BIREFRINGENCE AND LASER-DOPPLER VELOCIMETRY (LDV) STUDIES OF VISCOELASTIC FLOW-THROUGH A PLANAR CONTRACTION

Laser-Doppler velocimetry (LDV) and flow-induced birefringence (FIB) are used to measure the velocity and stress fields of a rheologically well characterized viscoelastic solution flowing through a 3.97:1 planar abrupt contraction. The FIB system is designed to give accurate, spatially resolved measurements of the birefringence through the contraction flow. The fluid is a 5.0 wt.% solution of polyisobutylene in tetradecane that exhibits shear-thinning of both the viscosity and the first normal stress coefficient. Shear-rate-dependent Deborah numbers for the contraction flow are accessible in the range 0 less-than-or-equal-to De(gamma overdot) less-than-or-equal-to 0.8. Measurements show the evolution of the shear stress and first normal stress difference throughout the converging flow upstream of the contraction plane as well as downstream of the contraction. Measurements of the stresses along lines emanating from the reentrant comer of the contraction follow the power-law scalings given by the analysis of the flow of Newtonian and power-law fluids near a comer, suggesting that the shear thinning of the fluid reduces the importance of ''elasticity'' near the comer.