INFLUENCE OF LARGE-AMPLITUDE OSCILLATIONS ON TURBULENT DRAG

The effect of imposed large-amplitude oscillations On turbulent drag i s studied. The system consists of water flow through a straight 5.08-c m pipe. The velocity gradient at the wall is measured with flush-mount ed electrochemical mass-transfer probes. Newly developed numerical alg orithms are used to analyze the probe performance in the presence of u nsteady flows. Sinusoidal oscillations are at large enough frequencies , omega+ = 0.0138-0.0506, that a pseudo-steady-state approximation can not be made. The ratio of the time-averaged velocity gradient at the w all, with and without oscillations, SBAR, varies between 1.00 and 1.03 , provided flow reversal does not occur. However, two experiments in w hich reversed flows existed at the wall for an appreciable period of t ime show drag reductions of 7 and 13%. Imposed nonsinusoidal oscillati ons are also studied for a period of favorable pressure gradient, abou t twice longer than that of unfavorable, and two sudden changes in the pressure gradient. Experiments at Re = 9,700 with T(o) of 2.00, 2.45, and 3.46 s, and at Re = 19,200 with T(o) = 3.46 give values of SBAR = 1.04-1.08. At Re = 19,200 and T(o) = 2.00, 1.50, 1.00 s, drag reducti ons are 10-15%. This phenomenon could be associated with the speed wit h which a flow adjusts to sudden changes in the pressure gradient.