A 3D NUMERICAL EXPERIMENTAL STUDY ON A STAGNATION FLOW OF A POLYISOBUTYLENE SOLUTION/

A combined 3D numerical/experimental method for the evaluation of visc oelastic constitutive equations is described. The method is applied to a three-dimensional (3D) stagnation flow, which exhibits strong elong ational deformations. A polyisobutylene solution is investigated. Lase r Doppler anemometry and flow induced birefringence are used to measur e pointwise velocities and linewise tin the depth of the flow cell) in tegrated stresses, respectively. The numerical simulations are decoupl ed: the 3D velocity field is derived from finite element calculations with the viscous Carreau model, whereafter the viscoelastic stresses a re calculated with a four mode Giesekus and Phan-Thien-Tanner (PTT) mo del. The validity of this approach is checked by comparing the calcula ted and measured velocities. The optical FIB signal is calculated from the integrated stresses, using the small retardance approximation. Th e approach is first evaluated for two slit flows with different depth/ height ratios (8:1 and 2:1). Calculated velocities and stresses show a good agreement with measurements. In the stagnation flow device, the calculated velocity field diverges from measured velocities at the hig her Weissenberg-numbers, at all Weissenberg numbers both viscoelastic models predict much too low stresses near the stagnation line. A modif ication of the PTT model is proposed to improve the stress predictions in the elongational flow domain. (C) 1998 Elsevier Science B.V. All r ights reserved.