Birefringence and computational studies of a polystyrene Boger fluid in axisymmetric stagnation flow

A polystyrene-based Boger fluid was studied in axisymmetric tube flow past an obstruction using flow birefringence. The shear and elongational rheology of the test fluid was typical of that observed in Boger fluids, and was fit to a FENE-P dumbbell model and an empirical two-mode Giesekus model. The FENE-P model predicts stress-optical failure owing to finite extensibility, and is capable of independent predictions of stress and flow birefringence in both simple and complex flows. Flow birefringence measurements in homogenous shear demonstrated surprising robustness of the stress-optical rule, even at rates where the polymer stress showed shear thinning and the FENE-P model predicts stress-optical failure. Viscoelastic flow simulations using the two-mode Giesekus model and assumption of the validity of the stress-optical rule were reasonably effective at predicting flow birefringence measured in forward stagnation flow. However, FENE-P simulations were less effective, due to the poor description of steady shear normal stress data which also obscured predicted consequences of finite extensibility to the flow birefringence. In rear stagnation flow, converged simulations could not be obtained at the experimental Weissenberg numbers. There are experimental indications of a flow instability in rear stagnation flow. (C) 2000 Elsevier Science B.V. All rights reserved.