Turbulent structure in low-concentration drag-reducing channel flows

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Abstract
A two-component laser-Doppler velocimeter was used to measure simultaneously velocity components parallel and normal to the wall in two fully developed, wellmixed, low-concentration (1-2 p.p.m.) drag-reducing channel flows and one turbulent channel flow of water. The mean velocity profiles, root-mean-square velocity profiles and the distributions of the ūv turbulent correlation confirm that the additives modify the buffer region of the flow. The principal influence of the additives is to damp velocity fluctuations normal to the wall in the buffer region.The structural results show that the average time between bursts increased for the drag-reducing flows. When compared to a water flow at the same wall shear stress, this increase in the timescale was equal to the increase in the average streak spacing. Conditionally averaged velocity signals of y+ = 30 centred on the leading edge of a burst, as well as those centred on the trailing edge, have the same general characteristics in all three flows indicating that the basic structure of the fundamental momentum transport event is the same in these drag-reducing flows. However, it was clearly shown that the lower-threshold Reynolds-stress-producing motions were damped while the higher-threshold motions were not damped. In the buffer region of the drag-reducing flows this yields a larger mean velocity gradient with damped fluctuations normal to the wall and increased fluctuations in the streamwise direction. It is hypothesized that some strong turbulent motions are required to maintain extended polymer molecules, which produce a solution with properties that can damp lower threshold turbulence and thereby reduce viscous drag.