H. C. Chan

Three-Dimensional Numerical Modeling of Secondary Flows in a Wide Curved Channel

Most natural rivers are curved channels, where the turbulent flows have a complex helical pattern, as has been extensively studied both numerically and experimentally. The helical flow structure in curved channels has an important bearing on sediment transport, riverbed evolution, and pollutant transport study. In this article, different turbulence closure schemes i.e., the mixing-length model and the k - ε model with different pressure solution techniques i. e., hydrostatic assumptions and dynamic pressure treatments are applied to study the helical secondary flows in an experiment curved channel. The agreements of vertically-averaged velocities between the simulated results obtained by using different turbulence models with different pressure solution techniques and the measured data are satisfactory. Their discrepancies with respect to surface elevations, superelevations and secondary flow patterns are discussed.

Experiments on hydraulic relations for flow over a compound sharp-crested weir

Basic experiments were conducted in a near full-scale compound sharp-crested weir. The compound sharp-crested weir composed of a trapezoidal weir, sloping crests and a rectangular weir. Detailed velocity measurements were performed for the effects of the compound weir geometry on the flow velocity distributions and the kinetic energy correction factors. Furthermore, a theoretical discharge equation for the proposed compound weir was derived and experimentally validated. The results showed that the kinetic energy correction factors decreased with increasing the head of the weir. The weir behaves as uniform flows passing over the weir under the conditions of the

Boundary treatment for 2D elliptic mesh generation in complex geometries

This paper presents a boundary treatment method for 2D elliptic mesh generation in complex geometries. Corresponding to Neumann-Dirichlet boundary conditions (sliding boundary conditions), the proposed method aims at achieving orthogonal and smooth nodal distribution along irregular boundaries. In this method, a three-lined auxiliary mesh is constructed which is composed of the boundary line, an auxiliary line generated on its inner side, and the reflection line of this auxiliary line. The movements of the boundary nodes are determined by solving this auxiliary mesh. The boundary nodes are further corrected and adjusted by a second-order parabolic interpolation method and a weighting parameter depending on the curvature of the boundary. The proposed method was demonstrated through examples, and applied to field cases with complex geometries. It has been shown that this method is stable and effective in producing high quality meshes.

Velocity and turbulence field around permeable structure: Comparisons between laboratory and numerical experiments

The results of a comparison between laboratory tests and numerical modeling for flow around permeable structures are presented. Three different structures are experimentally considered, first one is solid and second one is permeable structures. The permeable structures are represented by glass beads of two diameters, 2.5 and 1.5 cm, resulting in porosity equal to 0.475 and 0.349, respectively. A macroscopic model that solves the Reynolds-averaged Navier- Stokes equations with a non-Darcy resistance law is developed to simulate the flow around and within the permeable structures. The numerical predictions show good agreements with the experimental results. In the cases of permeable structures, recirculation regions are shown to be elongated in the downstream direction due to the bleed flow passing through the permeable structure. The turbulence intensity is highly reduced, compared to the situation with a solid structure, in the region near the surface and just behind the structures for the turbulence flow over a permeable structure. Discharge over the permeable structure indicates a significant reduction of the channel capacity and non-uniform distribution in the streamwise direction.

Numerical Calculation of Turbulent Channel Flow with Porous Ribs

The turbulent flow in a channel with periodic porous ribs on one wall is numerically studied. The numerical model utilizes the Reynolds averaged Navier-Stokes (RANS) equations with a k−e turbulent model for turbulence closure. Computational results show good agreements with experimental data in flows over a porous rib. The parameter effects, including the pitch ratio PR (1 ∼ 9) and porosity γ (0.4 ∼ 0.6), on flow fields are further examined in detail. Systematic variations of streamline, streamwise and vertical velocities, and turbulent kinetic energy are clearly identified. As to the PR effect, the interaction between outer flow and flow within the cavity is promoted by arranging ribs due to the penetration of the outer flow. Increasing porosity can reduce the downward outer flow by strong flows passing through the porous ribs. The numerical calculations suggest that the flow characteristics for porous ribs are not only a function of the rib geometry, i.e. pitch ratio, but also the porous property, i.e. porosity.

Hydraulic characteristics of flow over a highly permeable porous structure

permeable, and the vertical distributions of time-averaged velocities above permeable structures are more uniform than solid structure due to the slip velocity at the fluid/porous interface. The turbulence intensity above structure with large porosity are slightly greater than small porosity structure, and the permeable structures have smaller turbulence intensity than solid structure around the structures. The results show that the porous medium reduces the turbulence intensity, and then reduce the scour to structures.

Strategies for cutting management of riparian vegetation

This study presents the numerical predictions on the detailed hydrodynamic and habitat quality characteristics of riparian vegetation by using a depth-averaged two-dimensional flow model. The model solves the depth-averaged Reynolds Averaged Navier-Stokes Equation. The vegetative effect is considered by a drag force exerted by the flow on the vegetation, resulting in extra source terms included in the momentum equations. Simulated results of flows in a rectangular and a compound channel with vegetation along one side are coincided with the previous experimental data. Furthermore, the model is used to simulate the cutting management of riparian vegetation. Five different scenarios are proposed in this study, including original, as well as cutting along main-channel side, cutting along bank side, alternative cutting and reducing vegetative density. The influence of the proposed scenarios on hydrodynamic behaviors is investigated in a rectangular channel. Simulated results suggest that cutting along the main-channel side is the most effectively scenario among others in reducing the water depths and flow velocities of the original one.

Accelerating pollutant release from mud bed by sand drains

It is proposed to use sand drain systems in a channel mud bed to accelerate the pollutants release rate of the mud bed, based on the fact that the hydraulic conductivity of mud in the horizontal is usually larger than that of the vertical. Experiments are conducted in model channel bed system to assess this concept. This model is filled with various mud depths and has different sand drain arrangements. Ammonium nitrogen (NH4-N) and dissolved oxygen (DO) concentrations at the water body adj scent to the mud surface are measured. Experimental results show that the ammonium nitrogen concentration increases with the test time. It’s release rate increases with intensity of the sand drains. Deployment of a sand drain system is proved to be useful in accelerating the releasing of mudtrapped pollutant.