Momentum transfer in turbulent pipe flows (d = 51 mm) with and without a drag reducer was studied. Age distributions of coherent fluid structures at the pipe wall were determined at different Reynolds numbers (104<Re<6 × 104) using laser-Doppler velocimetry (LDV) and a suitable discriminating procedure. Mean ages, mean transfer thicknesses, friction factors, and time-averaged axial velocity profiles derived from LDV signals were compared with those from axial pressure gradients using the random surface-renewal model. The distribution of the ages was random, and drag reducers increased the mean age and the mean transfer thickness of the fluid elements at the wall, thus decreasing momentum and heat transfer at the fluid–wall interface. The mechanism of single-surface renewals was derived from conditionally averaged, local axial fluid velocities obtained from LDV signals.
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