title: The Transition to Asymmetry in Pipe Flow of Shear-thinning Fluids
Creators: name: Wen, CHAOFAN
Publisher: name: University of Liverpool Repository
Year: 2017

description: This thesis focuses on a particularly confusing aspect of the pipe flow of shear-thinning polymer solutions, which remains unexplained despite its ubiquity in all experimental results since its discovery around 2 decades ago. This is the observation that, time-averaged velocity profile was axisymmetric when the flow was purely laminar or fully turbulent, contrary to expectations it was asymmetric when flow is in the vicinity of transitional regime. This phenomenon was repeatable and many potential causes have been ruled out, such as the rotation of the earth, curvature of the pipe axis, major imperfection of flow geometry, upstream and downstream disturbances or temperature gradients. Therefore, this asymmetry is thought to be attributed to some, as yet unknown, fluid mechanism. In this work we provide new experimental results by stereoscopic particle image velocimetry that simultaneously capture the three-component velocity field across the entire pipe cross-section, which provides a much clearer picture of the behaviour of the asymmetric velocity profile and reveals that the asymmetry is produced by a pathway that has not previously been proposed. We first investigated the characteristics of flow asymmetry using an aqueous solution of 0.15% xanthan gum, which is essentially shear-thinning without large elastic effects. We experimentally observed that the asymmetry is not stationary but is actually time-varying, although it does have a preferred orientation in cross-sectional plane. The asymmetry occurs when Reynolds number (Re) exceeds a critical value Rec and the degree of asymmetry increases steadily with Re until the onset of turbulent puff. In fact, the asymmetry approximately grows with the square root of Re, suggesting a supercritical instability of the laminar base state leading to the flow asymmetry. The asymmetry is also non-hysteretic and reversible and contrary to what was previously believed, the classical laminar-turbulent transition is actually responsible for returning symmetry to the flow. Different concentrations of xanthan gum solutions, namely 0.07%, 0.08%, 0.1%, 0.125%, 0.15%, 0.2% were investigated to study the effect of shear-thinning characteristics on asymmetry. The asymmetry is prevalent throughout all xanthan gum solutions with different shearthinning characteristics and the greater the shear-thinning characteristics, the stronger the fluid asymmetry will appear. However, the critical Reynolds number for transition from axisymmetry to asymmetry remains unchanged ( 2000) when the shear-thinning characteristics of fluid varied. The shear-thinning characteristics also delay the onset of laminar turbulent transition compared with Newtonian fluids. We also investigated the effect of viscoelastic characteristics on asymmetry using polyacrylamide (PAA) aqueous solution. The asymmetry was absent in pure PAA solutions with concentration of 0.09%, 0.125% and 0.2%. However, once the shear-induced degradation was deliberately imposed on the 0.125% PAA solution to destruct the viscoelasticity while preserving the shear-thinning characteristics, the asymmetry reappears. The asymmetry in degraded PAA solution was also observed to evolve in a supercritical form which is similar to that of xanthan gum solutions. The 100, 200 and 500ppm PAA was added into 0.15% xanthan gum, the maximum degree of asymmetry was decreased with higher addition concentration of PAA, also suggesting the viscoelasticity inhibits asymmetry.