Numerical simulation of complex fluid flows with moving interfaces

• Autor(a) principal: França, Hugo Leonardo
• Data de Publicação: 2023
• Tipo de documento: Tese
• Idioma: eng
• Título da fonte: Biblioteca Digital de Teses e Dissertações da USP
• Texto Completo: https://www.teses.usp.br/teses/disponiveis/55/55134/tde-30082023-150314/
• topic: Complex fluids; Escoamentos com superfície livre; Fluidos complexos; Free surface flows; Numerical solution; Soluções numéricas

Flows of different types of non-Newtonian fluids are numerically investigated with a focus on complex problems, as for instance confined flows in geometries with singularities and moving interface flows with surface tension. For confined geometries, the novel natural stress formulation is used to represent the polymeric stress tensor in viscoelastic flows, and we show that greater accuracy is obtained near geometrical singularities in comparison to the traditional Cartesian formulation. For flows with a moving interface, we propose and validate a new algorithm based on machine learning to estimate the curvature in Front-Tracking interfaces, showing that it can provide similar results compared to more traditional approaches. Our viscoelastic implementation is tested with the Phan-Thien-Tanner model for the problem of binary droplet colisions. We provide maps of outcomes associated with the categories of Bouncing, Coalescence, and Separation as functions of the dimensionless numbers that govern the problem. In addition to the traditional Newtonian space defined by the Weber and the impact factor, associated with the collision angle, we also explore the Weissenberg number and the extensibility parameter in the PTT model. For non-bouncing scenarios, the results show that surface tension and elasticity act to maintain the integrity of the merged drop and avoid Separation. On the other hand, shearthinning effects induce the Separation outcome. Hence, in the PTT model there are opposite trends associated with elasticity and shear-thinning, what can lead to non-monotonic responses. We also study the spreading of an elastoviscoplastic droplet over a thin-film. By modelling an elastoviscoplastic material using Saramitos model, we perform a nondimensional analysis to understand the competition between surface tension and yield-stress, and how elasticity affects this balance. We can see that, for less viscous fluids, elasticity can greatly increase the spreading of a droplet, since the internal resisting stresses develop more slowly due to the polymeric relaxation time. This effect is more pronounced for materials of high yield-stress, which indicates elasticity has a greater impact for elastic solids than fluids. We believe the results in this thesis could shed light on the importance of elastic parameters in common industrial problems such as 3D printing with spreading of droplets or sprays with droplet coalescence and breakup.