The Molecular Stress Function (MSF) constitutive model with convective constraint release (CCR) mechanism has been shown to accurately fit a large range of viscometric data, and also shown to give strong vortex growth in flows of LDPE through planar and axisymmetric contractions. This work compares simulation and experimental results for 3-D flows of Lupolen 1840H LDPE through a contraction slit. Comparisons are made with vortex opening angles obtained from streak photography, and also with stress-birefringence measurements. The comparisons are made with two versions of the CCR mechanism. The simulated vortex angles for one version of the CCR mechanism are found to approach what is seen experimentally. The best-fit value for the stress optical coefficient was found to vary between CCRs and to decrease with flow rate. This is partially explained by different centreline elongational rates with the two CCRs, which in turn is related to different opening angles.
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