Abbasi Yazdi, Aliyeh (2024) Particle Dynamics in Elastoviscoplastic Fluids: Effect of Shape and Confinement. [Tesi di dottorato]

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Abstract

This thesis explores the dynamics of particles within elastoviscoplastic (EVP) fluids, which exhibit both yield-stress and viscoelastic properties. These unique characteristics of EVP fluids are essential in various industrial and geophysical applications, where controlling particle behavior can enhance process efficiency. This research investigates the settling, migration, and orientation dynamics of a particle with different shapes and under varying confinement conditions in EVP fluids. Using finite element method, the study investigates the behavior of a particle in an EVP fluid modeled through the Saramito-Giesekus constitutive equation, addressing key factors such as the effect of the Bingham and Weissenberg numbers on particle motion We start by studying the dynamics of a spherical particle settling near a flat wall in an EVP fluid. Direct numerical simulations reveal that particle-wall distance and the yield stress of the fluid significantly impact sedimentation and lateral migration. Additionally, the study identifies regions of yielded and unyielded fluid around the particle and examines the negative wake phenomenon in comparison to a purely viscoelastic case, highlighting how elasticity and plasticity interact to alter particle behavior near the boundary. Then, the investigation is extended to prolate spheroidal particle settling in an unbounded EVP material. The results indicate that particle shape strongly affects settling, lift, and angular velocities. With increasing aspect ratio, particle exhibits slower rotational dynamics and a higher sensitivity to the yield-stress properties. The study further explores the flow field around the particle, presenting results for the drag correction coefficient as a function of aspect ratio, Bingham number, and Weissenberg number, which collectively define the settling patterns and particle stoppage. Finally, the dynamics of circular and elliptical particles suspended in an EVP fluid flowing in a channel flow are examined. The main focus is on the particle migration and orientation, and interaction with yielded and unyielded fluid regions. The study demonstrates that the initial position, shape, and yield-stress characteristics of the fluid influence the equilibrium position and orientation attained by the particle.

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