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1	Linear stability analysis of viscoelastic fluid extrusion through a planar die Πέττας, Διονύσιος 02 June 2015 (has links) It is well-known that, increasing the flow rate in polymer extrusion, the flow becomes unstable and the smooth extrudate surface becomes wavy and disordered to an increasing degree. In order to investigate the mechanisms responsible for these instabilities we perform a linear stability analysis of the steady extrusion of a viscoelastic fluid flowing through a planar die under creeping flow conditions. We consider the Phan-Thien-Tanner (PTT) model to account for the viscoelasticity of the material. We employ the mixed finite element method combined with an elliptic grid generator to account for the deformable shape of the interface. The generalized eigenvalue problem is solved using Arnoldi’s algorithm. We perform a thorough parametric study in order to determine the effects of all material properties and rheological parameters. We investigate in detail the effect of interfacial tension and the presence of a deformable interface. It is found that the presence of a finite surface tension destabilizes the flow as compared to the case of the stick-slip flow. We recognize two modes which are found to become unstable beyond a critical value of the Weissenberg number and perform an energy analysis to examine the mechanisms responsible for the destabilization of the flow and compare against the mechanisms that have been suggested in the literature. / -- Finite element Numerical analysis Computational fluids Computational Moving boundaries Extrusion Linear stability Spurt flow Sharkskin Instabilities 532.053 3 Πεπερασμένα στοιχεία Αριθμητική ανάλυση Ρευστομηχανική Κινούμενα σύνορα Εκβολή Γραμμική ανάλυση Αστάθεια

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Linear stability analysis of viscoelastic fluid extrusion through a planar die