deformation.