Executive Summary : | Recent research on the transition to turbulence in polymer solutions reveals that the flow becomes linearly unstable at a high Reynolds number (Re), with a monotonic dependence of Reynolds number with increasing polymer concentration. High polymer concentrations destabilize the flow at a lower Re than flows with low polymer concentrations due to higher elasticity. Highly concentrated polymer solutions destabilize at very low Re, possibly due to the shear-thinning nature of these solutions or higher value elasticity compared to dilute polymer solutions. To understand the flow instability, it is necessary to separate the shear-thinning and elasticity effects. Dilute polymer solutions dissolved in a highly viscous solvent lead to a solution with high elasticity without a shear-thinning effect. Carboxymethyl cellulose is used to prepare a shear-thinning solution with no elasticity effect. Pressure drop measurements, pressure fluctuations, dye stream visualisation, and particle image velocimetry are used to detect the transition to turbulence. Moody charts are developed to detect the onset of transition and provide the extent of drag increase compared to laminar flow. Linear stability analysis predicts laminar-turbulent transitions for polymer solutions through tubes and channels accurately, and direct numerical simulation (DNS) can be used for subcritical transitions. Understanding the nature and cause of transitions could significantly contribute to the understanding of shear-thinning instability, which is present in additive manufacturing (3D printing) and could impact product quality, formation time, and production costs. |