RETRACTED: Experimental study of the drag reduction in turbulent pipe flow (Retracted article. See vol. 83, pg. 809, 2015)

In this work two-component LDV (Laser Doppler Velocimetry) technique was used to measure the effect of polymer on the drag reduction in a fully developed turbulent pipe flow. The Reynolds number based on the mean velocity, the pipe diameter and the local viscosity at the wall is approximately 10,000. The polymer solutions with three different concentrations are used and have been chosen such that maximum drag reduction occurs. The amount of drag reduction found is 60-70%. Our experimental results are compared with results obtained with water and with a very dilute solution which exhibits only a small amount of drag reduction. This study is focused on the observation of turbulence statistics (mean velocities and turbulence intensities) and on the various contributions to the total shear stress. The latter consists of a turbulent, a solvent (viscous) and a polymeric part. The polymers are found to contribute significantly to the total stress. With respect to the mean velocity profile, the results show a thickening of the buffer layer and an increase in the slope of the logarithmic profile. With respect to the turbulence statistics the results show that for the streamwise velocity fluctuations an increase of the root mean square at low polymer concentration but a return to values comparable to those for water at higher concentrations. The root mean square of the normal velocity fluctuations shows a strong decrease. Also the Reynolds (turbulent) shear stress and the correlation coefficient between the streamwise and the normal components are drastically reduced over the entire pipe diameter. In all cases the Reynolds stress stays definitely non-zero at maximum drag reduction. The consequence of the drop of the Reynolds stress is a large polymer stress, which can be 60% of the total stress. The kinetic-energy balance of the mean flow shows a large transfer of energy directly to the polymers instead of the route by turbulence. The kinetic energy of the turbulence suggests a possibly negative polymeric dissipation of turbulent energy. (C) 2013 Elsevier Ltd. All rights reserved.