Zhonghua Yang

Estimation of discharge in compound channels based on energy concept

A new model is developed to estimate the discharge capacity in straight, symmetric, compound open channels based on the energy concept. The energy loss and the transition mechanism in these channels are analysed based on mechanical analysis, in which the momentum transfer mechanism is taken into account as a product of the apparent shear stress. Four representative formulae of apparent shear stress are selected to validate the present model; the results indicate that the performance of the present model depends on our ability to accurately predict the apparent shear stress. The comparisons with experimental data indicate that the most appropriate apparent shear stress formula is that of Moreta and Martín-Vide; however, for the compound channels without roughness differences between the main channel and floodplains, Christodoulous formula is still recommended for its simplicity. Comparisons with other models indicate that the present models are simple in manipulation and have a reasonable accuracy for engineering purposes.

Curved Open Channel Flow on Vegetation Roughened Inner Bank

A RNG k − ε numerical model together with a laboratory measurement with Micro ADV are adopted to investigate the flow through a 180o curved open channel (a 4 m straight inflow section, a 180° curved section, and a 4m straight outflow section) partially covered with rigid vegetations on its inner bank. Under the combined action of the vegetation and the bend flow, the flow structure is complex. The stream-wise velocities in the vegetation region are much smaller than those in the non-vegetation region due to the retardation caused by the vegetation. For the same reason, no clear circulation is found in the vegetated region, while in the non-vegetation region, a slight counter-rotating circulation is found near the outer bank at both 90° and downstream curved cross-sections. A comparison between the numerical prediction and the laboratory measurement shows that the RNG k − ε model can well predict the flow structure of the bend flow with vegetation. Furthermore, the shear stress is analyzed based on the numerical prediction. The much smaller value in the inner vegetated region indicates that the vegetation can effectively protect the river bank from scouring and erosion, in other words, the sediment is more likely to be deposited in the vegetation region.

NUMERICAL STUDY OF FLOW AND DILUTION BEHAVIOR OF RADIAL WALL JET

The radial wall jet is a flow configuration that combines the radial jet and the wall jet. This article presents a simulation of the radial wall jet by applying the transition Shear-Stress Transport (SST) model. Tanaka’s experimental data are used for validation. The computed velocity profiles agree well with the experimental ones. The distributions of the velocity on cross-sections show a similarity in the main region and the profiles are different with those of the free radial jet or the wall jet, because the presence of the wall limits the expansion of the jet. By introducing the equivalent nozzle width, the maximum velocity decays and the half-width distributions are normalized, respectively. In addition to compare the flow field with experiments, this paper also analyzes the dilution effect of radial wall jets in terms of the concentration distributions. The concentrations on the wall keep constant within a certain distance from the nozzle. And the concentration distributions also show a similarity in the main region. Both the decays of the maximum concentration and the distributions of the concentration half-width fall into a single curve, respectively. The dilution effect of radial wall jets is thus verified.

Secondary flow coefficient of overbank flow

This paper presents a 2D analytical solution for the transverse velocity distribution in compound open channels based on the Shiono and Knight method (SKM), in which the secondary flow coefficient (K-value) is introduced to take into account the effect of the secondary flow. The modeling results agree well with the experimental results from the Science and Engineering Research Council-Flood Channel Facility (SERC-FCF). Based on the SERC-FCF, the effects of geography on the secondary flow coefficient and the reason for such effects are analyzed. The modeling results show that the intensity of the secondary flow is related to the geometry of the section of the compound channel, and the sign of the K-value is related to the rotating direction of the secondary flow cell. This study provides a scientific reference to the selection of the K-value.

Numerical simulation of initial mixing of marine wastewater discharge from multiport diffusers

Purpose – The purpose of this paper is to examine the initial mixing of wastewater discharged from submerged outfall diffusers and the influence of port configurations on wastewater distribution based on computational results. Design/methodology/approach – Marine wastewater discharges from multiport diffusers are investigated by numerically solving three-dimensional and uncompressible two-phase flow fields. A mixture model simulates this flow and the standard k-e model to resolve flow turbulence; inter-phase interactions were described in terms of relative slip velocity between phases. Computations were performed for two values of the port spacings s/H with different current Froude numbers F. Findings – Computational results compared well with previous laboratory measurements. Numerical results reveal that for both the closely spaced (s/H=0.21) and widely spaced (s/H=3.0) ports, the normalized dilution Sn becomes independent of F; further, the length of the near field xn and the spreading layer thickness ...

Two timescales for longitudinal dispersion in a laminar open-channel flow

At small dimensionless timescales T (= tD/H2), where t is the time, H is the depth of the channel and D is the molecular diffusion coefficient, the mean transverse concentration along the longitudinal direction is not in a Gaussian distribution and the transverse concentration distribution is nonuniform. However, previous studies found different dimensionless timescales in the early stage, which is not verified experimentally due to the demanding experimental requirements. In this letter, a stochastic method is employed to simulate the early stage of the longitudinal transport when the Peclet number is large. It is shown that the timescale for the transverse distribution to approach uniformity is T = 0.5, which is also the timescale for the dimensionless temporal longitudinal dispersion coefficient to reach its asymptotic value, the timescale for the longitudinal distribution to approach a Gaussian distribution is T = 1.0, which is also the timescale for the dimensionless history mean longitudinal dispersion coefficient to reach its asymptotic value.

Numerical study on the dynamics and mass transfer characteristics of a radial offset jet

Purpose – A radial offset jet has the flow characteristics of a radial jet and an offset jet, which are encountered in many engineering applications. The purpose of this paper is to study the dynamics and mass transfer characteristics of the radial offset jet with an offset ratio 6, 8 and 12. Design/methodology/approach – Three turbulence models, namely the SST k-? model, detached eddy simulation model, and improved delayed detached eddy simulation (IDDES), were applied to the radial offset jet with an offset ratio eight and their results were compared with experimental results. The contrasting results, such as the distributions of mean and turbulent velocity and pressure, show that the IDDES model was the best model in simulating the radial offset jet. The results of the IDDES were analyzed, including the Reynolds stress, turbulent kinetic energy, triple-velocity correlations, vertical structure and the tracer concentration distribution. Findings – In the axisymmetric plane, Reynolds stresses increase to...

Estimating the Transverse Mixing Coefficient in Laboratory Flumes and Natural Rivers

Transverse mixing is a complex process and important in understanding the transport of pollution in rivers. This study presents a genetic programming (GP)-based model for estimating the transverse mixing coefficient (TMC) in flumes and rivers. More than a hundred of data points from previous studies, including datasets on laboratory straight rectangular flumes and field measurements in natural rivers, are collected and used to develop the final formulae for estimating TMC. During the analysis, TMC is separated into the transverse turbulent diffusion coefficient and the transverse dispersion coefficient given that they represent two different processes. Before formula optimization and search are performed using GP software, the target formulae are semi-defined to reduce search time and ensure the physical basement of the final formulae. The model presented in this study exhibits good improvement in terms of accuracy and physical meaning compared with existing equations.

Evaluating the hydrodynamics of a round jet in a vegetated crossflow through large eddy simulation

Vegetation plays an important role on the turbulence structures of the effluent spreading in an open channel, which are insufficiently studied. This paper employs a large eddy simulation model to investigate the hydrodynamic processes of a round jet in a vegetated crossflow. The time-averaged simulated results are consistent with the experimental data. Analyses of mean flow characteristics including velocity distributions in different planes are showed that the array of rigid vegetation affects the averaged flow field and the jet structure, diminishing the velocity, with a significant increase of the jet penetration height. Moreover, this simulation successfully reproduces the coherent structures of classical and well-documented types based on the mean and instantaneous flow fields, including shear-layer vortices, wake vortices, counter-rotating vortex pair and windward vortex pair. The momentum transport mechanism is quantitatively elaborated by the quadrant analysis. Spectral analysis is adopted to obtain the dominant frequencies of vortex shedding and investigated the characteristic length of the large-scale vortex at different probes in the flow field.