Effect of self-stratification on channel flows and boundary layers: a study using direct numerical simulations

Channel Flow;Direct Numerical Simulation (DNS);Boundary Layer;Self Stratification;Suspended Sediment

Dutta, Som ; Garcia ; Marcelo H.

关键词: Channel Flow;    Direct Numerical Simulation (DNS);    Boundary Layer;    Self Stratification;    Suspended Sediment;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/42304/Som_Dutta.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Flows with suspended sediment are one of the most prevalent type of flows in nature.They can be found in nature as hypoconcentrated flows, mudflows, lahars and turbidity currents, to name a few. Effect of suspended sediment on the flow has wide ranging impact on geomorpholoy, ecological health, infrastructure health and contaminant transport. Thus,accurate prediction of suspended sediment load and velocity profiles havebeen a relevant problem for more than half a century. In the recent past new experimental and field observations have helped to differentiatethe effect due to cohesive and non-cohesive sediment; but an inherent property of suspended sediment irrespective of being cohesive or non-cohesiveis to evolve into a stable stratification, therebysuppressing turbulence in the flow. New insights into inherent mechanism of suppression of turbulence due to self-stratification was aided byDirect Numerical Simulation (DNS) simulations for a simplified formalism of the flow [Cantero et al. 2009] and the present study aims at furthering that work. In the present study, sediment is assumed to be non-cohesive in nature and possessing a constant settling velocity and no inertia. An Eulerian-Eulerian approach has been used for modeling the flow; which is driven by a constant pressure gradient. In this thesis, we have explored the effect of stratification on a pressure driven channel flow. We came across two distinct regimes of the flow, one in which turbulence is damped but the flow is still fully turbulent and the second in which the flow near the bottom wall locally relaminarizes. A flow being part of the first or the second regime was found to be dependent on the dimensionless number particle fall velocity multiplied by shear Richardson number (V*Ri_tau), if shear Reynolds number of the flow remains constant. We also explored the differences between the effect self-stratification has on a pressure driven channel flow and a pressure driven boundary-layer (similar to open channel flow). We found appreciable difference between the way channel flowand the boundary-layer reacted for exactly the samevalue of V*Ri_tau and shear Reynolds number. The channel flow was found to be more susceptible to the effect of increasing stratification. The results for the boundary-layer case was found to be in qualitative agreement with a recent study done for the same configuration but with the driving force being, gravity acting on suspended sediment (like in the case of a turbidity current).