Influence of large-amplitude oscillations on turbulent drag



The effect of imposed large-amplitude oscillations on turbulent drag is studied. The system consists of water flow through a straight 5.08-cm pipe. The velocity gradient at the wall is measured with flush-mounted electrochemical mass-transfer probes. Newly developed numerical algorithms are used to analyze the probe performance in the presence of unsteady flows. Sinusoidal oscillations are at large enough frequencies, ω+ = 0.0138–0.0506, that a pseudo-steady-state approximation cannot be made. The ratio of the time-averaged velocity gradient at the wall, with and without oscillations, S̄, varies between 1.00 and 1.03, provided flow reversal does not occur. However, two experiments in which reversed flows existed at the wall for an appreciable period of time show drag reductions of 7 and 13%.

Imposed nonsinusoidal oscillations are also studied for a period of favorable pressure gradient, about twice longer than that of unfavorable, and two sudden changes in the pressure gradient. Experiments at Re = 9,700 with T0 of 2.00, 2.45, and 3.46 s, and at Re = 19,200 with T0 = 3.46 give values of S̄ = 1.04–1.08. At Re = 19,200 and T0 = 2.00, 1.50, 1.00 s, drag reductions are 10–15%. This phenomenon could be associated with the speed with which a flow adjusts to sudden changes in the pressure gradient.