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

Laser-Doppler velocimetry (LDV) and flow-induced birefringence (FIB) a re used to measure the velocity and stress fields of a rheologically w ell characterized viscoelastic solution flowing through a 3.97:1 plana r abrupt contraction. The FIB system is designed to give accurate, spa tially resolved measurements of the birefringence through the contract ion flow. The fluid is a 5.0 wt.% solution of polyisobutylene in tetra decane that exhibits shear-thinning of both the viscosity and the firs t 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 throu ghout the converging flow upstream of the contraction plane as well as downstream of the contraction. Measurements of the stresses along lin es emanating from the reentrant comer of the contraction follow the po wer-law scalings given by the analysis of the flow of Newtonian and po wer-law fluids near a comer, suggesting that the shear thinning of the fluid reduces the importance of ''elasticity'' near the comer.