Taylor-Couette flow of shear-thinning fluids

Abstract

The flow between two concentric cylinder, one of which is rotating (Taylor-Couette flow), has been the focus of extensive research, due to the number of flow instabilities that may occur and its use in various industrial applications. We examine Taylor-Couette flow of Newtonian and shear-thinning fluids (solutions of xanthan gum in water/glycerol) using a combination of Particle-Image Velocimetry and flow-visualisation for a wide range of Reynolds number, spanning the Circular Couette Flow, Taylor Vortex Flow and Wavy Vortex Flow regimes. Shear-thinning is associated with an increase in the axial wavelength and has a non-monotonic effect on the critical Reynolds number for transition to Taylor Vortex Flow and Wavy Vortex Flow. The magnitude of vorticity and the strength of the radial jets transporting fluid away from the inner cylinder ('outward jets') are both reduced in shear-thinning fluids relative to the Newtonian case; the vorticity in the shear-thinning fluids also tends to concentrate at the edges of vortices, rather than in the cores. In the Wavy Vortex Flow regime for Newtonian fluids, the amplitudes of the waves at the 'inward jets' (moving towards the inner cylinder) are low compared to those at the outward jets. However, for the shear-thinning fluids, the amplitudes of the waves at both the inward and outward jets tend to be significantly larger. Finally, shear-thinning is associated with greater variations in time and space: we observe slow drifts in the axial positions of vortices and spatial variations in the amplitudes of the wavy instability, which are absent in Newtonian fluids.

Taylor–Couette flow of polymer solutions with shear-thinning and viscoelastic rheology

“…The tendency for shear-thinning to cause a reduction in the critical Reynolds number has been noted in previous studies (Lockett et al 1992;Escudier et al 1995;Alibenyahia et al 2012;Cagney & Balabani 2019b). The elasticity also appears to be associated with a progressive destabilisation of the flow (figure 6c), which is not always the case for Boger fluids (Muller 2008), although it is not straightforward to disentangle the effects of shear-thinning and viscoelasticity.…”

“…The no slip condition at the ends means that the first jet near the ends (i.e. one vortex spacing away from the end walls) move fluid radially outward (Imomoh et al 2010;Cagney & Balabani 2019b); using this information, it can be seen from figure 12(a) that the wavy instability is strongest at the inward jets and relatively weak at the outward jets, regardless of the axial location.…”

Taylor-vortex flow in shear-thinning fluids

“…Actually, there is an interplay between nonlinear inertia terms and nonlinear viscous terms. Based on the experimental results such the streamline patterns in Taylor vortices represented by Escudier et al [22] and the spatiotemporal plots reported by Cagney and Balabani [24] for different shear-thinning fluids, the Taylor vortex flow observed can be considered as periodic with a wavenumber k close to the critical value k c . Here we take k = k c and we concentrate on solutions that consist of this mode plus modes that can be generated from it through the nonlinearity.…”

Flow around an oscillating grid in water and shear-thinning polymer solution at low Reynolds number

“…This last uncertainty is maximal when the slope of the curve in Fig. 2 is the steepest, i.e., at the lowest effective flow index (as defined in Cagney and Balabani (2019)). This is here the case for shear rates around 4 s −1 .…”