ARE WEAKLY MOBILE-BED FLOWS A SPECIAL CLASS OF WALL-BOUNDED FLOWS?

摘要

Using an extensive experimental data set we show that although some turbulencernproperties of weakly mobile and fixed gravel-bed flows are similar (vertical distributionrnof broad-band characteristics, velocity spectra, cross-correlations, etc.), others arerndifferent. Weakly mobile bed flows differ from fixed bed flows in that: (1) the vonrnKarman constant is appreciably less than 0.40; (2) the hydrodynamic roughnessrnlength is appreciably lower; (3) the relative turbulence intensity (when normalised onrnthe local mean velocity) is less; and (4) the frequency of bursting events is reduced.rnThese differences reflect the effect of drag reduction due to the bed mobility andrnsupport Gust and Southard’s [1983] suggestion that mobile bed flows represent arnspecial class of wall-bounded flows. We propose a physical explanation for the dragrnreduction in weakly mobile gravel-bed flows, which is based on consideration of therneddy density and turbulence intensities and supported by the measurements.