Numerical simulation of three-dimensional viscoelastic flow using the openboundary condition method in coextrusion process

Three-dimensional numerical simulation of coextrusion process of two immisc ible polymers through a rectangular channel has been performed using the fi nite element method. The upper convected Maxwell (UCM) model and the Phan-T hien and Tanner (PTT) model were considered as viscoelastic constitutive eq uations. The elastic viscous stress splitting (EVSS) method was adopted to treat the viscoelastic stresses, and the streamline upwinding (SU) method w as applied to avoid the failure of convergence at high elasticity. The prob lem arising from the ambiguous outlet boundary condition that has previousl y been used in the three-dimensional simulation of a viscoelastic coextrusi on process could be avoided by introducing the open boundary condition (OBC ) method. The abrupt change or deviation of contact line position near the outlet that was observed when the fully developed outlet boundary condition was applied could be clearly removed by using the OBC method. The effects of viscoelastic properties, such as the shear viscosity ratio, the elastici ty, the second normal stress difference, and the extensional viscosity on t he interface distortion, the interface curvature, and the degree of encapsu lation along the downstream direction have been investigated. The shear vis cosity ratio between the polymer melts was the controlling factor of the in terface position and the encapsulation phenomena. The interface distortion seems to increase as the elasticity ratio increases under constant shear vi scosity, even though it is not so large. The degree of encapsulation seems to increase with increasing the ratio of the second normal stress differenc es. The extensional viscosity had minor effect on the encapsulation phenome na. The second normal stress difference was found to have a great influence on the increasing of the degree of encapsulation along the downstream dire ction as compared to the effect of the first normal stress difference.. (C) 2001 Elsevier Science B.V. All rights reserved.