EXPERIMENTAL REMOVAL OF THE REENTRANT CORNER SINGULARITY IN TUBULAR ENTRY FLOWS

Understanding the behavior of viscoelastic fluids in regions of high s tress is an important generic problem in viscoelastic fluid mechanics, while understanding the re-entrant corner singularity in tubular entr y 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 th e 4 to 1 circular contraction. The first fluid is the organically base d international test fluid M1, constructed by dissolving polyisobutyle ne in polybutene using kerosene as a third solvent. The second fluid i s a 0.03% polyacrylamide (Separan MG500) dissolved in com 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 v isualization in an abrupt and in a rounded entry with a 2 mm radius. T he nature of the secondary flow vortex is examined for both fluids wit h and without the rounded entry. Although both fluids are constant vis cosity 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 dra matic results and changes, as a result of the rounded corner, are obse rved for the polyisobutylene based fluid. The differences are explaine d in terms of the extendability of the molecules in a solution relativ e to their configuration at equilibrium, which is dependent upon the p olymer-solvent interaction.