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Turbulence modulation by suspended sediment within a mixing box
Experiments were performed to evaluate the effect of suspended sediment on turbulence characteristics in a mixing box where turbulence was generated with a vertically oscillating grid. Quartz sand of varying sizes and concentrations were used along with varying oscillation frequencies to generate a range of relative sediment and turbulence conditions. Particle image velocimetry (PIV) was used to characterize the fluid flow, where a fluorescent dye was used for the seed particles so that fluid and sediment motions could be differentiated. The study was designed to test the hypothesis that local turbulence could be either enhanced or suppressed, and that a criterion to distinguish between these two effects could be defined on the basis of relative size of particles compared with characteristic turbulence length scale. The critical value of this ratio is found to be approximately 0.2, agreeing closely with a value of 0.1 found previously for pipe flow conditions. On a macro (whole system) scale, total turbulence energy is shown to decrease with the presence of suspended sediment, as energy is expended to lift the particles.
Results are expected to advance our understanding of sediment interactions with turbulence in geophysically important flows. Further work is needed to better define the nature of these interactions, particularly for materials with different densities, and to develop procedures for extending these results to mean shear flows.