Guojian He

Calculations of Nonsubmerged Groin Flow in a Shallow Open Channel by Large-Eddy Simulation

AbstractRigid structures, such as groins or spur dikes, are constructed along riverbanks for various purposes, which pose computational challenges for unsteady flow in engineering mechanics. This paper presents a study of turbulent flow past a series of groins in a shallow, open channel by large-eddy simulation (LES). A direct-forcing immersed boundary method (IBM) was implemented to approximate complex boundaries around groins with round heads. The time-averaged velocities and turbulence intensities at the water surface obtained by an experiment using particle image velocimetry (PIV) were employed to validate the LES model, finding a satisfactory agreement between laboratory data and model results. Subsequently, the numerical model was employed to investigate the impact of groin parameters (i.e., head shape, aspect ratio L/D, and length L) on the flow properties. Model results showed that a rectangular-headed groin generates higher turbulence intensities and larger vortices than a round-headed groin. On ...

Analysis of biofilm bacterial communities under different shear stresses using size-fractionated sediment

Microorganisms are ubiquitous in aqueous environments and are crucial for biogeochemical processes, but their community structures and functions remain poorly understood. In this paper, a rotating reactor was designed to study the effects of substrata and flow conditions on sediment bacterial communities using 16S rRNA gene sequencing, assaying three groups of size-fractionated sediments and three different levels of applied shear stress. Proteobacteria, Firmicutes, and Bacteroidetes were the dominant phyla of the microbial communities, with more anaerobic bacteria and opportunistic pathogens being detected under static water conditions, while more aerobic bacteria were detected under dynamic water flow conditions. Most of the top 10 genera were present in all the samples; however, there were significant differences in the species abundance. Paludibacter and Comamonadaceae_unclassified were the most abundant genera under static and dynamic conditions, respectively. Under static water conditions, the medium-grained sediment had the highest microbial diversity, followed by the fine and coarse sediments. Under dynamic water flow conditions, a higher flow velocity corresponded to a greater microbial diversity. Overall, there was no significant difference in the community richness or diversity between the static and dynamic water flow conditions. This study is beneficial for further understanding the heterogeneities of microbial communities in natural aquatic ecosystems.

Phosphorus adsorption on natural sediments with different pH incorporating surface morphology characterization

Sediment samples from University Lake (U.L.) and Anacostia River (A.R.) were collected to study the phosphorus (P) adsorption with pH at 3.65, 4.75, and 5.65. The surface micro-morphology and pore structures of sediment particles were obtained using a scanning electron microscopy and gas adsorption method, respectively. Fourier analysis was then applied to approximate the surface morphology, which was incorporated into the Langmuir isotherm to directly derive the model parameters for P adsorption simulation. Meanwhile, an empirical function of pH was introduced to represent the pH effect on P adsorption. A stronger P adsorption was observed for the A.R. sediment due to the more clay minerals, smaller median diameter, and a greater percentage of large pores, and the increasing pH resulted in a decrease of adsorption equilibrium constant as well as the P adsorption capacity, which was well reproduced by the adsorption isotherms. This study would benefit the mechanism study of the interactions between sediment particles and pollutants, providing references for understanding the pollutants’ transport in aqueous systems.

Phosphorus adsorption onto clay minerals and iron oxide with consideration of heterogeneous particle morphology

Particle morphology plays an important role in solid-water interface adsorption, which affects the fate and behavior of phosphorus (P) in rivers and lakes and the resulting eutrophication. In this paper, three minerals including kaolinite, montmorillonite and hematite were considered to investigate the contributions of particle morphology to P adsorption using adsorption experiments and microscopic examinations. The Taylor expansion method is applied to quantitatively characterize the heterogeneity of surface morphology. The results reveal that local concave or convex micro-morphology characterized by the second order term of Taylor expansion F2, can affect the local adsorption capacity due to its effect on the distribution of surface charge and reactive sites. Moreover, the adsorbed P at different F2 here fits to a Weibull distribution, which can further define the representative average adsorption onto individual particles. A weighted average morphology factor F2a is derived to characterize the surface heterogeneity, and correlated with average P adsorption of particular mineral particles. In addition, the Sips model can successfully fit the experimental data of different minerals, and the heterogeneity parameters γ and adsorption capacity Qm in the model are proved to be functions with the basic mineral properties, including particle size, surface site density and morphology characterization as well. It is concluded that the complex surface morphology plays a significant role in particle adsorption and the morphological role need to be considered in the adsorption model in order to better describe the adsorption in system with heterogeneous solid surface.

A model of 90Sr distribution in the sea near Daya Bay Nuclear Power Plant in China

The impact on the environment of radionuclide release from nuclear power plants has attracted increased attention, especially after the accident at Fukushima Daiichi Nuclear Power Plant in Japan. Based on the mechanisms of adsorption/desorption at solid/liquid interfaces and a surface micromorphology model of sediments, a theoretical expression of the distribution coefficient Kd is derived. This coefficient has significant effects on the distribution of radionuclide in seawater, suspended sediment and seabed sediment. Kd is then used to simulate 90Sr transport in the sea near the Daya Bay Nuclear Power Plant. The simulation results are compared with field measurements of tidal level, current velocity, suspended sediment concentration and 90Sr concentrations in the same period. Overall, the simulated results agree well with the field measured data. Thus, the derived expression for Kd is capable of interpreting realistic adsorption/desorption processes. What’s more, conclusion is drawn that about 40% 90Sr released by Daya Bay Nuclear Power Plant will be adsorbed by suspended sediment and 20% by seabed sediment, only about 40% 90Sr will remain in the sea near Daya Bay Nuclear Power Plant in South China Sea.

3D Numerical Investigation of Distorted Scale in Hydraulic Physical Model Experiments

Abstract In some physical model experiments, it is necessary to use distorted models. It is difficult, however, to build in an optimal degree of distortion into the models to ensure a closer degree of similarity between the model and the prototype. Three-dimensional (3D) numerical simulation is used in this paper to model experiments with a straight channel under different distorted scales. The calculated results of stream-wise, lateral and vertical velocities and sediment concentration along the vertical direction are shown by comparing the deviations of the velocities and sediment concentration between the distorted model and a normal one. Generally the discrepancy between the distorted model and the normal in stream-wise velocity is acceptable, while in vertical and transverse directions, the velocity shows differences. Concerning sediment concentration and channel bed deformation, the effect of the distorted scale is mainly related to two different similarity criteria. The similarity ratio between the turbulence diffusion velocity and gravity settling velocity can reproduce better results of sediment concentration along the vertical direction. The better bed deformation results come, however, from the similarity ratio between the averaged flow velocity and gravity settling velocity.

A review on sediment bioflocculation: Dynamics, influencing factors and modeling

Sediment in a water column provides excellent substratum for microorganism colonization, and biological processes would alter the physical and chemical of sediment, resulting in substantial changes in sediment dynamics. The flocculation of sediment with biological processes are defined as sediment bioflocculation, which has been ubiquitously observed across aquatic ecosystems, activated sludge plants and bioflocculant applications, as a result of various processes involving particle aggregation and breakage under the complex effects of microorganisms and their metabolic products (e.g., extracellular polymeric substances EPS). EPS are complex high-molecular-weight mixtures of polymers, which are the primary components that hold microbial aggregates together by acting as a biological glue. Several mechanistic aggregation theories such as the alginate theory, adsorption bridging theory, divalent cation bridging theory, and Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, and a number of influencing factors (e.g., sediment properties, microbial activity, EPS quantities and components, and external environment conditions) have been proposed to elucidate the role of microorganisms and EPS in sediment aggregation, promoting the investigation of the sediment bioflocculation evolution and kinetics models. However, due to the complex interrelationships of multiple physical, chemical, and biological processes and the incomprehensive knowledge of microorganisms and EPS, considerable research should be further conducted to fully understand their precise roles in the sediment bioflocculation process. In this study, a review of dynamic characterizations, mechanism, influencing factors and models of sediment bioflocculation are given to obtain a more comprehensive understanding of sediment bioflocculation dynamics.