Drag reduction using surfactants in a rotating cylinder geometry

Turbulent Couette flow between two circular cylinders has been used for drag reduction experiments using surfactants. In the experiments presented here, only the outer cylinder rotates, the inner cylinder remains at rest and accurate measurements of the torque at the inner cylinder are measured. Water is used as a reference fluid. A drag reducing surfactant called Arquad S-50 (Akzo Nobel Surface Chemistry LLC, Chicago, Ill., USA) (5 mM)+NaSal (12.5 mM) was used as the drag reduction agent. This surfactant can reduce the drag up to 70% (a Reynolds number of about 70,000–150,000) as measured by pressure drop in a pipe flow. Experiments in Couette flow also show drag reduction in the turbulent range. Two arrangements were used, (1) one small trip-wire on the inner cylinder, and (2) four larger trip-wires on the outer cylinder. These trips reduce the critical Reynolds number for transition from laminar to turbulent flow. In case (1), we obtained 18% drag reduction at 5,000<Re<15,000 and in case (2), we obtained an average reduction of about 20% at 2,000<Re<10,000, increasing up to 30% at Re=15,000. The paper also discusses two important problems. First, the shear rate is not constant in the radial gap in circular Couette flow. For non-Newtonian fluids, where the molecular viscosity is a function of the shear rate, this effect must be considered. Second, which viscosity should be used in the Reynolds number? For pipe flow measurements, most authors use the viscosity of the solvent (generally water and Newtonian). For measurements in the Couette flow, we use a different approach, which is described in this paper. We conclude that Couette flow is a useful method for drag reduction investigations. Its advantage is the much smaller geometry in comparison to those of conventional test facilities such as wind tunnels, water, or oil channels or in tubes.