Observations of asymmetrical flow behaviour in transitional pipe flow of yield-stress and other shear-thinning liquids

The purpose of this brief paper is to report mean velocity profile data for fully developed pipe flow of a wide range of shear-thinning liquids together with two Newtonian control liquids. Although most of the data reported are for the laminar-turbulent transition regime, data are also included for laminar and turbulent flow. The experimental data were obtained in unrelated research programmes in UK, France and Australia, all using laser Doppler anemometry (LDA) as the measurement technique. In the majority of cases, axisymmetric flow is observed for the laminar and turbulent flow conditions, although asymmetry due to the Earth's rotation is evident for the laminar flow of a Newtonian fluid of low viscosity (i.e. low Ekman number). The key point, however, is that for certain fluids, both yield-stress and viscoelastic (all fluids in this study are shear-thinning), asymmetry to varying degrees is apparent at all stages of transition from laminar to turbulent flow, i.e. from the first indications to almost fully developed turbulence. The fact that symmetrical velocity profiles are obtained for both laminar and turbulent flow of all the non-Newtonian fluids in all three laboratories leads to the conclusion that the asymmetry must be a consequence of a fluid-dynamic mechanism, as yet not identified, rather than imperfections in the flow facilities.