LDV Measurements of Viscoelastic FlowTransitions in Abrupt Axisymmetric Contraction: Interaction of Inertia andElasticity

WILLIAM RAIFORD; LIDIA MARÍA QUINZANI; PAUL COATES; ROBERT ARMSTRONG; ROBERT BROWN

JOURNAL OF NON-NEWTONIAN FLUID MECHANICS

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 1989 vol. 32 p. 39 - 68

0377-0257

Laser Doppler velocimetry (LDV) measurements of the axial andradial velocity components are reported for the axisymmetric contraction flowof a viscoelastic solution of polyisobutylene dissolved in tetradecane. Thisfluid exhibits shear-thinning of both the viscosity and first normal stressdifference; the viscosity is well described by the Carreau-Yasuda model. Shear-rate-dependentReynolds and Deborah numbers are used to describe the flow conditions. Measurementsat low flow rates for contraction ratios of 2:1, 4:1, and 8:1 indicate that theflow near the contraction plane scales with the small tube radius. Aviscoelastic corner vortex is detected at moderate flow rates, yet vortexgrowth is not observed with increased flow rate, because inertial forces pressthe vortex into the outer comer. At high flow rates, where both the Reynoldsnumber and Deborah number are large, the flow near the contraction plane is dividedinto an accelerating core and an outer region where the flow retains itsupstream profile. The magnitude of the local rate-of-strain p is calculatedfrom the axial and radial velocity profiles. In the core, the magnitude of p> 150 s-l and its value is dominated by the axial gradients au,/& anda@~. Large extension rates along the centerline suggest that extensionalthinning occurs in this core region. Finite element calculations for aCarreau-Yasuda fluid with the same viscosity as the PIB/C14 solution and underthe same flow conditions do not predict the two-regions of the flow.