NUMERICAL SIMULATION OF TURBULENT FREE SURFACE FLOW OVER OBSTRUCTION

A two-dimensional hybrid numerical model, FEM-LES-VOF, for free surface flows is proposed in this study, which is a combination of three-step Taylor-Galerkin finite element method, large eddy simulation with the Smagorinsky sub-grid model and Computational Lagrangian-Eulerian Advection Remap Volume of Fluid （CLEAR-VOF） method. The present FEM-LES-VOF model allows the fluid flows involving violent free surface and turbulence subject to complex boundary configuration to be simulated in a straightforward manner with unstructured grids in the context of finite element method. Numerical simulation of a benchmark problem of dam breaking is conducted to verify the present model. Comparisons with experimental data show that the proposed model works well and is capable of producing reliable predictions for free surface flows. Using the FEM-LES-VOF model, the free surface flow over a semi-circular obstruction is investigated. The simulation results are compared with available experimental and numerical results. Good performance of the FEM-LES-VOF model is demonstrated again. Moreover, the numerical studies show that the turbulence plays an important role in the evolution of free surface when the reflected wave propagates upstream during the fluid flow passing the submerged obstacle.     

An analytical model for solute mixing in surcharged manholes

A mathematical model has been developed to describe solute mixing in surcharged manholes using the submerged jet theory. The model has been applied for straight-through flow manholes and for manholes with a difference in the level of the inlet and outlet pipes. The model is applicable to dissolved substances, i.e. the model has not been validated for high sediment concentrations (above 1 g/l) and care should be taken in such cases. Simulation results from the new model are compared with laboratory measurements and are further compared to conventional modelling techniques currently available in commercial software specially developed for modelling the water quality in sewers. The results prove that the new model produces considerably better results compared to the traditional assumption of full mixing in a manhole. The new model compares well to the laboratory measurements and hence improves the accuracy of modelling soluble pollutant transport in sewers. The applicability of the new modelling approach is discussed and further studies are recommended.     

Spectral characterization of mixing properties of annular MHD micromixers

We develop a quantitative analysis of mixing regimes in an annular MHD-driven micromixer recently proposed by Gleeson et al. as a prototype for biomolecular applications. The analysis is based on the spectral properties of the advection–diffusion operator, with specific focus on the dependence of the dominant eigenvalue–eigenfunction on the Peclet number and on the system geometry. A theoretical prediction for the dominant eigenvalue encompassing all mixing regimes is developed and validated by comparison with numerical simulations. The theoretical prediction is extended to an open inflow–outflow version of the reactor, which shows the occurrence of new regimes associated with the existence of a nonuniform axial flow.     

三亚市崖城水厂净水工艺流程设计

新建三亚市崖城水厂是三亚市多水源水厂之一,供水量10万m3/d,引大隆水库水为源水(Ⅰ类源水水质,属贫营养水库,仅有时长藻),经比较采用成熟,常规水处理工艺流程——机械搅拌混合,竖向折板絮凝,平流沉淀,气水反冲V型过滤,出厂水可达到或优于国标《生活饮用水卫生标准》GB5749-2006要求。     

Fully transient analytical solution for degradable organic contaminant transport through GMB/GCL/AL composite liners

In this study, analytical solution for degradable organic contaminant transport through a composite liner consisting of a geomembrane (GMB) layer, a geosynthetic clay liner (GCL) and an attenuation layer (AL) is derived by the separation of variables method. The transient contaminant transport in the whole composite liner can be well described avoiding some weird phenomena in existing analytical solutions. The results of parametric study show that GCL has significant effect on improving the barrier efficiency especially for scenarios with high leachate head. The biodegradation and adsorption in GCL have significant influence on the contaminant transport through the composite liner when the half-life of contaminant in GCL is less than 5 years. Otherwise, the effect can be neglected.     

Anti-Dissipative Schemes for Advection and Application to Hamilton–Jacobi–Bellmann Equations

We propose two new antidiffusive schemes for advection (or linear transport), one of them being a mixture of Roe’s Super-Bee scheme and of the “Ultra-Bee” scheme. We show how to apply these schemes to treat time-dependent first order Hamilton–Jacobi–Bellman equations with discontinuous initial data, possibly infinitely-valued. Numerical tests are proposed, in one and two space dimensions, in order to validate the methods AMS subject classifications. Primary 65M12, Secondary 58J47     

Numerical Solution of Fractional Advection-Dispersion Equation

Numerical schemes and stability criteria are developed for solution of the one-dimensional fractional advection-dispersion equation (FRADE) derived by revising Fick’s first law. Employing 74 sets of dye test data measured on natural streams, it is found that the fractional order F of the partial differential operator acting on the dispersion term varies around the most frequently occurring value of F = 1.65 in the range of 1.4 to 2.0. Two series expansions are proposed for approximation of the limit definitions of fractional derivatives. On this ground, two three-term finite-difference schemes?“1.3 Backward Scheme” having the first-order accuracy and “F.3 Central Scheme” possessing the F-th order accuracy?are presented for fractional order derivatives. The F.3 scheme is found to perform better than does the 1.3 scheme in terms of error and stability analyses and is thus recommended for numerical solution of FRADE. The fractional dispersion model characterized by the FRADE and the F.3 scheme can accurately simulate the long-tailed dispersion processes in natural rivers.     

Gas Permeability of Partially Hydrated Geosynthetic Clay Liners

A series of gas permeability tests were performed on four partially hydrated geosynthetic clay liners (GCLs) (GCL1, GCL2, GCL3, and GCL4). All GCLs consisted of essentially dry bentonite (powder or granular) sandwiched between geotextile layers. The geotextiles were held together as a composite material by needle-punching, except GCL-4, which was stitch bonded. GCL-2 had a special characteristic, which consisted of a cover nonwoven geotextile layer impregnated with powdered bentonite. The gas permeability was found to be very sensitive to the change of moisture content and volumetric water content. The results also highlighted the effects of the GCL structures (bentonite impregnation, needle punching, and stitch bonding) and bentonite forms (granular and powdered) on the gas permeability. The needle punched GCLs tended to have lower gas permeability than the stitch bonded GCLs, and the GCLs containing granular bentonite tended to have higher gas permeability than the GCLs containing powdered bentonite. The bentonite impregnation of the nonwoven geotextile also contributed to lower gas permeability. For comparable conditions, these effects resulted in a reduction of up to three orders of magnitude of gas permittivity from one GCL to another. However, the effect of the differences between the GCLs on gas permeability, at high volumetric water content (>70%), was overridden by the presence of the overburden pressure during hydration. Furthermore, the overburden pressure also had an important role in the reduction of gas permeability, which implies that the GCL should be subjected to confinement at the time of installation or hydration in order to obtain a low gas permeability.     

Characteristics Method Using Cubic–Spline Interpolation for Advection–Diffusion Equation

The characteristics method by using the cubic-spline interpolation is comparable to the Holly–Preissmann scheme in solving the advection portion of the advection–diffusion equation. In order to conduct a cubic-spline interpolation, an additional constraint must be specified at each endpoint. In general, four types of endpoint constraints are available, i.e., the first derivative, second derivative, quadratic, and not-a-knot constraints. The goal of this paper is to examine each type of endpoint constraints. Two hypothetical cases are used to conduct the investigation. Among the four types of constraints examined herein, the not-a-knot constraint and the first derivative constraint with high-order finite difference approximation yield the best results. However, as far as accuracy and simple implementation are concerned the not-a-knot constraint should be the best choice in solving the advection–diffusion equation