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Numerical Heat Transfer And Fluid Flow

Part I presents the statistical theory of turbulence, and Part 2 the coherent structures in open-channel flows and boundary layers. The book is intended for advanced students and researchers in hydraulic research, fluid mechanics, environmental sciences and related disciplines. References Index.

Turbulence in open-channel flows

Turburence structure and coherent motion in vegetated canopy open-channel flows

This paper presents results of several large-eddy simulations (LES) of turbulent flow in an open channel through staggered arrays of rigid, emergent cylinders, which can be regarded as idealized vegetation. In this study, two cylinder Reynolds numbers, RD=1,340 and RD=500, and three vegetation densities are considered. The LES of the lowest density and at RD=1,340 corresponds to a recently completed laboratory experiment, the data of which is used to validate the simulations. Fairly good agreement between calculated and measured first- and second-order statistics along measurement profiles is found, confirming the accuracy of the simulations. The high resolution of the simulations enables an explicit calculation of drag forces, decomposed into pressure and friction drag, that are exerted on the cylinders. The effect of the cylinder Reynolds number and the cylinder density on the drag and hence on the flow resistance is quantified and in agreement with previous experimental studies. Turbulence structures are visualized through instantaneous pressure fluctuations, isosurfaces of the Q-criterion and contours of vertical vorticity in horizontal planes. Analysis of velocity time signals and distributions of drag and lift forces over time reveals that flow and turbulence are more influenced by the vegetation density than by the cylinder Reynolds number.

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Turbulent Flow through Idealized Emergent Vegetation

A new model for the depth-averaged velocity for flow in presence of submerged vegetation is developed. The model is based on a two-layer approach, where flow above and through the vegetation layer is described separately. Vegetation is treated as a homogeneous field of identical cylindrical stems, and the flow field is considered stationary and uniform. It is demonstrated that scaling considerations of the bulk flow field can be used to avoid complications associated with smaller scale flow processes and that still the behavior of depth-averaged flow over vegetation is described accurately. The derived scaling expression of the average flow field is simple in form, it follows fundamental laws of fluid flow, and it shows very good agreement with laboratory flume experiments. The new model can be used for quick evaluation of a river’s hydraulic response in cases where vegetated floodplains are inundated.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2006WR005625

Analytical solution of the depth-averaged flow velocity in case of submerged rigid cylindrical vegetation

The Reynolds stress model is applied to open-channel flows with vegetation. For the computation of pressure-strain term, the Speziale, Sarkar, and Gatski's model is employed. Mellor and Herring's model and Rotta's model are used for diffusion and dissipation rate of Reynolds stress, respectively. Flow structures of open-channels under two vegetative conditions are simulated, namely submerged and emergent plants. Plain open-channel flows are also simulated for comparisons. Computed profiles are compared with the results from the κ-e model and the algebraic stress model as well as measured data available in the literature. For the plain open-channel flow and the open-channel flow with emergent vegetation, the Reynolds stress model is observed to simulate the non-isotropic nature of the flows better than the algebraic stress model and the κ-e model. For the open-channel flow with submerged vegetation, it is found that the Reynolds stress model predicts the mean flow and turbulence quantities best compared wi...

Reynolds stress modeling of vegetated open-channel flows

Changes of river morphology and physical fish habitat following weir removal

Turbulence modeling of compound open-channel flows with and without vegetation on the floodplain using the Reynolds stress model

A Reynolds stress model for the numerical simulation of partly-vegetated flows is presented. The model uses Speziale, Sarkar, and Gatski's model for the pressure–strain correlation, Mellor and Herring's model and Rotta's model for the diffusion and the dissipation rate of the Reynolds stress, respectively. The model is applied to partly-vegetated rectangular open-channel flows, and simulated results are compared with experimental data. The model satisfactorily predicts mean flow and turbulence statistics. Through numerical experiments, the evolution of secondary current patterns and mean flow structure are presented for different densities of vegetation. A budget analysis of the streamwise vorticity equation is also performed to investigate the mechanism by which secondary currents in a partly-vegetated open-channel flow are generated. In the vegetated zone, the production by anisotropy is important in generating secondary currents over the entire depth, except for regions close to the free surface and th...

Numerical investigations of mean flow and turbulence structures of partly-vegetated open-channel flows using the Reynolds stress model

A Reynolds stress model for the numerical simulation of uniform 3D turbulent open-channel flows is described. The finite volume method is used for the numerical solution of the flow equations and transport equations of the Reynolds stress components. The overall solution strategy is the SIMPLER algorithm, and the power-law scheme is used to discretize the convection and diffusion terms in the governing equations. The developed model is applied to a flow at a Reynolds number of 77000 in a rectangular channel with a width to depth ratio of 2. The simulated mean flow and turbulence structures are compared with measured and computed data from the literature. The computed flow vectors in the plane normal to the streamwise direction show a small vortex, called inner secondary currents, located at the juncture of the sidewall and the free surface as well as the free surface and bottom vortices. This small vortex causes a significant increase in the wall shear stress in the vicinity of the free surface. A budget analysis of the streamwise vorticity is carried out. It is found that both production terms by anisotropy of Reynolds normal stress and by Reynolds shear stress contribute to the generation of secondary currents. Copyright © 2006 John Wiley & Sons, Ltd.

Hyeongsik Kang

Papers

Reynolds stress modeling of vegetated open-channel flows

Changes of river morphology and physical fish habitat following weir removal

Turbulence modeling of compound open-channel flows with and without vegetation on the floodplain using the Reynolds stress model

Numerical investigations of mean flow and turbulence structures of partly-vegetated open-channel flows using the Reynolds stress model

Reynolds stress modelling of rectangular open‐channel flow