Turbulence properties in the region of maximum drag reduction

The influence of drag reducing polymers on the turbulence structure close to a wall was studied by electrochemical techniques. By measuring the electric current flowing under conditions where the reaction rate at the electrolysis cell test electrode is controlled by the mass transfer rate, it is found that the addition of drag reducing polymers decreases the frequency of the velocity fluctuations close to the wall. However, a comparison made with Newtonian fluids with the same wall velocity gradient shows no change, a conclusion supported by measurements of the spectral density function up to drag reductions of 60%. The results suggest that drag reducing polymers increase dampening of the small scale turbulence and that up to the 60% drag reduction the rise can be correlated with the larger scale of wall oriented eddies. The relation between close to the wall velocity fluctuations and drag reduction ceases at large polymer concentrations where the pressure loss is approximated by Virk's asymptote for maximum drag reduction.