Abstract
Abstract In this work, turbulent drag reduction in a pipe is investigated by using laser Doppler velocimetry. The effect of decreasing the friction factor of the flow is obtained by addition of high molecular weight polymers. The mechanism of drag reduction is explained in terms of a stress anisotropy that inhibits the transversal transport of momentum by turbulent fluctuations. Semi-theoretical models based on a nonlinear constitutive equation, which takes into account an extra extensional rate of strain in the flow produced by the local additive orientation, are presented. The semi-theoretical models used to predict the friction factor of the flow in the presence of the polymer have successfully described the experimental measurements. The results have revealed a reduction in the friction factor of 65% for a concentration of 350ppm in volume of polyacrylamide (PAMA) in an aqueous solution. In addition, the flow statistics, such as the axial and radial velocity fluctuations, the normalized autocorrelation functions as well as the power spectra for both velocity fluctuation components, are examined for the Newtonian flow of pure water and the flow of a 120ppm solution of PAMA at the same friction velocity. Next, the results are compared in order to characterize the effect of the additive on the turbulent flow.
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