A previous paper from this laboratory presented measurements of turbulence that were made for water flowing over a wavy wall. The system was a 50 mm×600 mm rectangular channel for which the top wall was flat and the bottom wall was a train of sinusoidal waves with a wave length of 5 mm and a wave height of 0.50 mm. The Reynolds number, defined with the bulk velocity and the half-height of the channel, was 46,000. These conditions were such that a sand covered surface would be fully rough and characterized with a dimensionless sand roughness of k s +=104. The present paper presents measurements with Reynolds numbers of 11,000 and 3,200 for which the surface corresponds to the intermediate regime (k s +=22.4) and the hydraulically smooth regime (k s +=6.89). The measurements in the outer flow for k s +=22.4 differ from what is observed for k s +=104 in that the quadrant analysis of the Reynolds stress and the skewness coefficients for u i and v i are the same as found for flow over a flat boundary. Measurements of mean velocities, wall drag and turbulent intensities for k s +=6.89 are consistent with defining this surface as hydraulically smooth. Surprisingly the quadrant analysis of the Reynolds stress and measurements of the skewness factor for normal velocity fluctuations for k s +=6.89 are different from what is found for a flat surface. The contributions of quadrants 2 and 4 and their ratio are larger than what is found for a flat surface. Measurements for all three surfaces show smaller relative contributions for large wavenumber fluctuations. However, this is much more pronounced for k s +=6.89 and k s +=22.4.
Experiments in Fluids – Springer Journals
Published: Sep 26, 2003
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