Chunbo Jiang

A Study of Drag Coefficient Related with Vegetation Based on the Flume Experiment

This study is focused on the effects of ecological factors (diameter and flexibility) and vegetation community composition on the drag coefficient related with vegetation. The single leafy shrub and three mixed communities (including shrub-grass, shrub-reed and reed-grass community) were studied. The flow velocity and water level were measured and used to calculate the drag coefficient based on the Bernoulli’s equation, Darcy drag formula and the expression for the drag coefficient related with Darcy drag factor. The trend of the drag coefficient in the vertical direction was analyzed against flow depth, diameter, diameter Reynolds number, flow depth Reynolds number and relative roughness height in different discharges. The results show that beside the dense leafy shrubs community, the vertical trend of the drag coefficient among other cases against flow depth, diameter, diameter Reynolds number, flow depth Reynolds number and relative roughness height can be approximately expressed by power law functions under different flow discharges. Moreover, in a mixed community with two plants with distinctly different ecological factors, the one with the most distinct variations of ecological factors determines the vertical trend of the drag coefficient; the other one only affects the magnitude of the drag coefficient. Furthermore, if the ecological factors of the vegetation in the vertical direction are kept almost not changed, the drag coefficient can be approximately regarded as a constant.

Predicting near-field dam-break flow and impact force using a 3D model

A three-dimensional (3D) numerical model based on the unsteady Reynolds equations was used to simulate near-field dam-break flows and estimate the impact force on obstacles. The model employs a projection method to solve the governing equations and the method of volume of fluid (VOF) to capture the water surface movement. The model is first applied to simulate two physical model experiments of dam-break flows. Model-predicted pressure, water depth and velocity distributions are compared with laboratory measurements. For the second case, the 3D-VOF model predictions are also compared with predictions made by a two-dimensional model. The 3D-VOF model is then used to calculate the impact force of dam-break flow on a steady obstacle. A physical model experiment is set up to assist the numerical model study. The model-predicted impact force on the obstacle and the critical condition for it to move are compared with the measurements from the experiment.

A review on TVD schemes and a refined flux-limiter for steady-state calculations

This paper presents an extensive review of most of the existing TVD schemes found in literature that are based on the One-step Time-space-coupled Unsteady TVD criterion (OTU-TVD), the Multi-step Time-space-separated Unsteady TVD criterion (MTU-TVD) and the Semi-discrete Steady-state TVD criterion (SS-TVD). The design principles of these schemes are examined in detail. It is found that the selection of appropriate flux-limiters is a key design element in developing these schemes. Different flux-limiter forms (CFL-dependent or CFL-independent, and various limiting criteria) are shown to lead to different performances in accuracy and convergence. Furthermore, a refined SS-TVD flux-limiter, referred to henceforth as TCDF (Third-order Continuously Differentiable Function), is proposed for steady-state calculations based on the review. To evaluate the performance of the newly proposed scheme, many existing classical SS-TVD limiters are compared with the TCDF in eight two-dimensional test cases. The numerical results clearly show that the TCDF results in an improved overall performance.

A refined volume-of-fluid algorithm for capturing sharp fluid interfaces on arbitrary meshes

This paper presents a new volume-of-fluid scheme (M-CICSAM), capable of capturing abrupt interfaces on meshes of arbitrary topology, which is a modification to the compressive interface capturing scheme for arbitrary meshes (CICSAM) proposed in the recent literature. Without resort to any explicit interface reconstruction, M-CICSAM is able to precisely model the complex free surface deformation, such as interface rupture and coalescence. By theoretical analysis, it is shown that the modified CICSAM overcomes three inherent drawbacks of the original CICSAM, concerning the basic differencing schemes, the switching strategy between the compressive downwind and diffusive high-resolution schemes, and the far-upwind reconstruction technique on arbitrary unstructured meshes. To evaluate the performance of the newly proposed scheme, several classic interface capturing methods developed in the past decades are compared with M-CICSAM in four test problems. The numerical results clearly demonstrate that M-CICSAM produces more accurate predictions on arbitrary meshes, especially at high Courant numbers, by reducing the numerical diffusion and preserving the interface shape.

Comparison between empirical formulae of intake vortices

The hydrodynamic properties of free surface vortices at hydraulic intakes were investigated. Based on the axisymmetric Navier–Stokes equations and empirical assumptions, two sets of formulations for the velocity distributions and the free surface profiles are proposed and validated against measurements available in the literature. Compared with previous formulae, the modifications based on Mihs formula are found to greatly improve the agreement with the experimental data. Physical model tests were also conducted to study the intake vortex of the Xiluodu hydroelectric project in China. The proposed velocity distribution formula was applied to the solid boundary as considered by the method of images. A good agreement was again observed between the prediction and the measurements.

Computation of shallow wakes with the fractional step finite element method

The depth-integrated shallow-water equations are used to simulate shallow wake flow around a solid plate-like obstacle. Influences of different terms in the equations are analyzed by normalizing the momentum equation. A fractional step finite element method is used to discretize the shallow-water equations with third-order accuracy and uniform Courant-Friedrichs-Lewy (CFL) property. The established numerical model is used to simulate the shallow wake in three typical cases and the computational flow fields are compared with digital Particle Image Velocimetry (PIV) measurements and flow visualization pictures. Three turbulence models are considered: The results of the standard k–ε model agree with the experimental results better than those of zero equation and sub-depth scale turbulence models. Furthermore, the results of the simulation also show that it is the wake stability parameter that mainly controls the flow patterns of shallow wakes.

Modelling Graded Sediment Transport and Bed Evolution in a Tidal Harbour

ABSTRACT Yang, C.; Jiang, C., and Lin, B. 2013. Modelling graded sediment transport and bed evolution in a tidal harbour. This paper presents the development of a sediment transport model to predict the bed evolution processes in estuarine and coastal waters. The model is based on an existing hydrodynamic and sediment transport model, with significant refinements being made to enhance its capability for simulating the transport of graded sediments under nonequilibrium conditions. A multifraction sediment transport model has been developed to replace the existing single-fraction model. The sediment mixture is divided into several fractions according to the grain size. For each fraction, the particle size is considered to be uniform, and its transport form, either as suspended load or bed load, is determined by the magnitude of a suspension index. A bed evolution model is developed to simulate the processes of bed level change and sediment grain size sorting. The model is applied to a laboratory model harbour, for which measurements of tidal currents and bed level inside the harbour are available. The water level, velocity distributions, and bed level changes predicted by the numerical model are compared with the laboratory data. Comparisons are also made between predictions made by the fractional model and the single-size model. The effect of bed sediment size change on the erosion process has been investigated.

Investigation of air-core vortex at hydraulic intakes

Hydrodynamic properties of the surface vortex have been investigated. Based on the Navier-Stokes equations, three sets of the new formulations for the tangential velocity distributions are derived, and verified against the experimental measurements in the literature. It is shown that one modification greatly improves the agreement with the experimental data. Physical model experiments were carried out to study the intake vortex related to the Xiluodu hydropower project. The velocity fields were measured using the Particle Tracking Velocimetry (PTV) technique. The proposed equation for tangential velocity distribution is applied to the Xiluodu project with the solid boundary being considered by the method of images. Good agreement has been observed between the formula prediction and the experimental observation.

Investigation of the surface vortex in a spillway tunnel intake

Abstract The surface vortex in a spillway tunnel intake was investigated in a physical model of the Xiluodu hydropower station. The velocity fields were measured using the particle tracking velocimetry technique. The tangential velocity formula of the surface flow field was derived based on the Navier-Stokes equations, and this formula greatly improved the consistency between the numerical and experimental data. Also, the formula has the advantage of describing the tangential velocity more accurately than previous formulas. The current research is based on established engineering practices, and the results provide a valuable reference for actual projects designed to prevent and eliminate surface vortexes.

A NEW MODEL FOR PREDICTING BED EVOLUTION IN ESTUARINE AREA AND ITS APPLICATION IN YELLOW RIVER DELTA

This article discusses the process of sediment transport and proposes a morphological model to predict the bed evolution in estuaries. The hydrodynamic module is based on an existent model-Depth Integrated Velocity And Solute Transport (DIVAST) and the wetting and drying method is adopted to deal with the moving boundary. Both cohesive sediment and non-cohesive sediment are taken into consideration in the sediment transport module with the capability of simulating the transport of graded sediments under non-equilibrium conditions. The fall velocity of the suspended sediment is modified in the present model due to the high sediment concentration. A 3-layer approach is adopted to simulate the variations of sediment gradations of bed materials. Furthermore, the model is used to simulate the bed evolution in the Yellow River Delta (YRD) from 1992 to 1995. Field data are used to calibrate the parameters. The numerical results show how the morphology was developed in the Yellow River Estuary with a good agreement with the field data.