Flow Dynamics Modulation by Suspended Sediment
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Flow dynamics modulation by suspended sediment is present in nearly all geophysical flows. Many studies have sought to define the interactions among time-mean flows, turbulence, and suspended sediment. Yet the mechanisms by which suspended sediment modulates the flow dynamics are still poorly understood. The overall objectives of this study are to experimentally examine the effects of suspended sediments on the carrier fluid phase in systematic controlled lab-scale conditions and to quantify the physical processes responsible for sediment suspension in turbulent flows. Experiments were carried out in a mixing box to evaluate the effects of suspended sand on the turbulent flow generated by an oscillating grid near the bottom of the box. Two-phase particle image velocimetry (PIV) was used to obtain velocity characteristics of both the sediment and fluid phases separately. Experiments were performed for clear water flow as a reference base and sediment-laden flows with different sediment volumetric concentration (v) of 0.3%, 0.45%, 0.6%, 0.9%, 1.2%, and 1.4%. To obtain instantaneous velocities for both fluid carrier and sediment phases, an image-analysis method was developed to discriminate fluid tracer particles from sediment particles. The image-based method was applicable for dilute suspension regimes; fluorescent tracer particles along with optical filters were used for phase discrimination in dense suspension regimes. Results show that (1) the turbulent characteristics of clear water condition were consistent with previous studies, but non-negligible secondary circulations were observed, (2) in the presence of suspended sediment, fluid-phase total mean and turbulent kinetic energy and fluid eddy viscosity all decreased, while the turbulent integral length scales became larger and more anisotropic, resulting in flat-shaped energetic eddies, (3) three important length scales were recognized in sediment-laden flows, the depth of secondary circulation dampening (Ds), the depth of turbulent mixing confinement (D), and the depth of a uniform high-concentrated suspension layer (Dc), and (4) three non-dimensional numbers controlling flow were identified as the Reynolds number (Re), Rouse number (Ro) and Richardson number (Ri). As sediment loading increased, Re decreased, Ro increased and, Ri decreased. In all sediment-laden experiments, Ri value was larger than Ro value. This result implies that suspended sediment buoyancy flux was stronger than suspended sediment settling flux. Hence, sediment remained in suspension and formed a uniform high-concentrated suspension layer with depth Dc producing flow stratification within the box. As sediment loading increased, the effect of this stratified layer on flow dynamics modulation intensified. The results presented here demonstrate remarkable flow dynamics modulation by suspended sediment, and are expected to be applicable to a wide range of sediment-laden geophysical flows.