Stress- and Velocity-Field Evolution in Viscoelastic Planar Contraction Flow

Planar contraction flow was used as a model system to investigate two topics in vis-coelastic fluid mechanics: 1) the origin and structure of transitions in viscoelastic flow and 2) the transient elongational response of a polymer solution to shearfree flow. The flow cell was adjustable to contraction ratios of 2, 8, or 32. A semidilute polymer solution com-posed of high molecular weight (MW) polyisobutylene in medium MW polybutene was used as the test fluid. This "Boger " fluid was characterized by a long relaxation time and nearly constant viscosity; inertia had negligible influence on the flow. Laser Doppler velocimetry (LDV) and light sheet visualization were used to character-ize the spatiotemporal structure of, and onset conditions for transition from two-dimen-sional to three-dimensional, steady flow. The three-dimensional flow had the form of "viscoelastic Gtirtler " vortices previously observed in other flow geometries. Characteris-tic length scales of the three-dimensional flow indicated an influential role of curve