Author(s): Vladimir I. Nikora; Derek G. Goring
Linked Author(s):
Keywords: Turbulence; Bed-load; Drag reduction; Coherent structures
Abstract: Using an extensive experimental data set we show that although some turbulence properties of weakly mobile and fixed gravel-bed flows are similar (vertical distribution of broad-band characteristics, velocity spectra, cross-correlations, etc. ), others are different. Weakly mobile bed flows differ from fixed bed flows in that: (1) the von Karman constant is appreciably less than 0. 40; (2) the hydrodynamic roughness length is appreciably lower; (3) the relative turbulence intensity (when normalised on the local mean velocity) is less; and (4) the frequency of bursting events is reduced. These differences reflect the effect of drag reduction due to the bed mobility and support Gust and Southard’s[1983]suggestion that mobile bed flows represent a special class of wall-bounded flows. We propose a physical explanation for the drag reduction in weakly mobile gravel-bed flows, which is based on consideration of the eddy density and turbulence intensities and supported by the measurements.
Year: 1999