Uncertainty and Sensitivity Analyses of the Simulated Seawater—Freshwater Mixing Zones in Steady-State Coastal Aquifers

The uncertainty and sensitivity of predicted positions and thicknesses of seawater-freshwater mixing zones with respect to uncertainties of saturated hydraulic conductivity, porosity, molecular diffusivity, longitudinal and transverse dispersivities were investigated in both head-control and flux-control inland boundary systems. It shows that uncertainties and sensitivities of predicted results vary in different boundary systems. With the same designed matrix of uncertain factors in simulation experiments, the variance of predicted positions and thickness in the flux-control system is much larger than that predicted in the head-control system. In a head-control system, the most sensitive factors for the predicted position of the mixing zone are inland freshwater head and transverse dispersivity. However, the predicted position of the mixing zone is more sensitive to saturated hydraulic conductivity in a flux-control system. In a head-control system, the most sensitive factors for the predicted thickness of the mixing zone include transverse dispersivity, molecular diffusivity, porosity, and longitudinal dispersivity, but the predicted thickness is more sensitive to the saturated hydraulic conductivity in a flux-control system. These findings improve our understandings for the development of seawater-freshwater mixing zone during seawater intrusion processes, and give technical support for groundwater resource management in coastal aquifers.

Sedimentation in the Submarine Outfall and in the Mixing Zones （Avoiding, Diagnosis and Remediation）

Some practical design tips and important recommendations are given to minimize the negative effect of discharge of wastewater laden with solid particles via submarine outfall. This study emphasizes the role of respecting the hydraulic conditions in the outfall to prevent sedimentation in the outfall or their accumulation in adjacent areas; also it includes the ways used to improve the outfall hydraulic capacity that decreases with time. The diagnostics and remediation procedures of mixing zones are discussed, especially in the case of previous toxic discharge that results in toxic sediments at the bed load. A literature review of techniques used to assess sediment quality near discharge points and locate effluent-affected sediment deposit is presented that include using acoustic profiles and images, chemical analysis, toxicity tests and multivariate indicators.

Mixing Zone Guidelines Application in a Smelter in Northern Sweden with Discharge to the Baltic Sea

This pilot study intended to investigate the application of Mixing Zone Guidelines in northern Sweden. The EC （European Commission）-Mixing Zone Guidelines were applied to seven effluent discharges. The effluents were from industrial processes used in the Ronnskar smelter, in northern Sweden. Each outlet in the smelter area discharges water into the Bothnian bay of the Baltic Sea. Cadmium （Cd）, mercury （Hg）, nickel （Ni） and lead （Pb） were the primary substances present in the effluents. A ＂Tiered Approach＂ was followed for mixing zone assessments in each of the discharge points. Discharge Test was used at Tier 2 and CORMIX （Cornell Mixing Zone Expert System model）, version 7, at Tier 3. At each discharge point, the AA-EQS （annual average-environmental quality standard） for each metal was met within a distance of 500 m from the outfalls. This distance was exceeded to meet the MAC-EQS （maximum allowable concentration-environmental quality standard） criteria at points 1 and 3 for total Hg concentrations. However, for the proper application of Mixing Zone Guidelines, a version of the Discharge Test for coastal waters should be developed and used. The decision at which tier the dissolved metal concentration should be compared with EQS values could depend on the effluent characteristics. For Swedish coastal waters, some consideration should be given to the background concentration of metals.

Impact of tropical cyclone Matmo on mixed zone of the Yellow and Bohai Seas

The Bohai Sea is a low-lying semi-enclosed sea area that is linked to the Yellow Sea via the Bohai straits(mixed zone). Its of fshore seabed is shallow, which makes it vulnerable to serious marine meteorological disasters associated with the northward passage of Pacific tropical cyclones. Analyses on data of remote sensing and buoy of the mixed zone of the Yellow and Bohai seas indicate that all the wind speed, significant wave height, and salinity(SAL) increased, sea surface temperature decreased, and wind energy density changed considerably during the passage of tropical cyclone Matmo on July 25, 2014. It was found that the SAL inversion layer in the mixed zone of the Yellow and Bohai Seas was caused by the tropical cyclone. Furthermore, it was found that the tropical cyclone transported the northern Yellow Sea cold water mass(NYSCWM) into the mixed zone of the Yellow and Bohai Seas. The NYSCWM has direct influence on both the aquaculture and the ecological environment of the region. Therefore, further research is needed to establish the mechanism behind the formation of the SAL inversion layer in the mixed zone, and to determine the influence of tropical cyclones on the NYSCWM.

CIVB flashback analysis of hydrogen flame based on azimuthal vorticity at mixing zone exit

Swirl-premixed combustion systems exhibit potential to meet future regulations on pollution emissions. However, combustion induced vortex breakdown(CIVB) flashback is frequently observed in these systems, especially for high hydrogen content fuel. In this study, a swirl-premixed burner with diverging centerbody was used to investigate CIVB flashback based on azimuthal vorticity at mixing zone exit. Through 2D axisymmetric model, it was found that there was a maximal azimuthal vorticity at mixing zone exit for each equivalence ratio. The physical meaning of these maximal azimuthal vorticity values was the minimally required azimuthal vorticity to trigger CIVB flashback. At the same time, the required azimuthal vorticity declined with the increase of equivalence ratio since turbulent burning velocity started to control flashback. Nevertheless, azimuthal vorticity offered by heat release increased with the increase of equivalence ratio, which promoted flame propagating upstream continually.

Experimental and numerical investigation of inclined air/SF_6 interface instability under shock wave

The shock tube experiments of inclined air/SF6 interface instability under the shock wave with the Mach numbers 1.23 and 1.41 are conducted. The numerical simulation is done with the parallel algorithm and the multi-viscous-fluid and turbulence （MVFT） code of the large-eddy simulation （LES）. The developing process of the interface accelerated by the shock wave is reproduced by the simulations. The complex wave structures, e.g., the propagation, refraction, and reflection of the shock wave, are clearly revealed in the flows. The simulated evolving images of the interface are consistent with the experimental ones. The simulated width of the turbulent mixing zone （TMZ） and the displacements of the bubble and the spike also agree well with the experimental data. Also, the reliability and effectiveness of the MVFT in simulating the problem of interface instability are validated. The more energies are injected into the TMZ when the shock wave has a larger Mach number. Therefore, the perturbed interface develops faster.

Numerical Simulation and Analysis of Migration Law of Gas Mixture Using Carbon Dioxide as Cushion Gas in Underground Gas Storage Reservoir

One of the major technical challenges in using carbon dioxide( CO2) as part of the cushion gas of the underground gas storage reservoir( UGSR) is the mixture of CO2and natural gas. To decrease the mixing extent and manage the migration of the mixed zone,an understanding of the mechanism of CO2and natural gas mixing and the diffusion of the mixed gas in aquifer is necessary. In this paper,a numerical model based on the three dimensional gas-water two-phase flow theory and gas diffusion theory is developed to understand this mechanism. This model is validated by the actual operational data in Dazhangtuo UGSR in Tianjin City,China.Using the validated model,the mixed characteristic of CO2and natural gas and the migration mechanism of the mixed zone in an underground porous reservoir is further studied. Particularly,the impacts of the following factors on the migration mechanism are studied: the ratio of CO2injection,the reservoir porosity and the initial operating pressure. Based on the results,the optimal CO2injection ratio and an optimal control strategy to manage the migration of the mixed zone are obtained. These results provide technical guides for using CO2as cushion gas for UGSR in real projects.

Mechanical Properties and ITZ Microstructure of Recycled Aggregate Concrete Using Carbonated Recycled Coarse Aggregate

The effect of carbonation treatment and mixing method on the mechanical properties and interfacial transition zone（ITZ） properties of recycled aggregate concrete（RAC） was investigated. Properties of recycled concrete aggregate（RCA） were tested firstly. Then, five types of concretes were made and slump of fresh concrete was measured immediately after mixing. Compressive strength and splitting tensile strength of hardened concrete were measured at 28 d. Meanwhile, the microstructure of RAC was analyzed by backscattered electron（BSE） image. It was found that the water absorption ratio of carbonated recycled concrete aggregate（CRCA） was much lower when compared to the untreated RCA. Comparatively, the apparent density of CRCA was not significantly modified. The concrete strength results indicate that the mix CRAC-2 prepared with CRCA by adopting two-stage mixing approach shows the highest compressive strength value compared to the other mixes. The microstructural analysis demonstrate that the mix CRAC-2 has a much denser old ITZ than the untreated RAC because of the chemical reaction between CO2 and the hydration products of RCA. This study confirms that the ITZ microstructure of RAC can be efficiently modified by carbonation treatment of RCA and encourages broadening the application of construction and demolition wastes.

A numerical study of shock-interface interaction and prediction of the mixing zone growth in inhomogeneous medium

The growth of mixing zone on an interface induced by Richtmyer-Meshkov(RM)instability occurs frequently in natural phenomena and in engineering applications.Usually,the medium on which the RM instability happens is inhomogeneous,the effect of medium inhomogeneity on the growth of the mixing zone during the RM instability is still not clear.Therefore,it is necessary to investigate the RM instability in inhomogeneous medium.Based on a high-order computational scheme,the interactions of a density interface with an incident shock wave(ISW)in inhomogeneous medium are numerically simulated by solving the compressible Navier-Stokes equations.The effect of the inhomogeneity on the interface evolution after the passage of ISW through the interface is investigated.The results show that the interface morphology develops in a distinctive "spike-spike"structure in inhomogeneous medium.Particularly,the spike structure on the bottom of the interface is due to the reverse induction of RM instability by curved ISW or reflected shock wave.With the increase of inhomogeneity,the growth rate of the mixing zone width on interface increases,and the wave patterns caused by interaction between the shock wave and interface are more complex.Compared with RM instability in homogeneous medium,the inhomogeneous distribution of the density in medium further enhances the baroclinic effect and induces larger vorticity in flow field.Therefore,the interface is stretched much more significantly under the induction of enhanced vorticity in inhomogeneous medium.Based on above analyses,a model for predicting the growth of mixing zone width on the interface after the passage of ISW is proposed,in order to provide a useful method for evaluations of perturbation growth behavior during the RM instability in inhomogeneous medium.

Numerical Analysis of the Mixing Zone for a Vertical Discharge into a Tidal River

This paper analyzes the mixing zone of a vertical discharge of sewage into a natural tidal river with strong tidal currents. The paper presents a numerical model, which combines 1 D and 2 D models to compute the mixing zone for the Sibao Segment of the Qiantang River. The simple 1 D model was used to model the flow for the entire river using field data as the boundary conditions. The complete depth averaged turbulence model was used for the 2 D computation. The calculated results agree well with the field observations. The analysis provides a practical method for the computation of mixing zones in tidal rivers.