Author(s): Zhixian Cao; Shinji Egashira
Linked Author(s):
Keywords: Sediment-laden flow; Sediment transport; Turbulence modulation; Turbulence eddy viscosity; Stokes number; Particle size; Stratified flow analogy
Abstract: Suspended sediments are known to alter the structure of turbulent flow by which they are carried, which in turn affects the transport of sediments. Previous studies mostly are based either on the reduced von Karman coefficient, the law of the wake, stratified flow analogy, modified Prandtl’s mixing length, or similarity argument. Current understanding of the mechanism of turbulence interaction with sediment particles is still far from complete. This paper deals with turbulence modulation by suspended sediment in open channel flow analytically as a complement to experiments. Turbulence modulation is interpreted in terms of turbulent eddy viscosity as compared to its counterpart of clear-water flow. Attenuation (enhancement) of turbulence is believed to occur in relation to decreased (increased) turbulent eddy viscosity. Existing laboratory data of mean velocity and sediment concentration are exploited. It is found that turbulence modulation by suspended sediment is closely associated with particle Stokes number and the ratio of particle size-to-turbulence length scale. In particular, within the range of data considered, turbulence attenuation is found while the values of particle Stokes number and the ratio of particle size-to-turbulence Taylor micro-scale vary under 0. 35 and 0. 24respectively. Also, turbulence attenuation is intensified away from the bed. This is the case as the particle Stokes number or the ratio of particle size-to-turbulence length scale decreases, or as sediment concentration increases below the equilibrium value for a specific particle diameter. The above results agree well with prior experimental finding of the general solid particle-fluid two-phase flow covering a wide spectrum of conditions. Moreover, under equilibrium conditions for different particle diameters, roughly consistent correlations are found between turbulence attenuation and particle Stokes number or the ratio of particle size-to-turbulence length scale. Further, substantial variability of the flux Richardson parameter is found related to turbulence attenuation. This fact appears to challenge the applicability of the stratified flow analogy for suspended sediment-laden flow, which involves the flux Richardson parameter or alternatively, the Monin-Obukhov length scale commonly used for constant flux atmospheric boundary layer. This study provides useful background for the improvement of turbulent eddy viscosity-based closure model for suspended sediment-laden flow.
Year: 1999