Characterization of the non-Newtonian flow behavior of drag-reducing fluids

Abstract

The steady streaming flow induced by a cylinder oscillating in a fluid otherwise at rest has been employed to characterize the rheological properties of drag-reducing fluids. Flow reversal and several features common with turbulence make this flow on alternative to conventional viscometric measurements, which do not provide adequate information at very low polymer concentrations. Relevant fluid relaxation times are determined by measuring the steady secondary velocity profiles and then fitting theoretical predictions to the data. Laser Doppler velocimetry has been used to measure steady streaming velocities of well-characterized solutions of polyacrylamide, polyethylene oxide, and a surfactant mixture. Results show that the effective fluid elasticity, or relaxation time, increases with concentration or with frequency of oscillation of the primary flow. Furthermore, the relaxation time appropriate for characterizing steady streaming has been found to be far smaller than the longest relaxation time obtained from viscometric flows. The sensitivity to frequency is important, especially when associating a relaxation time with drag-reducing capability. A comparison with drag-reduction experiments suggests that ranking additives according to elasticity provides information about drag-reducing effectiveness. Steady streaming is proposed as a useful tool for characterizing the response of polymers and surfactants in a variety of unsteady nonviscometric flows.