Numerical study on internal flow structures of subaqueous density underflows and stratified open-channel flows

turbulence modeling;turbidity current;density stratification;suspended sediment;sediment transport;fish-trap effect;internal wave;flow similarity

Yeh, Tzu-Hao

关键词: turbulence modeling;    turbidity current;    density stratification;    suspended sediment;    sediment transport;    fish-trap effect;    internal wave;    flow similarity;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/44392/Tzu-Hao_Yeh.pdf?sequence=1&isAllowed=y

美国|英语

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

【 摘 要 】

The internal structure of subaqueous density underflows and stratified open channel flows are examined in this study. Three different flow configurations are used to investigate the effects of density stratification on the internal structure and the responses of the structure as flow conditions vary. The first configuration describes an open-channel flow carrying sediment suspension, whereas the second and third configurations are used to investigate the flow structure of subaqueous density underflows. For each flow configurations, the internal flow structure is solved numerically using finite volume method with various turbulence closures and benchmarked with experimental or DNS data. We demonstrated that the choice of turbulence closures is crucial in capturing the stratification effects and internal structure. Results showed that closures that incorporate the dependency of density gradients in the stability functions, such as QE k-epsilon and Mellor-Yamada models, yield better performance than the standard k-epsilon and constant eddy viscosity models in capturing stratification effects.Two key limitations are identified for all the two-equation RANS models presented in this study when applied to density underflows. The first limitation can be known as the “fish-trap” effect, which is manifested in the sharp density gradient near the velocity maximum which separates the sediment concentration into two distinct regions. Such a phenomenon is associated with the overemphasized reduction of the eddy diffusivity caused by the structure of the closures. The second limitation is to the inability of the RANS models to describe energy dissipation due to internal waves, a mechanism substantially different from the turbulent energy cascade theory. Analyses showed that the dimensionless settling velocity and shear Richardson number are the most dominant dimensionless parameters in causing stratification and modifying the flow structures. The role of the former is to initiate density gradient and stratification effects whereas increasing value of the latter act to amplify these effects. For natural density currents, the effect of slope becomes important. As the slope decreases, the flow becomes more subcritical and the flow entrainment rate decreases. Such model predictions are consistent with various experimental studies and field observations. The numerical models for the open-channel flow configuration are developed into a user-friendly software, StratSedOC. This tool provides simple evaluation and visualization on how stratification causes the velocity and suspended sediment concentration to vary from the standard log law and the Vanoni-Rouse formulation, respectively.