Numerical simulation of wind-driven circulation and pollutant transport in Taihu Lake based on a quadtree grid

In this study,a two-dimensional flow-pollutant coupled model was developed based on a quadtree grid.This model was established to allow the accurate simulation of wind-driven flow in a large-scale shallow lake with irregular natural boundaries when focusing on important smallscale localized flow features.The quadtree grid was created by domain decomposition.The governing equations were solved using the finite volume method,and the normal fluxes of mass,momentum,and pollutants across the interface between cells were computed by means of a Godunov-type Osher scheme.The model was employed to simulate wind-driven flow in a circular basin with non-uniform depth.The computed values were in agreement with analytical data.The results indicate that the quadtree grid has fine local resolution and high efficiency,and is convenient for local refinement.It is clear that the quadtree grid model is effective when applied to complex flow domains.Finally,the model was used to calculate the flow field and concentration field of Taihu Lake,demonstrating its ability to predict the flow and concentration fields in an actual water area with complex geometry.

Pollutant mixing and transport process via diverse transverse release positions in a multi-anabranch river with three braid bars

A multi-anabranch river with three braid bars is a typical river pattern in nature, but no studies have been conducted to describe mixing characteristics of pollutants in the river. In this study, a physical model of a typical multi-anabranch river with three braid bars was established to explore the pollutant mixing characteristics in different branches. The multi-anabranch reach was separated into seven branches, B1, B2, B3, B4, B5, B6, and BT, by three braid bars. Five tracer release positions located 2.9 m upstream from the inlet section of the multi-anabranch reach were adopted, and the distances from the five positions to the left bank of the upstream main channel were 1/6B, 1/3B, 1/2B, 2/3B, and 5/6B （B is the width of the upstream main channel）, respectively. The longitudinal velocities and pollutant concentrations in the seven branches were measured. The planar flow field and mixing characteristics of pollutants from the bottom to the surface in the multi-anabranch river were obtained and analyzed. The results show that the pollutant release positions are the main influencing factors in the pollutant transport process, and the diversion points and pollutant release positions jointly influence the percentage ratios of the pollutant fluxes in branches B 1, B2, and B3 to the pollutant flux in the upstream main channel.

Flow patterns and critical criteria of thermally stratified shear flow in braided rivers

Flow characteristics of thermally stratified shear flow in braided rivers are particularly complicated and poorly understood. In this study, a series of typical flow patterns was examined and their critical criteria were determined. Four flow patterns were identified: mixed, locally unstable, continuously stratified, and two-layer flow. Temperature distributions of the four types of flow patterns were analyzed and compared.The critical Froude numbers for unstable flow, FDcr1, and stable flow, FDcr2, were determined to be 6 and 1, respectively, and comparison of FDcr1 and FDcr2 to the peak Froude numbers, FD1 at the outer bank and FD2 at the inner bank along the anabranch, allowed the flow patterns to be assessed. Then, a discriminant based on initial Jeffreys-Keulegan stability parameters was established to distinguish the flow stages from twolayer flow to completely mixed flow. It is indicated that the three critical Jeffreys-Keulegan parameters increased with the diversion angle of braided rivers. Results also show that, compared to the stratified flow in straight and curved channels, it was more difficult for braided stratified flow to maintain as two-layer flow, and it more easily became mixed flow. Consequently, empirical expressions for stability criteria of the thermally stratified shear flow in braided rivers are presented.

Laboratory simulation of soil respiration response to environmental conditions in intertidal zones of Jiangsu Province,China

As major contributor to the blue carbon sink,intertidal zones play a crucial role in the global carbon cycle.In recent years,more attention has been given to the carbon cycle in intertidal wetlands.However,due to highly variable and uncertain environmental conditions,it is difficult to clarify the quantitative relationship between soil respiration and environmental factors through in-situ experiments.In this study,the response of soil respiration characteristics to variations in the temperature and water table was investigated using a monitoring apparatus of CO_(2)flux at the soil-air interface in the intertidal zone.The results showed that soil respiration flux was significantly correlated with temperature,and the correlation best fitted the DoseResp function.Meanwhile,the respiration flux was enhanced with the descent of water table,a relationship could be described by a quadratic function.The effect of the water table on soil respiration became more pronounced with the rise of temperature.These results provide significant clarification of the impact of human activities on the carbon cycle in bare intertidal zones and as well as support for numerical simulations of the carbon cycle in bare intertidal zones.

Longitudinal dispersion coefficients of pollutants in compound channels with vegetated floodplains

The characteristics of the longitudinal dispersion of pollutants in compound channels remain unclear. This study examines the relationships among the vegetation density, the width of the floodplain, the water depth ratio, the cross-sectional mean velocity, and the longitudinal dispersion coefficient of a symmetrical compound channel with a rigid non-submerged vegetated floodplain. The longitudinal dispersion coefficient is found to increase significantly with the presence of vegetation on floodplains, and is positively correlated with the plant density. When the density of the vegetation on the floodplains exceeds a certain value, the dispersion coefficient no longer changes with the vegetation density. The longitudinal dispersion coefficient is found to increase with the increase of the width of the floodplain. Moreover, the combined effects of the mean velocity and the water depth ratio have a positive correlation with the dispersion coefficient. The effects of the vegetation on the longitudinal dispersion coefficient in the channels with various cross-sections are also compared. The compound channels with a vegetated floodplain are found to differ significantly from the channels with a rectangular cross-section.

Identification of pollution sources in river based on particle swarm optimization

The pollution sources identification model is presented by the coupling of the river water quality model and particle swarm optimization(PSO)algorithm to estimate pollution sources from the measured/simulated contaminant concentration in the river.The“twin experiment”is adopted to verify the feasibility of the identification model,and three cases are constructed to explore the results of the identification model in different situations.The experiment test demonstrated that the identification model is effective and efficient,while the model can accurately figure out the quantities of the pollutants and position of a single pollution source or multiple sources,with the relative error of the mean is less than 3%.Many factors are explored,including the level of random disturbance and the impact of particle population size.The outcome showed that the disturbance level is less than 5%,thus the precision is preferable,and when the number of particles is three,the identification is the best.When performing multiple sources,identification of multiple sets of monitoring sections respectively can obtain more accurate results with less error.In this paper,the optimization method of the inverse problem is applied to the identification of river pollution sources,which can help us to identify pollution sources and provide us a scientific basis for subsequent water pollution control and prevention.