Modelling the responses of the Lagoon of Venice ecosystem to variations in physical forcings

A Finite Element Ecological Model for the Lagoon of Venice (VELFEEM) has been used to test the responses of the Lagoon of Venice ecosystem to variations in physical conditions.The model is obtained by coupling a finite element hydrodynamic model, that computes the velocity fields of water, an energetic model to compute the water temperature fields, and an ecological model that simulates the dynamic of phytoplankton, zooplankton, nutrients (ammonia, nitrate and phosphate) organic detritus (organic nitrogen, organic phosphorous and CBOD) and dissolved oxygen.The transport model is a two-dimensional barotropic finite element model which allows for a better resolution of the lagoon morphology.The ecological model has been developed by starting from the ecological module EUTRO of WASP (Water Analysis Simulation System released by US EPA), and by adapting it to the peculiarity of the Lagoon of Venice.A reference condition has been identified by running a 1-year simulation under climatologic condition. Then, the sensitivity to physical forcing (tide and wind) and to the input of macronutrients has been investigated, by comparing model predictions of spatial and temporal evolution of major state variables and of an aggregate index of Water Quality Trophic Index (TRIX).     

Environmental Impact of Undular Tidal Bores in Tropical Rivers

A tidal bore impacts significantly on the estuarine ecosystem, although little is known on the flow field, mixing and sediment motion beneath tidal bores. In the absence of detailed systematic field measurements, a quasi-steady flow analogy was applied to investigate undular tidal bores with inflow Froude numbers between 1.25 and 1.6. Experimental results indicated that rapid flow redistributions occur beneath the free-surface undulations, with significant variations in bed shear stress between wave crests and troughs. Dynamic similarity was used to predict detailed flow characteristics of undular tidal bores. The effects of periodic loading on river sediments, scour of river bed and flow mixing behind the bore are discussed. A better understanding of these processes will contribute to better management practices in tidal bore affected rivers, including the Styx and Daly rivers in tropical Australia.     

森林-水资源系统的理论分析

本文认为,应当从森林-水资源系统出发研究森林与水资源的关系。并提出了一个分析森林-水资源系统产流过程的仿真模拟方法,得出在一定条件下,有森林植被的地方可能会比无森林植被的地方产生更多的能够为人类所利用的水资源的结论;进一步提出了构成最佳水资源生成系统的四项条件。作者分析了森林对降雨的影响,认为二次降雨和水平降雨对水资源量的直接影响很小;由于森林是大气的特殊下垫面具有软边界和强烈的物质能量变换的特点,加之森林蒸散的水汽和释放的凝结核等因素,可能会影响大气垂直降雨的空间分布,或提高产生降雨的可能性,从而使有林地区有增雨效果。最后对森林-水资源的研究方法提出了几点建议。     

Mesoscale meteorological modelling for Hong Kong-application of the MC2 model

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核电厂大型自然通风冷却塔对气载流出物扩散的影响

为深入研究大型自然通风冷却塔及其湿热羽对内陆核电厂气载流出物扩散影响,应用计算流体力学软件STAR-CCM+提供的k-ε湍流模型模拟了单一冷却塔的运行和停机对不同位置、不同释放高度污染物扩散的影响,结果表明：当释放高度为10m,释放点位于停机冷却塔迎风侧时,释放点下风向的地面轴线弥散因子相比于没有冷却塔时普遍降低1/3~1/2.当释放高度为75m,冷却塔运行时,若释放点位于冷却塔迎风侧时,轴线弥散因子相比于没有冷却塔时普遍增大1~2倍;若释放点位于冷却塔背风侧,则相比于没有冷却塔时普遍降低约1/2.当冷却塔停机时,无论75m高度释放点位于迎风侧还是背风侧,其轴线弥散因子均高于没有冷却塔时.当迎风侧释放高度达到150m时,在释放点下风向约800m的范围内,冷却塔湿热羽使得轴线弥散因子显著增大,但到了800m范围以外,冷却塔湿热羽使得轴线弥散因子减小.     

基于AERMOD线源模式的城市路网一次PM2.5排放扩散特征研究

机动车排放的一次细颗粒物(PM_(2.5))易在城市交通密集区域和高峰时段扩散积累,形成高浓度的排放热点区域,对人体健康影响显著.本研究基于本地化机动车路网排放清单,应用AERMOD扩散模型对北京市六环内区域进行线源扩散模拟,探讨城市路网一次PM_(2.5)扩散浓度的时空变化规律与排放特征、气象条件和临时控制措施的关系.研究表明,在典型工作日的排放水平下,冬季工作日的机动车源一次PM_(2.5)模拟浓度日均值为2.94μg·m-3,夏季工作日为1.95μg·m-3.两季24 h浓度变化均呈夜间浓度高于日间的特点,但两季峰值在气象条件和排放强度的双重作用下又有所区别.冬季早晚高峰峰值浓度分别为日均浓度的2.3和1.7倍,而夏季早晚高峰由于扩散条件相对较好并未明显形成一次PM_(2.5)峰值.研究还以APEC峰会为例,评估峰会期间临时交通管控措施的影响.结果发现,在相似的季节气象条件下,APEC峰会期间机动车源一次PM_(2.5)模拟浓度日均值较峰会前削减50.1%,凌晨5:00达到小时最大削减率66.6%.冬夏季工作日机动车源一次PM_(2.5)扩散浓度的空间分布呈偏态分布,65%的浓度集中在30%的网格面积上.以冬季工作日为例,高浓度热点区域主要集中在各环路快速路、易拥堵主干道及省道、国道和高速路上,以上道路应成为排放控制的重点对象.     

Gas explosion analysis of safety gap effect on the innovating FLNG vessel with a cylindrical platform

After investigating gas dispersion on a cylindrical Floating Liquefied Natural Gas (FLNG) platform (Li et al, 2016), this second article focuses on assessment of gas explosion by using Computational Fluid Dynamics (CFD). Gas explosion simulations are carried out to evaluate the explosion overpressure mitigating effect of safety gap. The Data-dump technique, which is an effective tool in resetting turbulence length scale in gas explosion overpressure calculation, is applied to ensure simulation accuracy for the congestion scenario with safety gap. Two sets of different safety gaps are designed to investigate the safety gap on the cylindrical FLNG platform, the overall results indicate that the safety gap is effective in reducing overpressure in two adjacent congestions. However, for the explosion scenario where the flame is propagating through several safety gaps to the far field congestion, the safety gap mitigates overpressure only in certain explosion protecting targets. Two series of artificial configurations are modeled to further investigate the explosion scenarios with more than two safety gaps in one direction. It is concluded that the optimal safety gap design in overpressure mitigation for the cylindrical FLNG platform is to balance the safety gap distance ratio in the congested regions.     

Modelling of dust lifting process behind propagating shock wave

Dust dispersion from a layer is a complicated problem, which has not been completely solved yet, especially if an Eulerian–Eulerian approach has to be used to model the two-phase dusty flow. In previous investigation, a phenomenological model of the dust dispersion process from a layer was developed, but the evaluation of the model revealed some weaknesses. In the current paper, the model of the dust dispersion process was presented and three improvements of the model were studied: Saffman force, Magnus force and particles collisions. The implementation of Magnus and Saffman forces into the code did not improve the numerical results and it was shown that it had very little influence on the dust lifting process, in case the phenomenological model of the layer is used. Some explanations were proposed in the paper. Besides, an empirical model of particles collisions was also added to the code and its influence on the results was studied. It was shown that the particles collisions model improved the obtained results, but further modifications are to be studied in the future.     

Numerical simulation of landfill aeration using computational fluid dynamics

The present study is an application of Computational Fluid Dynamics (CFD) to the numerical simulation of landfill aeration systems. Specifically, the CFD algorithms provided by the commercial solver ANSYS Fluent 14.0, combined with an in-house source code developed to modify the main solver, were used. The unsaturated multiphase flow of air and liquid phases and the biochemical processes for aerobic biodegradation of the organic fraction of municipal solid waste were simulated taking into consideration their temporal and spatial evolution, as well as complex effects, such as oxygen mass transfer across phases, unsaturated flow effects (capillary suction and unsaturated hydraulic conductivity), temperature variations due to biochemical processes and environmental correction factors for the applied kinetics (Monod and 1st order kinetics). The developed model results were compared with literature experimental data. Also, pilot scale simulations and sensitivity analysis were implemented. Moreover, simulation results of a hypothetical single aeration well were shown, while its zone of influence was estimated using both the pressure and oxygen distribution. Finally, a case study was simulated for a hypothetical landfill aeration system. Both a static (steadily positive or negative relative pressure with time) and a hybrid (following a square wave pattern of positive and negative values of relative pressure with time) scenarios for the aeration wells were examined. The results showed that the present model is capable of simulating landfill aeration and the obtained results were in good agreement with corresponding previous experimental and numerical investigations.     

Description and evaluation of a model of deposition velocities for routine estimates of air pollutant dry deposition over North America.: Part I: model development

This paper describes the development of a detailed dry deposition model for routine computation of dry deposition velocities of SO₂, O₃, HNO₃ and fine particle SO₄²⁻ across much of North America. Four different dry deposition/surface exchange sub-models have been combined with the current Canadian weather forecast model (Global Environmental Multiscale model) with a 3 h time resolution and a horizontal spatial resolution of 35 km. The present model uses the US Geological Survey North American Land Cover Characteristics data to obtain fourteen different land use and five seasonal categories. The four sub-models used are a multi-layer model for gaseous species over taller canopy land-use types, a big-leaf model for gaseous species over lower canopies (including bare soil and water) and for HNO₃ under all surface types and, two different models for SO₄²⁻, one for tall canopies and the other for short canopies. All necessary parameters for each sub-model, chemical species, land-use and seasonal categories have been selected from available data libraries or from the values reported in the literature. The purpose for developing this model (referred to as the Routine Deposition Model (RDM)), when coupled with air concentration data, is to provide estimates of seasonal dry deposition, which can be combined with wet deposition to produce total deposition estimates. Model theory is discussed in this paper and model sensitivity tests and results will be presented in a companion paper.