Taylor vortices in Newtonian and shear-thinning liquids

Abstract

The flow structure in a concentric annular geometry with a radius ratio of 0.506 has been investigated for inner cylinder (centrebody) rotation. Detailed velocity measurements made with a laser Doppler anemometer for an aqueous solution of glucose (Newtonian), at a Taylor number well above the critical value, reveal that the tangential velocity component has a periodic structure of the same wavelength as that for the axial component, but with the extrema corresponding to zero axial velocity. Higher values of the maximum axial velocity and velocity gradient are observed closer to the centrebody than at the outer wall, whilst the radial location of zero axial velocity in the vortex interior (i. e. the eye of the vortex) moves towards the outer wall with increasing Taylor number. Similar measurements for an aqueous solution of Xanthan gum, which is strongly shear thinning and slightly elastic, revealed that the asymmetry in the maximum axial velocities was more marked than for the Newtonian case with a significant radial shift in the location of the vortex eye towards the centrebody. It was also found that the vortices exhibit a slow axial drift in the direction opposite to the centrebody rotation vector, whereas there was zero drift in the Newtonian case. The vortex cell structure for the second non-Newtonian fluid, a Laponite/CMC aqueous blend, which is shear thinning and also thixotropic, was very similar to that for the Xanthan gum suggesting that the shear-thinning aspect of the fluid rheology for both non-Newtonian fluids was far more significant than either thixotropy or viscoelasticity. An axial drift was again apparent, but in the opposite direction to that for the Xanthan gum, indicating that this effect is associated with the differences in the rheological characteristics of the two fluids.