Dispersal of fish populations in dams: modelling and simulation

A mathematical model is introduced for the analysis of changes in the habitat of certain species and consequent migration processes. The weak formulation for the resulting partial differential non-linear equation is presented, as well as a linearization method. Existence and uniqueness results are analyzed. In addition, a numerical method is suggested. Finally, results from numerical simulations with empirical parameters are shown and discussed from an ecological point of view.     

Numerical simulation of sand dune erosion

Erosion of sand or other granular material is a subject of utmost importance in several fields of practical interest, including industrial processes or environmental issues. Resulting from intricate interaction between the incident flow field and localized body forces responsible for the granular material cohesion, erosion is a particularly complex phenomenon. The present work addresses this problem, proposing a numerical method to compute the time evolution of a sand dune subjected to aeolian erosion, along with the associated entrainment and deposition fluxes. Turbulent fluid flow is computed through a three-dimensional Navier-Stokes solver based on a generalized coordinate system. A Lagrangian approach is adopted for tracking the trajectories of particles entrained in the saltation regime, thus allowing prediction of the corresponding deposition locations. Different models for saltation fluxes are tested, along with several formulations for the creeping-to-saltation flux ratio, creeping threshold and creeping distance. Comparison with results from wind tunnel experiments is very encouraging, stressing the relative importance of creeping in the erosion process for the presently studied conditions.     

Numerical simulation of particle-driven gravity currents

Particle-driven gravity currents frequently occur in nature, for instance as turbidity currents in reservoirs. They are produced by the buoyant forces between fluids of different density and can introduce sediments and pollutants into water bodies. In this study, the propagation dynamics of gravity currents is investigated using the FLOW-3D computational fluid dynamics code. The performance of the numerical model using two different turbulence closure schemes namely the renormalization group (RNG) ${k-\epsilon}$ scheme in a Reynold-averaged Navier-Stokes framework (RANS) and the large-eddy simulation (LES) technique using the Smagorinsky scheme, were compared with laboratory experiments. The numerical simulations focus on two different types of density flows from laboratory experiments namely: Intrusive Gravity Currents (IGC) and Particle-Driven Gravity Currents (PDGC). The simulated evolution profiles and propagation speeds are compared with laboratory experiments and analytical solutions. The numerical model shows good quantitative agreement for predicting the temporal and spatial evolution of intrusive gravity currents. In particular, the simulated propagation speeds are in excellent agreement with experimental results. The simulation results do not show any considerable discrepancies between RNG ${k-\epsilon}$ and LES closure schemes. The FLOW-3D model coupled with a particle dynamics algorithm successfully captured the decreasing propagation speeds of PDGC due to settling of sediment particles. The simulation results show that the ratio of transported to initial concentration C _o /C _i by the gravity current varies as a function of the particle diameter d _s . We classify the transport pattern by PDGC into three regimes: (1) a suspended regime (d _s is less than about 16 μm) where the effect of particle deposition rate on the propagation dynamics of gravity currents is negligible i.e. such flows behave like homogeneous fluids (IGC); (2) a mixed regime (16 μm < d _s <40 μm) where deposition rates significantly change the flow dynamics; and (3) a deposition regime (d _s ?> 40 μm) where the PDGC rapidly loses its forward momentum due to fast deposition. The present work highlights the potential of the RANS simulation technique using the RNG ${k-\epsilon}$ turbulence closure scheme for field scale investigation of particle-driven gravity currents.     

Application of Markov-chain model for vegetation restoration assessment at landslide areas caused by a catastrophic earthquake in Central Taiwan

The 921 earthquake caused a catastrophic disaster in Central Taiwan. Ten years have passed since the earthquake occurred. Vegetation succession is the basis for establishing a restoration reference which plays an important role in vegetation restoration at landslide sites. Generally, growth conditions for grass are easier and the growth rate is faster than that for trees. Therefore, grass can be considered a pioneer species or an important reference for the early vegetation succession stage. This is the reason why grass is required to be extracted from other land covers. Integrating remote sensing, geographic information system and image classification into vegetation succession models is very important. In this study, the Markov chain model was applied for vegetation restoration assessment and discussion. Chiufenershan and Ninety-nine peaks were selected as the study areas. Five SPOT satellite images are used for land cover mapping and vegetation restoration simulations. Four categories of land covers were extracted, including forest, grass, bare land and water, respectively. From the transitive probability matrix (derived from any two land covers), the results show that vegetation restoration at the Chiufenershan and Ninety-nine peaks landslide areas is ongoing, but that has been disturbed by natural disasters.     

Multi-parameter renewal processes and simulation of forest fires

The concept of the renewal property is extended to processes indexed by a multidimensional time parameter. The definition given includes not only partial sum processes, but also Poisson processes and many other point processes whose jump points are not totally ordered. Various properties of renewal processes are discussed. Renewal processes are proposed as a basis for modelling the spread of a forest fire under a prevailing wind.

Numerical simulation of benzene transport in shoreline groundwater affected by tides under different conditions

● An approach for assessing the transport of benzene on the beach was proposed. ● The behavior of benzene in the subsurface of the beach was impacted by tide. ● Tidal amplitude influenced the travel speed and the benzene biodegradation. ● Hydraulic conductivity had the impact on plume residence time and biodegradation. ● Plume dispersed and concentration decreased due to high longitudinal dispersivity. The release and transport of benzene in coastal aquifers were investigated in the present study. Numerical simulations were implemented using the SEAM3D, coupled with GMS, to study the behavior of benzene in the subsurface of tidally influenced beaches. The transport and fate of the benzene plume were simulated, considering advection, dispersion, sorption, biodegradation, and dissolution on the beach. Different tide amplitudes, aquifer characteristics, and pollutant release locations were studied. It was found that the tide amplitude, hydraulic conductivity, and longitudinal dispersivity were the primary factors affecting the fate and transport of benzene. The tidal amplitude influenced the transport speed and percentage of biodegradation of benzene plume in the beach. A high tidal range reduced the spreading area and enhanced the rate of benzene biodegradation. Hydraulic conductivity had an impact on plume residence time and the percentage of contaminant biodegradation. Lower hydraulic conductivity induced longer residence time in each beach portion and a higher percentage of biodegradation on the beach. The plume dispersed and the concentration decreased due to high longitudinal dispersivity. The results can be used to support future risk assessment and management for the shorelines impacted by spill and leaking accidents. Modeling the heterogeneous beach aquifer subjected to tides can also be further explored in the future study.     

Numerical simulation of non-breaking solitary wave run-up using exponential basis functions

A meshless method based on exponential basis functions (EBFs) is developed to simulate the propagation of solitary waves and run-up on the slope. The presented method is a boundary-type meshless method applying the exponential basis functions with complex exponents. The solution of governing equations is considered as a series of these basis functions. Boundary conditions are satisfied through a point-wise collocation approach. Based on the presented EBF meshless method, a new formula is introduced for the maximum run-up height on different slopes, valuable for engineering applications. The results obtained through the numerical method in the prediction of solitary wave propagation and estimation of run-up are verified through the comparison with experimental data. The comparison with 159 experimental data indicates that this new formula is more accurate than the preceding formulas in predicting the maximum run-up of non-breaking solitary waves. Minimum calculation time and convenient performances are the other advantages of this method.     

Numerical simulation of stratified flow around a tall building of a complex shape

Environmental Fluid Mechanics - Results of large-eddy simulations of stably stratified atmospheric flow around an isolated, complex-shaped tall building are presented. The study focuses on the...     

Understanding of the electrochemical behaviors of aqueous zinc-manganese batteries: Reaction processes and failure mechanisms

As one of the most common cathode materials for aqueous zinc-ion batteries(AZIBs),manganese oxides have the advantages of abundant reserves,low cost,and low toxicity.However,the electrochemical mechanism at the cathode of aqueous zinc-manganese batteries(AZMBs) is complicated due to different electrode materials,electrolytes and working conditions.These complicated mechanisms severely limit the research progress of AZMBs system and the design of cells with better performance.Hence,the mechanism ...     

Numerical simulation and optimization study of the wind flow through a porous fence

Three turbulence closure models (RNG k-ε, SST k-ω and RSM) were used to investigate the flow characteristics around a two-dimensional isolated porous fence. The comparison between the numerical results and the experimental measurements indicated that RSM model shows a better performance than the other two models. The aim of this paper is to accurately and efficiently determine the optimum porosity that attain the best shelter effect of the wind fence in the near wake region (0–4h_b) and in the far wake region (4h_b–10h_b) respectively, where h_b is the height of the fence. The gradient algorithm was adopted as the optimization algorithm and the RSM model was used to model turbulent features of the flow. The shelter effect was parameterized by the peak velocity ratio involving velocity and turbulence. The objective was to reduce the peak velocity ratio in the near or far wake region by changing the design variable porosity (?) of the fence, which ranged between 2 and 60%. The results revealed that a porosity of 10.2% was found as the optimum value giving rise to the best shelter effect in the near wake region, and ? = 22.1% was determined in the case of the far wake region. In addition, based on the proposed optimization method, it is found that the recirculating bubble behind the fence can only be detected when ? < 29.9%.     

Degradation of bisphenol A by photo-fenton processes

The photo-Fenton reactions, which could yield hydroxyl radicals via the catalytic degradation of H₂O₂ by Fe(II), were focused as one of the abiotic degradation processes of bisphenol A (BPA) in surface waters. At pH 6, in the presence of H₂O₂ only, 32% of BPA was degraded after 120?min of irradiation. However, 97% of BPA was degraded in the presence of both H₂O₂ and Fe(II). Without light irradiation, no BPA degradation was observed even in the presence of Fe(II) and H₂O₂. These results show that photo-Fenton processes are effective in the natural attenuation of BPA in surface water. In addition, the presence of humic acids (HAs), which were of more aliphatic nature, resulted in enhancing BPA degradation via the photo-Fenton processes. Therefore, HAs can be one of the important factors in enhancing the degradation of BPA in surface water via the photo-Fenton processes.     

Numerical simulation of pedestrian level wind conditions: effect of building shape and orientation

Environmental Fluid Mechanics - Pedestrian level wind comfort, natural ventilation and pollutant dispersion are strongly influenced by building shape and orientation. In the recent years, the trend...     

Numerical simulation of scour around pipelines using an Euler–Euler coupled two-phase model

The scour around a long fixed pipeline placed just above a non-cohesive sandy bed is numerically simulated using an Eulerian two-phase model that implements Euler–Euler coupled governing equations for fluid and solid phases and a modified k−ɛ turbulence closure for the fluid phase, the modeling system being a part of the CFD software package FLUENT. Both flow–particle and particle–particle interactions are considered in the model. During the simulations, the interface between sand and water is specified using a threshold volume fraction of sand, and the evolution of the bedforms is studied in detail. The predictions of bedform evolution are in good agreement with previous laboratory measurements. Investigations into the mechanisms of scour reveal that three sediment transport modes (bed-load, suspended-load and laminated-load) are associated with the scour development. While some previously proposed scour development formulae for cylindrical objects are in good general agreement with the simulations, scour prediction based on a commonly used operational mine-burial model (DRAMBUIE) shows disparities with present simulations.     

Warm discharges in cold fresh water: 2. Numerical simulation of laminar line plumes

The behaviour of a discharge of warm water upwards into a homogeneous body of cold fresh water was investigated by means of a numerical model. The discharge has a parabolic velocity profile, with Reynolds number \(Re=50\), Prandtl number \(Pr=7\) and Froude number varied over the range \(0.2 \le {\rm Fr} \le 2.5\). Water density is taken to be a quadratic function of temperature, so that an initially positively buoyant discharge will experience buoyancy reversal as it mixes with an ambient below the temperature of maximum density. The resulting plume has some similarities to a fountain resulting from injection of negatively buoyant fluid upward into a less dense ambient. The plume is initially symmetric, but then its head detaches as it approaches its maximum height. The detached head is denser than the fluid in the plume below it, and the interaction between the sinking head and the rising plume causes a sideways deflection; as this cycle is repeated, the plume displays side-to-side flapping motion and vertical bobbing. As Froude number is increased (i.e. buoyancy reduced) the growth of the plume becomes slower, but the plume eventually reaches a greater height. We obtain empirical power-law scalings for maximum height and time taken to reach that height as functions of Froude number; these scalings are simlar to those for fountains with a linear dependence of density on temperature in the very weak regime.     

Laboratory simulation of light-focusing by water-surface waves

A laboratory system has been developed to simulate wave-induced irradiance fluctuations that occur in the top few metres of the sea under sunny surface conditions. A principle of operation is that the fluctuations are produced after refraction of light by water waves generated in the tank. Simulated irradiance consists of repetitive high-amplitude flashes resulting from a lens-effect of the waves. Statistical properties of flashes are similar to those recorded at sea. In our laboratory simulation, the flashes that exceed an amplitude level of 1.5 (where is the time-averaged irradiance) have a mean frequency of about 230 min^-1, and a typical duration of 5 to 20 ms. The frequency of the flashes decreases exponentially with increasing amplitude level. The system simulates timeaveraged irradiance comparable to full sunlight at shallow depths (800 mol quanta m^-2 s^-1 over the visible spectral region), and provides light with a reasonable spectral composition. The simulation method was designed for studying responses of phytoplankton, but applications involving optical aspects can also be foreseen.     

Numerical simulation of water mixing and renewals in the Barcelona harbour area: the winter season

In the present paper, we use numerical simulation to investigate currents, mixing and water renewal in Barcelona harbour under typical conditions of wind forcing for the winter season. This site is of particular importance due to the interplay between touristic and commercial activities, requiring detailed and high-definition studies of water quality within the harbour. We use Large Eddy Simulation (LES) which directly resolves the anisotropic and energetic large scales of motion and parametrizes the small, dissipative, ones. Small-scale turbulence is modelled by the anisotropic Smagorinsky model (ASM) to be employed in presence of large cell anisotropy. The complexity of the harbour is modelled using a combination of curvilinear, structured, non-staggered grid and the immersed boundary method. Boundary conditions for wind and currents at the inlets of the port are obtained from in-situ measurements. Analysis of the numerical results is carried out based on both instantaneous and time-averaged velocity fields. First- and second-order statistics, such as turbulent kinetic energy and horizontal and vertical eddy viscosities, are calculated and their spatial distribution is discussed. The study shows the presence of intense current in the narrow and elongated part of the harbour together with sub-surface along-shore elongated rolling structures (with a time scale of a few hours), and they contribute to the vertical water mixing. Time-averaged velocity field reveals intense upwelling and downwelling zones along the walls of the harbour. The analysis of second-order statistics shows strong inhomogeneity of turbulent kinetic energy and horizontal and vertical eddy viscosities in the horizontal plane, with larger values in the regions characterized by stronger currents. The water renewal within the port is quantified for particular sub-domain regions, showing that the complexity of the harbour is such that certain in-harbour basins have a water renewal of over five days, including the yacht marina area. The LES solution compares favourably with available current-meter data. The LES solution is also compared with a RANS solution obtained in literature for the same site under the same forcing conditions, the comparison demonstrating a large sensitivity of properties to model resolution and frictional parametrization.     

天津城市热岛及其对污染物扩散影响的数值模拟

城市热岛直接影响城市风场结构和污染物扩散路径,以能量平衡模式得到的地面温度作为下边界条件、中尺度气象模式MM5作初始场和侧边界条件,建立了天津市500 m的细网格城市边界层模式并用其研究天津市秋季热岛及其对污染物扩散的影响.结果表明,模式可以较好地模拟城市热岛现象,地面温度日变化规律及近地层的温度廓线与实际观测值较一致,能够反映夜间出现的逆温.模式成功地再现了城市温度场和流场的三维结构,结果表明17:00时热岛强度在地面最强,到300m存在个别的弱热岛中心,500m高度城郊不存在温度差异;地面高温中心有一个弱的辐合中心,直接影响污染物在城区的扩散,模拟的污染物浓度在地面较低,150～300 m高度最大.该研究结果可为了解天津地区局地气候的形成、污染物的扩散及城市规划提供参考和帮助.     

Numerical simulation for impacts of hydrodynamic conditions on algae growth in Chongqing Section of Jialing River, China

Hydrodynamic conditions are important factors for planktonic algae growth, through introducing two parameters which express the optimal velocity and the velocity range for planktonic algae growth, a new velocity factor was put forward for the formula of growth rate. Therefore, the two-dimensional unsteady ecological dynamic model for algae growth was established to analyze the effects of hydrodynamic conditions on algae growth in Chongqing Reach of Jialing River in China. The temporal and spatial distribution of Chlorophyll-a (Chl-a) concentration was simulated numerically for various water levels, under climate conditions in period of high frequency for algae blooms of Three Gorges Reservoir and nutrition status at present in the research reach. The corresponding locations and areas of likely algae blooms were analyzed and forecasted. The results showed that about 0.04 m s⁻¹ was the optimal velocity for algae growth, and the occurrence of algae blooms in large scale is almost impossible because of relatively high water flow velocity for Jialing River.     

Optimization of anaerobic processes by speciation and bioavailability of trace metals

The deficiency trials of some trace metals (copper, nickel, zinc) were made in laboratory Upflow Anaerobic Sludge Blanket (UASB) reactors containing the inoculum of an industrial anaerobic sludge, a basal medium with the rest of nutrients and vitamins in comparison with a control reactor with the optimal doses of trace elements. From the main control parameters (alkalinity, acidity, DQO), no inactivation by deficit of essential elements was detected, but the control reactor showed greater mineralization and solid content, being necessary to make purge to avoid bulking. The inoculum has high metallic element contents, so the additional contribution produces high bacterial growth, also purge being necessary to avoid bulking. The purge, made after 120 days in continuous operation, eliminates a great quantity of solids and increases the volatile solids (VS)/total solids (TS) ratio. The comparison of the initial content of metallic elements in the solid and liquid phases with the contents after 120 days showed a progressive depletion of metals in all reactors without a clear appreciation of the lack of the studied metals. Finally, in 210 days, the reactors without addition of a metallic element accumulated greater quantity of major (Ca, Fe, Mg) and trace (Co, Ni, Cu) elements in the solid phase with respect to the control.     

Effects of flow regimes altered by dams on survival, population declines, and range-wide losses of California river-breeding frogs

Widespread alteration of natural hydrologic patterns by large dams combined with peak demands for power and water delivery during summer months have resulted in frequent aseasonal flow pulses in rivers of western North America. Native species in these ecosystems have evolved with predictable annual flood-drought cycles; thus, their likelihood of persistence may decrease in response to disruption of the seasonal synchrony between stable low-flow conditions and reproduction. We evaluated whether altered flow regimes affected 2 native frogs in California and Oregon (U.S.A.) at 4 spatial and temporal extents. We examined changes in species distribution over approximately 50 years, current population density in 11 regulated and 16 unregulated rivers, temporal trends in abundance among populations occupying rivers with different hydrologic histories, and within-year patterns of survival relative to seasonal hydrology. The foothill yellow-legged frog (Rana boylii), which breeds only in flowing water, is more likely to be absent downstream of large dams than in free-flowing rivers, and breeding populations are on average 5 times smaller in regulated rivers than in unregulated rivers. Time series data (range = 8 - 19 years) from 5 populations of yellow-legged frogs and 2 populations of California red-legged frogs (R. draytonii) across a gradient of natural to highly artificial timing and magnitude of flooding indicate that variability of flows in spring and summer is strongly correlated with high mortality of early life stages and subsequent decreases in densities of adult females. Flow management that better mimics natural flow timing is likely to promote persistence of these species and others with similar phenology.