Experimental removal of the re‐entrant corner singularity in tubular entry flows

Abstract

Understanding the behavior of viscoelastic fluids in regions of high stress is an important generic problem in viscoelastic fluid mechanics, while understanding the re‐entrant corner singularity in tubular entry flow is of significant importance in the numerical solution of this important test problem. Two constant viscosity elastic liquids (Boger fluids) are used for making streakline photographic observations in the 4 to 1 circular contraction. The first fluid is the organically based international test fluid M1, constructed by dissolving polyisobutylene in polybutene using kerosene as a third solvent. The second fluid is a 0.03% polyacrylamide (Separan MG500) dissolved in corn syrup using water as the third solvent. Both fluids exhibit significant elasticity, while at the same time a constant viscosity. Each is used for flow visualization in an abrupt and in a rounded entry with a 2 mm radius. The nature of the secondary flow vortex is examined for both fluids with and without the rounded entry. Although both fluids are constant viscosity elastic liquids with about the same zero shear relaxation time, each reacts very differently to the rounded corner. Dramatic changes are observed for the polyacrylamide based elastic fluid while less dramatic results and changes, as a result of the rounded corner, are observed for the polyisobutylene based fluid. The differences are explained in terms of the extendability of the molecules in a solution relative to their configuration at equilibrium, which is dependent upon the polymer–solvent interaction.