Large-scale modeling of reactive solute transport in fracture zones of granitic bedrocks

Final disposal of high-level radioactive waste in deep repositories located in fractured granite formations is being considered by several countries. The assessment of the safety of such repositories requires using numerical models of groundwater flow, solute transport and chemical processes. These models are being developed from data and knowledge gained from in situ experiments such as the Redox Zone Experiment carried out at the underground laboratory of Äspö in Sweden. This experiment aimed at evaluating the effects of the construction of the access tunnel on the hydrogeological and hydrochemical conditions of a fracture zone intersected by the tunnel. Most chemical species showed dilution trends except for bicarbonate and sulphate which unexpectedly increased with time. Molinero and Samper [Molinero, J. and Samper, J. Groundwater flow and solute transport in fracture zones: an improved model for a large-scale field experiment at Äspö (Sweden). J. Hydraul. Res., 42, Extra Issue, 157–172] presented a two-dimensional water flow and solute transport finite element model which reproduced measured drawdowns and dilution curves of conservative species. Here we extend their model by using a reactive transport which accounts for aqueous complexation, acid–base, redox processes, dissolution–precipitation of calcite, quartz, hematite and pyrite, and cation exchange between Na⁺ and Ca²⁺. The model provides field-scale estimates of cation exchange capacity of the fracture zone and redox potential of groundwater recharge. It serves also to identify the mineral phases controlling the solubility of iron. In addition, the model is useful to test the relevance of several geochemical processes. Model results rule out calcite dissolution as the process causing the increase in bicarbonate concentration and reject the following possible sources of sulphate: (1) pyrite dissolution, (2) leaching of alkaline sulphate-rich waters from a nearby rock landfill and (3) dissolution of iron monosulphides contained in Baltic seafloor sediments. Based on these results, microbially mediated processes are postulated as the most likely hypothesis to explain the measured increase of dissolved bicarbonates and sulphates after tunnel construction.     

Numerical modeling of the 26th December 2004 India Ocean tsunami at Andaman and Nicobar Islands

A numerical simulation of the 26th December 2004 Indian Ocean tsunami for the Andaman and Nicobar Islands case study is presented. The simulation approach is based on a fully nonlinear Boussinesq tsunami propagation model and included an accurate computational domain and a robust coseismic source. The simulation is first confronted to available tide gauge and run-up observations. The agreement between observations and the predicted wave heights allowed a reasonable validation of the simulation. As a result a full picture of the tsunami impact is provided over the entire coastal zone of Andaman and Nicobar Islands. The processes responsible for coastal vulnerability are discussed.     

Influence of hydrodynamic conditions on the production and fate of Posidonia oceanica in a semi-enclosed shallow basin (Stagnone di Marsala, Western Sicily)

An integrated approach using hydrodynamic and transport numerical models, lepidochronology and stable isotope analysis was used to investigate how local hydrodynamic conditions influence the primary production and fate of the seagrass Posidonia oceanica in a Mediterranean semi-enclosed marine system (Stagnone di Marsala). The water mass exchange aptitude of different sectors of the basin was analysed, and data collected were used to select two sectors (colonized by Posidonia oceanica showing the lowest and highest water exchange values) for biological analyses. According to the mean dispersal coefficient differences simulated by the hydrodynamic model, growth rate and primary production of P. oceanica differed between sectors, with average values lower in the central sector where water exchange is lower than in the southern sector. Although P. oceanica coverage and primary production were higher in the southern sector, carbon and nitrogen stable isotope analysis suggests that the transfer of seagrass organic matter to higher trophic levels of the food web was higher in the central sector. The possibility of a link between hydrodynamism, production and fate of organic matter is proposed to explain the observed patterns.     

Numerical simulation of flow hydrodynamics of struvite pellets in a liquid–solid fluidized bed

Phosphorus recovery in the form of struvite has been aroused in recent decades for its dual advantages in eutrophication control and resource protection.The usage of the struvite products is normally determined by the size which is largely depended on the hydrodynamics.In this study,flow behavior of struvite pellets was simulated by means of Eulerian–Eulerian two-fluid model combining with kinetic theory of granular flow in a liquid–solid fluidized bed reactor(FBR).A parametric study including the mesh size,time step,discretization strategy,turbulent model and drag model was first developed,followed by the evaluations of crucial operational conditions,particle characteristics and reactor shapes.The results showed that a cold model with the mesh resolution of 16 × 240,default time step of 0.001 sec and first order discretization scheme was accurate enough to describe the fluidization.The struvite holdup profile using Syamlal–O'Brien drag model was best fitted to the experimental data as compared with other drag models and the empirical Richardson–Zaki equation.Regarding the model evaluation,it showed that liquid velocity and particle size played important roles on both solid holdups and velocities.The reactor diameter only influenced the solid velocity while the static bed height almost took no effect.These results are direct and can be applied to guide the operation and process control of the struvite fluidization.Moreover,the model parameters can also be used as the basic settings in further crystallization simulations.     

A numerical model of wave- and current-driven nutrient uptake by coral reef communities

We developed a numerical model capable of simulating the spatial zonation of nutrient uptake in coral reef systems driven by hydrodynamic forcing (both from waves and currents). Relationships between nutrient uptake and bed stress derived from flume and field studies were added to a four-component biogeochemical model embedded within a three-dimensional (3-D) hydrodynamic ocean model coupled to a numerical wave model. The performance of the resulting coupled physical-biogeochemical model was first evaluated in an idealized one-dimensional (1-D) channel for both a pure current and a combined wave-current flow. Waves in the channel were represented by an oscillatory flow with constant amplitude and frequency. The simulated nutrient concentrations were in good agreement with the analytical solution for nutrient depletion along a uniform channel, as well as with existing observations of phosphate uptake across a real reef flat. We then applied this integrated model to investigate more complex two-dimensional (2-D) nutrient dynamics, firstly to an idealized coral reef-lagoon morphology, and secondly to a realistic section of Ningaloo Reef in Western Australia, where nutrients were advected into the domain via alongshore coastal currents. Both the idealized reef and Ningaloo Reef simulations showed similar patterns of maximum uptake rates on the shallow forereef and reef crest, and with nutrient concentration decreasing as water flowed over the reef flat. As a result of the cumulative outflow of nutrient-depleted water exiting the reef channels and then being advected down the coast by alongshore currents, both reef simulations exhibited substantial alongshore variation in nutrient concentrations. The coupled models successfully reproduced the observed spatial-variability in nitrate concentration across the Ningaloo Reef system.     

C2H6/CO2组分构成对瓦斯着火延迟时间的影响规律研究

为进一步研究多种不同性质的气体对瓦斯着火过程的影响,采用CHEMKIN-PRO Release软件,选择CH_4燃烧化学反应机理USC Mech 2.0模型对不同组分构成的C_2H_6/CO_2与甲烷混合气体进行数值模拟,然后分析不同组分下瓦斯着火延迟时间的变化趋势,并利用SENKIN程序对其进行敏感性分析。计算结果表明:当C_2H_6百分比小于CO_2百分比时,随着CO_2百分比的增加,瓦斯着火延迟时间略有增加,且其在T=1200K下延长了17.0%,在T=2200K下延长了8.4%,同时抑制CH_4生成的关键反应步敏感性系数下降幅度略大,其协同抑制瓦斯爆炸;当C_2H_6百分比大于CO_2百分比时,随着C_2H_6百分比的增加,瓦斯着火延迟时间大幅缩短,且其在T=1200K下缩短了63.7%,在T=2200K下缩短了35.5%,同时促进CH_4生成的关键反应步敏感性系数下降幅度大,其协同促进瓦斯爆炸。     

氢气爆燃作用下核电站安全壳力学响应数值模拟分析

在核电站严重事故中,由氢气爆燃产生的压力载荷会危及安全壳完整性致其失效,进而造成放射性物质泄漏的严重危害。通过ANSYS/Fluent有限元数值模拟软件,建立了安全壳有限元模型,并对安全壳内氢气爆燃过程以及其力学特性进行了数值模拟研究,获得了氢气爆炸过程中的超压值、升压速率、安全壳变形以及压应力分布。结果表明:爆燃波传递引起压强升高,火焰阵面处压强最高,爆燃波所经区域超压疾速上升随后快速下降;爆燃作用下,顶部壳体和下部筒体连接区域混凝土位移最大,最大压应力也集中分布在该区域,最易受到破坏。获得的结论可为安全壳结构抗爆设计和安全性研究提供理论参考。     

Assessment of the effects of a port expansion on algae appearance in a costal bay through mathematical modelling. Application to San Lorenzo Bay (North Spain)

In the last years several episodes of algae appearance affecting bathing areas have been observed in San Lorenzo Bay (north of Spain). The analysis of the collected algae revealed that they might come from near intertidal or shallow subtidal zones due to eutrophication processes or through drift algae movement by the action of marine currents. In the vicinity of this area, the expansion of the Port of Gijón (now under construction) supposes a significant modification of the coastal geometry. The magnitude of such an expansion could cause changes in the patterns of currents in the bay, with the consequent alteration of the observed algal appearance phenomena. A mathematical modelling study to evaluate the risk of generation of eutrophication processes in the San Lorenzo Bay area and the transport of drift algae from near sea bed areas was developed. This study required the use of different hydrodynamic models in order to characterize the currents caused by tides, winds and waves. The eutrophication processes in the bay were analyzed with a depth-averaged two-dimensional eutrophication model which deals with eight water quality variables. Calibration of model parameters with the observed data from a field survey was performed. A reasonable agreement with the field measurements was achieved. Model results showed that the maximum phytoplankton concentrations were below eutrophic conditions. Although, the port expansion has led to an increment of phytoplankton concentrations, chlorophyll a levels were not representative of eutrophic conditions. To analyse the transport of drift algae, a methodology based on the utilization of a two-dimensional model which solves the depth-averaged advection-diffusion equation considering seaweed as a conservative tracer was developed and applied. Numerical modelling allowed the identification of the coastal areas that seems to be the source of the seaweed found on the beach. It was also proven that port expansion does not significantly affect drift algae transport in the area.     

非预混碳氢火焰结构条件矩封闭模拟

采用Smooke 35步机理和条件矩封闭模型,数值方法应用二阶精度中心差分和刚性系统算法器VODE,对一甲烷对撞火焰结构进行了模拟,获得了非预混火焰中反应物、主产物和微量组分随混合分数的变化关系.结果表明,该方法具有较好预测效果,具有较大潜力.     

树冠火蔓延模型和数值分析

本文计及森林多孔性燃烧床含水量和有机物含量，以及渗流气体的受热膨胀和质量源，建立了树冠火蔓延的一维物理模型，给出火蔓延的控制方程组和定解条件。通过数值分析，得到树冠火蔓延速度特性曲线，研究水份蒸发和有机物热解对树冠火蔓延速度的影响。