Experiments in turbulent pipe flow with polymer additives at maximum drag reduction

In this paper we report on (two-component) LDV experiments in a fully devel oped turbulent pipe flow with a drag-reducing polymer (partially hydrolyzed polyacrylamide) dissolved in water. The Reynolds number based on the mean velocity, the pipe diameter and the local viscosity at the wall is approxim ately 10000. We have used polymer solutions with three different concentrat ions which have been chosen such that maximum drag reduction occurs. The am ount of drag reduction found is 60-70%. Our experimental results are compar ed with results obtained with water and with a very dilute solution which e xhibits only a small amount of drag reduction. We have focused on the observation of turbulence statistics (mean velocitie s 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 we find a thickeni ng of the buffer layer and an increase in the slope of the logarithmic prof ile. With respect to the turbulence statistics we find for the streamwise v elocity fluctuations an increase of the root mean square at low polymer con centration but a return to values comparable to those for water at higher c oncentrations. The root mean square of the normal velocity fluctuations sho ws a strong decrease. Also the Reynolds (turbulent) shear stress and the co rrelation coefficient between the streamwise and the normal components are drastically reduced over the entire pipe diameter. In all cases the Reynold s stress stays definitely non-zero at maximum drag reduction. The consequen ce 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 sho ws a large transfer of energy directly to the polymers instead of the route by turbulence. The kinetic energy of the turbulence suggests a possibly ne gative polymeric dissipation of turbulent energy.