Flow of a Boger fluid around an isolated cylinder

Abstract

The flow around an isolated cylinder with a Boger fluid was studied experimentally by partially immersing a small vertical cylinder in a test fluid contained in an annular tank slowly rotating on a turntable. To approximate isolation, the ratio of the tank width to cylinder diameter varied from 40/1 to 300/1, and Reynolds numbers ranged from 10−5 to 0.5. Drag was measured by a custom-made mechanical/optical system and the flow field was mapped using particle image velocimetry (PIV). The experimental liquids were two Newtonian fluids and two polyisobutylene/polybutene Boger fluids, and the cylinders had various diameters. The Newtonian drag data, corrected for end and wall effects, agree with Kaplun's low-Re asymptotic formula. Elasticity increased the drag, starting at a Deborah number of 0.6, by 50% at a De of 2.5, and by more at higher Deborah numbers. PIV measurements were made around a cylinder—upstream, downstream, and transversely—and at 180° from it. The velocity measurements showed that the wake increased with De, among other findings. To relate the increased drag to fluid elasticity, normal stresses due to elasticity were found from deformation rates derived from the PIV data, but these stresses appear to be insufficient to account for the effect.