VISCOELASTIC FLOW PAST A CONFINED CYLINDER OF A LOW-DENSITY POLYETHYLENE MELT

The capabilities of the exponential version of the Phan-Thien-Tanner ( PTT) model and the Giesekus model to predict stress fields for the vis coelastic flow of a low density polyethylene melt around a confined cy linder are investigated. Computations are based on a newly developed v ersion of the discontinuous Galerkin method. This method gives converg ent results up to a Deborah number of 2.5 for the falling sphere in a tube benchmark problem. Moreover, the specific implicit-explicit imple mentation allows the efficient resolution of problems with multiple re laxation times which are mandatory for polymer melts. Experimentally, stress fields are related to birefringence distributions by means of t he stress optical rule. Three different fits, of equal quality, to ava ilable viscometric shear data are used: two for the PTT model and one for the Giesekus model. Comparison of computed and measured fringes re veals that neither of the models is capable of describing the full bir efringence pattern sufficiently well. In particular it appears difficu lt to predict both the birefringent tail at the wake of the cylinder t hat is dominated by elongational effects and the fringe pattern betwee n cylinder and the walls where a combined shear-elongational flow is p resent. (C) 1997 Elsevier Science B.V.