The structure of turbulent shear-induced countercurrent flow
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abstract
Countercurrent flow is a generalized plane Couette flow in which a shear-induced drift current is opposed by a pressure-driven return flow such that the resulting mass flux is zero. This type of flow is encountered in both environmental fluid mechanics and tribology. Measurements are described that were undertaken in steady turbulent countercurrent flow, generated in a novel type of apparatus between smooth walls, at Reynolds numbers, expressed in terms of surface velocity and depth of the flow, from 200 to 20000; the critical Reynolds number of laminar to turbulent transition as determined herein is approximately 1750. The laboratory facility is explained, and experimental data are presented on mean velocities, pressure gradient, turbulence intensities, Reynolds stresses, and energy spectra. It is found that the velocity distributions follow the universal law of the wall in the drift current, but that the flow is undeveloped in the return portion of the flow. The non-dimensional longitudinal pressure gradient increases with Reynolds number, and a semi-empirical law of resistance is proposed and experimentally verified. Turbulence intensities in the drift current increase toward the shearing surface but are essentially constant in the opposing pressure-driven flow. The distribution of the Reynolds stress is found to be consistent with previous measurements obtained in this type of flow using different apparatus, but only approximately follows the theoretically linear distribution. Energy spectra in the shear-induced portion of the flow involve higher frequencies closer to the shearing surface than in the return current. There, the spectral energy is essentially the same throughout the flow, reflecting the constancy of turbulent intensity in this region.
