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

Abstract

In this paper we report on (two-component) LDV experiments in a fully developed tur-bulentpipe flow with a drag-reducing polymer (partially hydrolyzed polyacrylamide) dissolved inwater. The Reynolds number based on the mean velocity, the pipe diameter and the local viscosity atthe wall is approximately 10000. We have used polymer solutions with three different concentrationswhich have been chosen such that maximum drag reduction occurs. The amount of drag reductionfound is 60 70%. Our experimental results are compared with results obtained with water and witha very dilute solution which exhibits only a small amount of drag reduction.We have focused on the observation of turbulence statistics (mean velocities and turbulenceintensities) and on the various contributions to the total shear stress. The latter c! onsists of a turbulent,a solvent (viscous) and a polymeric part. The polymers are found to contribute significantly to thetotal stress. With respect to the mean velocity profile we find a thickening of the buffer layer and anincrease in the slope of the logarithmic profile. With respect to the turbulence statistics we find for thestreamwise velocity fluctuations an increase of the root mean square at low polymer concentrationbut a return to values comparable to those for water at higher concentrations. The root mean square ofthe normal velocity fluctuations shows a strong decrease. Also the Reynolds (turbulent) shear stressand the correlation coefficient between the streamwise and the normal components are drasticallyreduced over the entire pipe diameter. In all cases the Reynolds stress stays definitely non-zero atmaximum drag reduction. The consequence of the drop of the Reynolds stress is a large polymerstress, which can be 60% of t! he total stress. The kinetic-energy balance of the mean flow shows alarge transfer of energy directly to the polymers instead of the route by turbulence. The kinetic energyof the turbulence suggests a possibly negative polymeric dissipation of turbulent energy