http://www.hach.ulg.ac.be/cms/system/files/Mignot_2006.pdf

Modeling floods in a dense urban area using 2D shallow water equations

In highly curved river bends, flow may separate at the inner bank to create a recirculation eddy with weak upstream flow. Very little is known about how recirculation eddies connect with the downstream flow or how the latter is affected by their presence. We investigate these questions using three-dimensional time-averaged computational fluid dynamics models of two natural bends with inner-bank separation. Test measurements of velocity in one bend agree well with the simulation. Common points in the two simulations are that (1) an outer-bank helix is only present in the upstream part of the bend, (2) maximum near-bank velocities are highest here rather than beyond the apex as in most bends, (3) reverse flow extends farther across the channel at the surface than the bed, and (4) flow within the recirculation eddy has a three-dimensional structure, linked with that in the outer-bank free stream. Substantial differences in detail between the two bends appear to be due to differences in upstream planform, manifested through the lateral distribution of inflow velocity.

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Flow in meander bends with recirculation at the inner bank

The energy based least action principle (LAP) has proven to be very successful for explaining natural phenomena in both classical and modern physics. This paper briefly reviews its historical development and details how, in three ways, it governs the behaviour and stability of alluvial rivers. First, the LAP embodies the special stationary equilibrium state of motion and so its incorporation with the principle of energy conservation explains why so many optimizing hypotheses have been proposed in fluvial geomorphology. Second, the variational approach underlying the LAP provides a more straightforward and simpler fuzzy-object orientated method for solving river regime problems than do the various complex Newtonian formulations. Third, it is shown that in fluvial systems with surplus energy the surplus can be expended with slope and/or channel geometry adjustments, with the degree of channel geometry adjustment quantified by the dimensionless numbers F for depth dominated adjustment and H for width/depth dominated adjustment. Different planforms are preferred at different energy levels, with H providing a quantitative measure of the flow's efficiency for moving sediment. In rivers with insufficient energy, the interactions of endogenous and exogenous factors are shown to be capable, in certain circumstances, of achieving a stationary equilibrium condition which acts as the attractor state. Importantly, this study describes how iterative changes enable systems to achieve such a stable equilibrium. Copyright © 2007 John Wiley & Sons, Ltd.

/pdf/least-action-principle-equilibrium-states-iterative-2wj64z0m6k.pdf

Least action principle, equilibrium states, iterative adjustment and the stability of alluvial channels

Lateral mixing of a pollutant is considered as a slow process that is usually complete within 100-300 river widths. Recent studies on flow dynamics at river confluences revealed that lateral mixing can be markedly enhanced when the tributary channel is shallower than the main channel. This study uses a three-dimensional model to examine mixing processes immediately downstream of confluences as well as further downstream in the mainstream. Simulations are presented for a concordant and discordant laboratory junction and a field confluence for a low and a high flow condition. The decrease in standard deviation at a cross section of a tracer over a distance of 5 channel widths is 30% for discordant beds but only 10% for concordant beds in the laboratory simulation. At the natural site, bed discordance is more important at the low flow than at the high flow with corresponding decreases in the standard deviation of 31 and 18% over 3.5 channel widths. Mixing is completed after a distance of 25 and 37 channel widths for the low and high flow conditions, respectively. Further downstream, mixing is mainly affected by planform curvature of the channel.

/pdf/three-dimensional-numerical-modeling-of-mixing-at-river-1u3yf1tg8v.pdf

Three dimensional numerical modeling of mixing at river confluences

Dividing flows in open channels are commonly encountered in hydraulic engineering systems. They are inherently three-dimensional (3D) in character. Past experimental studies were mostly limited to the collection of test data on the assumption that the flow was 1D or 2D. In the present experimental study, the flow is treated as 3D and test results are obtained for the flow characteristics of dividing flows in a 90° , sharp-edged, rectangular open-channel junction formed by channels of equal width. Depth measurements are made using point gauges, while velocity measurements are obtained using a Dantec laser Doppler anemometer over grids defined throughout the junction region. A 3D turbulence model is also developed to investigate the dividing open-channel flow characteristics. The predicted flow characteristics are validated using experimental data. Following proper model validation, the numerical model developed can yield design data pertaining to flow characteristics for different discharge and area ratios...

Numerical and experimental study of dividing open-channel flows

An open-channel junction flow is encountered in many hydraulic structures ranging from wastewater treatment facilities to fish passage conveyance structures. An extensive number of experimental studies have been conducted but a comprehensive three-dimensional numerical study of junction flow characteristics has not been performed and reported. In this paper, a three-dimensional numerical model is developed to investigate the open-channel junction flow. The main objective is to present the validation of a three-dimensional numerical model with high-quality experimental data and compare additional simulations with classical one-dimensional water surface calculations. The three-dimensional model is first validated using the experimental data of a 90° junction flow under two flow conditions. Good agreement is obtained between the model simulation and the experimental measurements. The model is then applied to investigate the effect of the junction angle on the flow characteristics and a discussion of the results is presented.

Three-Dimensional Numerical Study of Flows in Open-Channel Junctions

Verification and validation of an improved three-dimensional Reynolds-averaged Navier Stokes equation solver, developed for general hydraulic flows in ducts, open channels, and natural rivers, are described. This work is motivated by the need for building confidence in using numerical models to solve practical hydraulic engineering problems. The rigorous numerical verification procedure is based on the ASME editorial policy statement. The flow in an S-shaped open channel with sloping banks is used as benchmark. The numerical results are compared with available experimental data and previous numerical solutions. The level of agreement between the model predictions and laboratory data indicates the limits to which the solver is verified and validated for this test problem.

Verification and validation of a 3-d numerical model for open-channel flows

A three-dimensional numerical hydrodynamic model was developed by the writers to simulate flows through natural river reaches. The model has been successfully applied to and validated for a number of open channel flows. In this study, it is further applied to a flow through a reach of the Chattahoochee River, near Atlanta, GA, for which field and laboratory measurements were performed. The region of interest contains two piers of the Holcomb Bridge, a pier of an abandoned bridge, an old in-stream water intake, and six pump intakes. Good agreement is obtained between the model simulation and the experimental measurements demonstrating the ability of the model to capture important flow features.

Jianchun Huang

Papers

Three-Dimensional Numerical Study of Flows in Open-Channel Junctions

Verification and validation of a 3-d numerical model for open-channel flows

Simulation study of flow through a reach of the Chattahoochee River