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Title: Numerical study of filament-stretching and step-strain in viscoelastic fluid flows
Author: Ban, Mohammad Javad
Awarding Body: Swansea University
Current Institution: Swansea University
Date of Award: 2007
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This thesis is concerned with the numerical prediction of two-dimensional viscoelastic filaments under stretching and step-strain within cylindrical-like domains. A hybrid finite element/finite volume (fe/fv) scheme has been implemented in this study to solve the governing equations (mass and momentum conservation and constitutive model). A time-stepping procedure is utilised in the fe/fv algorithm. A number of rheological models have been employed to stimulate the desired rheological response. Amongst these is the Oldroyd-B model. This is considered as a strong strain-hardening model being widely used due to its sound physical background and its ability to reproduce qualitative response of polymer melts in rheometrical flows. The linear version of Phan-Thien/Tanner (LPTT) and Giesekus models are also studied to compare simulation results for both dilute and concentrated polymeric systems against the Oldroyd-B model. For fluids with higher degree of strain-hardening, larger stress values cause a reduction in stretching period. In addition, Boger-like response has been represented under increasing levels of solvent within the system. Filament-stretching has been studied under two modes of stretching, exponential and linear for multi-mode and single-mode representations, that has included a numerical study on mesh refinement and algorithms developed for free-surface movement. Bead-like structure formation has been studied for a variety of surface tension coefficients in the absence/presence of body forces. ALE methods and free-surface techniques have been analysed for Volume-of-Fluid (VOF) mesh and Compressed-Mesh (CM) procedures. VOF mesh procedures are outperformed by their CM counterparts. For free-surface curvature to be determined precisely, a particle-tracking approach has been found to be preferable to a kinematic condition for surface-level. Variation of anisotropy levels and xi-parameter settings has been studied for the Gieskus and LPTT models, respectively. A further chapter is included where the recently addressed subject of step-strain is considered, to simulate sudden cessation of stretching across the three viscoelastic models. Sudden decline and sharp rise in axial stress have been observed and interpreted alongside filament radial evolution in the context of step-strain. The effect of inertia has been neglected but the effect of capillary and body forces has been brought into consideration. Larger stress values are observed for fluids with a higher degree of strain-hardening, and consequently, cause an increase in the step- strain period. Similar dynamic trends are followed for LPTT fluids with parameter settings of xi={0.0,0.13} under the context of step-strain. Here, rheological differences would emerge in shear. A paper which has been recently submitted for publication is included in the appendix. There, different aspects of gradual plate halt are discussed.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available