EFFECTS OF STRATIFICATION BY SUSPENDED SEDIMENTS ON TURBULENT SHEAR FLOWS

Sediments suspended in turbulent flows of water over plane beds are known to influence the structure of the flows by which they are carried. Past attempts to model this effect have been based almost exclusively on a theoretical framework in which the dense solid particles stratify the flow and have an influence analogous to that of a downward heat flux in the stably stratified atmospheric surface layer. We compare results from a model based on this theoretical framework with laboratory measurements of ensemble-averaged velocity and ensemble-averaged particle concentration, obtained by previous investigators, in order to test the applicability of the theoretical approach to dilute suspensions of sand in turbulent flows of water. We find that the stratification effect can be observed qualitatively in measured velocity profiles, except in one series of experiments in which systematic measurement errors may have obscured the effect. Estimates of model constants based on measured velocity profiles are, overall, roughly consistent with expectations based on measurements in thermally stratified flows, although there is substantial variability in individual profiles. Some of this variability is explainable as a consequence of a parameter range in which stratification effects are too weak to be detected accurately, other variability is explainable as a consequence of approximations in the model, and still other variability is possibly a consequence of weak dependence of model constants on sediment grain size, which was not expected from the theoretical development. The stratification effect is not observed in individual particle concentration profiles but must instead be observed in experiments in which the flow conditions and particle properties are held fixed while the particle load is increased.