A Comparison of Rheological Drag Reduction in Wall Turbulence Using Different Additives
Creators: Warwaruk, Lucas
Publisher: name: University of Alberta Library
Year: 2023

<p>It is well known that long-chain polymers and surfactants can significantly reduce the skin-friction drag of turbulent liquid flows; a phenomenon often referred to as rheological drag reduction. However, it is unclear if the mechanism for drag reduction is common among different types of polymers and surfactants. In the present dissertation the rheology and drag-reducing capabilities of three different additives are compared, including a flexible polymer, a rigid polymer and a cationic surfactant. Educated predictions regarding each additives mechanism for drag reduction are made.</p>

<p>Aqueous solutions of flexible polymers exhibit viscoelastic non-Newtonian rheology, and a good ability to reduce drag in turbulent channel and boundary layer flows. Measurements of steady shear rheology indicate that drag-reducing flexible polymer solutions are only marginally shear thinning. That being said, the same solutions have an appreciable extensional relaxation time, as demonstrated by extensional rheology measurements using a capillary break up extensional rheometer (CaBER) and dripping onto substrate (DoS) rheometer. In a turbulent channel flow with a Reynolds number Re of approximately 30 000, flexible polymer solutions achieve drag reduction (DR) percentages as large as 70% and a mean velocity profile that straddles the maximum drag reduction (MDR) limit. A turbulent boundary layer comprised of flexible polymers with low amounts of DR, indicate that skin-friction drag is reduced from a near-wall attenuation of vorticity and extensional flow motions - particular biaxial extension. As a result, the polymer-laden boundary layer exhibits more two-dimensional and shear-dominate flow within the conventional limits of the buffer layer, indicative of an expansion in the viscous sublayer and flow parobolization.</p>

<p>Similar to flexible polymers, solutions of rigid polymers can exhibit large amounts of DR in a turbulent channel flow; however, the mechanism for reducing drag in rigid polymers is seemingly different. A rigid polymer solution that is capable of imparting the same amount of DR as a flexible polymer solution in a turbulent channel flow tends to have a larger overall shear viscosity, more shear thinning, but no measurable extensional relaxation time using CaBER and DoS. Therefore, drag reduction using rigid polymers is largely driven by the shear thinning rheology of the solution. Gradients in the mean velocity coupled with the solutions shear thinning rheology generate an effective slip within the buffer layer and a reduction in skin friction drag.</p>

<p>Unlike the polymeric solutions, drag-reducing solutions of cationic surfactants do not have a shear thinning viscosity, nor do they have a measurable extensional relaxation time from CaBER and DoS rheometry. Instead, surfactant solutions have a shear and extensional rheology similar to water, despite their ability to achieve a large DR of 70% in a turbulent channel flow. To discern the non-Newtonian qualities of the surfactant solution, the laminar flow of the drag-reducing fluids were compared in a periodically constricted tube (PCT), where the tube walls vary sinusoidally with respect to the streamwise direction. Although the PCT flow is not rheometric, it is also not as complex as wall turbulence and provides a comparison among the polymeric and surfactant fluids in a nontrivial flow with mixed kinematics. Above an Re of 100 within the PCT, certain surfactant solutions exhibit a similar inertioelastic flow pattern as flexible polymers. Due to the sudden onset of inertioelastic flow with increasing Re, evidence is provided that flow-induced structures develop within the surfactant solutions. These flow-induced structures produce similar rheological features as flexible polymers, and most likely, a common means for reducing drag.    </p>