Instantaneous transport of a passive scalar in a turbulent separated flow

The results of large-eddy simulations of flow and transient solute transport over a backward facing step and through a 180° bend are presented. The simulations are validated successfully in terms of hydrodynamics and tracer transport with experimental velocity data and measured residence time distribution curves confirming the accuracy of the method. The hydrodynamics are characterised by flow separation and subsequent recirculation in vertical and horizontal directions and the solute dispersion process is a direct response to the significant unsteadiness and turbulence in the flow. The turbulence in the system is analysed and quantified in terms of power density spectra and covariance of velocity fluctuations. The injection of an instantaneous passive tracer and its dispersion through the system is simulated. Large-eddy simulations enable the resolution of the instantaneous flow field and it is demonstrated that the instabilities of intermittent large-scale structures play a distinguished role in the solute transport. The advection and diffusion of the scalar is governed by the severe unsteadiness of the flow and this is visualised and quantified. The analysis of the scalar mass transport budget quantifies the mechanisms controlling the turbulent mixing and reveals that the mass flux is dominated by advection.     

A non-negative and high-resolution finite volume method for the depth-integrated solute transport equation using an unstructured triangular mesh

This paper proposes a new high-resolution finite volume method for solving the two-dimensional (2D) solute transport equation using an unstructured mesh. A new simple r-factor algorithm is introduced into the Total Variation Diminishing flux limiter to achieve a more efficient yet accurate high-resolution scheme for solving the advection term. To avoid the physically-meaningless negative solutions resulted from using the Green–Gauss theorem, a nonlinear two-point flux approximation scheme is adopted to deal with the anisotropic diffusion term. The developed method can be readily coupled with a two-dimensional finite-volume-based flow models under unstructured triangular mesh. By integrating with the ELCIRC flow model, the proposed method was verified using three idealized benchmark cases (i.e., advection of a circle-shaped solute field, advection in a cyclogenesis flow field and transport of a initially square-shaped solute plume), and further applied to simulate the non-reactive solute transport process driven by irregular tides in the Deep Bay, eastern Pearl River Estuary of China. These cases are also simulated by models using other existing methods, including different r-factor for advection term and the Green–Gauss theorem for diffusion term. The comparison between the results from the new method and those from other existing methods demonstrated the new method could describe advection induced concentration shock and discontinuities, and anisotropic diffusion at high resolution without providing spurious oscillations and negative values.     

A comparison of three Eulerian numerical methods for fractional-order transport models

Tracer transport in complex systems like turbulent flows or heterogeneous porous media is now more and more regarded as a non-local process that can hardly be represented by second-order diffusion models. In this work, we consider diffusion models that assume that tracer particles follow a heavy-tail Lévy distribution, which allows for large displacements. We show that such an assumption leads to a fractional-order diffusion operator in the governing equation for tracer concentration. A comparison of three Eulerian numerical methods to discretize that equation is then performed. These consist of the finite difference, finite element and spectral element methods. We suggest that non-local methods, like the spectral element method, are better suited to transport models with fractional-order diffusion operators.     

A fractal derivative model to quantify bed-load transport along a heterogeneous sand bed

Environmental Fluid Mechanics - Bed-load transport is a complex process exhibiting anomalous dynamics, which cannot be efficiently described using the traditional advection–diffusion...     

CFCs地下水年龄示踪技术的原理、应用及展望

CFCs是一种合成的含氯离子和氟离子的有机化学品,主要组成包括CFC-11、CFC-12与CFC-113等。其自1930年以来开始被大量生产,年生产量达106t。由于其具有挥发性与良好的化学稳定性,通过量化地下水中CFCs的平衡分压,将该平衡分压与大气CFCs浓度变化标准曲线进行对比,即可确定目标区域地下水的年龄。自20世纪70年代中期美国首次尝试利用CFCs用于研究地下水年龄以来,国际上日益重视,广泛开展了其在确定地下水年龄、揭示浅层地下水和深层地下水之间的水力联系、确定地下水各种来源水的混合比例等地下水循环演化方面的研究,但目前中国等发展中国家对该技术的研究及应用还处于初步发展阶段。从CFCs的特性、生产及环境释放入手,阐述CFCs测试技术的关键环节——地下水的野外采样、CFCs的提取、测定及CFCs方法测定地下水年龄的原理与计算方法,并以全球视角对CFCs年龄示踪技术在国内外的应用进行综述。在比较分析目前测定地下水年龄测定的主要方法中,指出CFCs环境示踪技术相比SF6、3H/3He等其它环境示踪技术具有分析便捷、价格适中、精度较高的显著优点,特别是在已经做过历年大气CFCs浓度监测等相关研究的地区的应用。     

Calibration and predictive ability analysis of longitudinal solute transport models in mountain streams

The first step in developing travel time and water quality models in streams is to correctly model solute transport mechanisms. In this paper a comparison between two solute transport models is performed. The parameters of the Transient Storage model (TS) and the Aggregated Dead Zone model (ADZ) are estimated using data of thirty seven tracer experiments carried out under different discharges in five mountain streams of Colombian Los Andes. Calibration is performed with the generalized uncertainty estimation method (GLUE) based on Monte-Carlo simulations. Aspects of model parameters identifiability and model parsimony are analyzed and discussed. The TS model with four parameters shows excellent results during calibration but the model parameters present high interaction and poor identifiability. The ADZ model with two independent and clearly identifiable parameters gives sufficiently precise calibration results. As a conclusion, it is stated that the ADZ model with only two parameters is a parsimonious model that is able to represent solute transport mechanisms of advection and longitudinal dispersion in the studied mountain streams. A simple model parameter estimation methodology as a function of discharge is proposed in this work to be used in prediction mode of travel time and solute transport applications along mountain streams.     

Solute transport in a semi-infinite homogeneous aquifer with a fixed point source concentration

This study derives an analytical solution of the advection–dispersion (AD) equation commonly used to describe the transport of pollutants in a semi-infinite homogeneous aquifer. When an extra constant source term is added to the AD equation, it changes the solution of the equation. The AD equation is solved analytically using Laplace transform. Also, the equation is solved numerically using an explicit finite difference method and its stability condition is presented with the aid of matrix method. For the solution of the AD equation the following considerations are made: (1) The dispersion and velocity are considered as time-dependent; (2) dispersion is expressed as directly proportional to the square of velocity; (3) there is also diffusion; (4) there is some initial concentration and the aquifer domain is, therefore, not pollutant-free; (5) There is a time-dependent exponentially decreasing input source; and (6) the concentration gradient is assumed to be zero at the exit boundary. It is found that the contaminant concentration decreases with time contrary to what happens when the extra term is not included.     

Simulating transport of 129I and idealized tracers in the northern North Atlantic Ocean

Transport of the radioactive tracer Iodine-129 (¹²⁹I, T _1/2 = 15.7 Myr) in the northern North Atlantic Ocean has been investigated using a global isopycnic Ocean General Circulation Model (OGCM) and observed data. ¹²⁹I originates mainly from the nuclear fuel reprocessing plants in Sellafield (UK) and La Hague (France), and is transported northwards along the Norwegian coast, and then into surface and intermediate layers in the Arctic Ocean through the Barents Sea and the Fram Strait, but also partly recirculating south along the eastern coast of Greenland. In the North Atlantic Subpolar Seas, ¹²⁹I is mainly found in dense overflow waters from the Nordic Seas being exported southwards in the Deep Western Boundary Current, and to a lesser extent in surface and intermediate layers circulating cyclonically within the Subpolar Gyre. Observed concentration of ¹²⁹I along a surface transect in the eastern Nordic Seas in 2001 is captured by the OGCM, while in the Nansen Basin of the Arctic Ocean the OGCM overestimates the observed values by a factor of two. The vertical profile of ¹²⁹I in the Labrador Sea, repeatedly observed since 1997 to present, is fairly realistically reproduced by the OGCM. This indicates that the applied model system has potential for predicting the magnitude and depth of overflow waters from the Nordic Seas into the North Atlantic Subpolar Seas. To supplement the information obtained from the ¹²⁹I distribution, we have conducted a number of idealized tracer experiments with the OGCM, including tracers mimicking pure water masses, and instantaneous pulse releases. New insight into time-scales of tracer transport in this region is obtained by utilizing a few recently developed methods based on the theory of Transit Time Distribution (TTD) and age of tracers. Implications for other types of “anomalies” in the northern North Atlantic Ocean, being anomalous hydrography or chemical tracers, and how they are interpreted, are discussed.     

Suspended particulate matter dynamics in a particle framework

Suspended particulate matter (SPM) dynamics in ocean models are usually treated with an advection–diffusion equation for one or more sediment size classes coupled to the hydrodynamical part of the model. Numerical solution of these additional partial differential equations unavoidably introduces numerical diffusion, i.e. in the case of sharp gradients the possible occurrence of artificial oscillations and non-positivity. A Lagrangian particle-tracking model has been developed to simulate short-term SPM dynamics. Modelling individual sediment particles allows a straightforward physical interpretation of the processes. The tracking of large numbers of individual and independent particles (up to 25 million in total in a single sediment class) can be achieved on high performance computer clusters, due to efficient parallelisation of particle tracking. The movement of the particles is described by a stochastic differential equation, which is consistent with the advection–diffusion equation. Here, the concentration profile is represented by a set of independent moving particles, which are advected according to the 3D velocity field, while the diffusive displacements of the particles are sampled from a random distribution, which is related to the eddy diffusivity field. To account for erosion a new parameterisation is proposed. Three numerical particle tracking schemes (EULER, MILSTEIN and HEUN) are presented and validated in idealised test cases. Finally, the particle tracking algorithms are applied to a realistic scenario, a severe winter storm in the East Frisian Wadden Sea (southern North Sea). The comparison with observations and an Eulerian SPM transport model seems to indicate a somewhat better fidelity of the Lagrangian approach.     

Solution of fully-coupled shallow water equations and contaminant transport using a primitive-variable Riemann method

A Riemann-solver scheme, using primitive variables rather than conserved variables, is configured and tuned for the solution of the fully-coupled two-dimensional shallow water and contaminant transport equations. This scheme is based on the unstructured finite volume discretization using primitive-variable Roe-flux approximation with an entropy fix. The primitive-variable flux associated with the exact source-term balancing is well-behaved and well-balanced for both still-water and dry regions with arbitrary bed topography. Second-order accuracy is used in space and time. The present study uses a nonlinear implicit scheme based on Newton-iterative algorithm for the time integration. In order to show the accuracy of the scheme, numerical results are verified by different test cases for contaminant advection and diffusion. A scenario of contaminant transport in a complex geometry with wet and dry elements is also simulated to demonstrate that the present work can be implemented on practical applications involving flooding and contaminant transport.     

A σ-coordinate transport model coupled with rotational boussinesq-type equations

This paper describes a σ-coordinate scalar transport model coupled with a Boussinesq-type hydrodynamic model. The Boussinesq model has the ability to calculate both three-dimensional velocity distributions and the water surface motion. To capture ‘dispersion’ processes in open channel flow, horizontal vorticity effects induced by a bottom shear stress are included in the Boussinesq model. Thus, a reasonable representation of vertical flow structure can be captured in shallow and wavy flow fields. To solve the coupled Boussinesq and scalar transport system, a finite-volume method, based on a Godunov-type scheme with the HLL Riemann solver, is employed. Basic advection and advection–diffusion numerical tests in a non-rectangular domain were carried out and the computed results show good agreement with analytic solutions. With quantitative comparisons of dispersion experiments in an open channel, it is verified that the proposed coupled model is appropriate for both near and far field scalar transport predictions. From numerical simulations in the surf zone, physically reasonable results showing expected vertical variation are obtained.     

A model diagnostic study of age of river-borne sediment transport in the tidal York River Estuary

As nutrients and organic matters are transported preferentially in an adsorbed state and tend to bind to the sediments, sediment transport plays an important role on eutrophication processes in the estuaries. The timescale of sediment transport is of significance for studying the retention of pollutants and eutrophication processes in the estuaries. Unlike transport of dissolved substances that is mainly controlled by advection and diffusion processes, the sediment transport is significantly affected by the intermittent settling and resuspension processes. A three-dimensional model with suspended sediment transport was utilized to investigate the transport timescale of river-borne sediment in the tidal York River Estuary. The results indicate that river discharge dominantly determines the age of river-borne sediment in the estuary. High river discharge results in a low sediment age compared to that under mean flow. The intermittent effects of settling and resuspension events greatly affect the river-borne sediment age. Both settling velocity and critical shear stress are shown to be key parameters in determining the sediment transport timescale. The sediment age decreases as settling velocity and/or critical shear stress decrease, while it increases with the increase of settling velocity that prevents the sediment to be transported out of the estuary.     

A review of source tracking techniques for fine sediment within a catchment

Excessive transport of fine sediment, and its associated pollutants, can cause detrimental impacts in aquatic environments. It is therefore important to perform accurate sediment source apportionment to identify hot spots of soil erosion. Various tracers have been adopted, often in combination, to identify sediment source type and its spatial origin; these include fallout radionuclides, geochemical tracers, mineral magnetic properties and bulk and compound-specific stable isotopes. In this review, the applicability of these techniques to particular settings and their advantages and limitations are reviewed. By synthesizing existing approaches, that make use of multiple tracers in combination with measured changes of channel geomorphological attributes, an integrated analysis of tracer profiles in deposited sediments in lakes and reservoirs can be made. Through a multi-scale approach for fine sediment tracking, temporal changes in soil erosion and sediment load can be reconstructed and the consequences of changing catchment practices evaluated. We recommend that long-term, as well as short-term, monitoring of riverine fine sediment and corresponding surface and subsurface sources at nested sites within a catchment are essential. Such monitoring will inform the development and validation of models for predicting dynamics of fine sediment transport as a function of hydro-climatic and geomorphological controls. We highlight that the need for monitoring is particularly important for hilly catchments with complex and changing land use. We recommend that research should be prioritized for sloping farmland-dominated catchments.     

The wave-induced solute flux from submerged sediment

The issue of the transport of dissolved nutrients and contaminants between the sediment in the bottom of a lake or reservoir and the body of water above it is an important one for many reasons. In particular the biological and chemical condition of the body of water is intricately linked to these mass transport processes. As the review by Boudreau (Rev Geophys 38(3):389–416, 2000) clearly demonstrates those transport processes are very complex involving mechanisms as diverse as the wave-induced flux between the sediment and the overlying water and the effect of burrowing animals on the transport within the sediment as well as basic diffusion mechanisms. The present paper focuses on one facet of these transport processes; we re-examine the balance of diffusion and wave-induced advection and demonstrate that the wave-induced flux of a solute from submerged sediment is not necessarily purely diffusive as suggested by Harrison et al. (J Geophys Res 88:7617–7622, 1983) but can be dominated by a mean or time-averaged flux induced by the advective fluid motion into and out of the sediment caused by the fluctuating pressure waves associated with wave motion. Indeed along the subtidal shoreline where the fluctuating bottom pressures are greatest, wave-induced advection will dominate the mean, time-averaged transport of solute into or out of the sediment as suggested in the work of Riedl et al. (Mar Biol 13:210–221, 1972). However, the present calculations also indicate that this advective flux decreases rapidly with increasing depth so that further away from the shoreline the advective flux becomes negligible relative to the diffusive flux and therefore the latter dominates in deeper water.     

Temperature,salinity and water-age variations in a tidal creek network,Bushehr Port,Iran

The spatio-temporal variability of temperature and salinity was studied for a creek network and its adjacent coastal waters along the northwestern coast of the Persian Gulf during warm and cold months. Salinity variations and tidal fluctuations were found to be out of phase throughout the creek. Temperature variations at the creek were exhibit a direct correlation with tidal fluctuations during cold months and were inversely related during warm months. The creek water was colder (warmer) than offshore water during the cold (warm) season. The salinity values observed inside this inverse estuary were higher during the warm season than the corresponding values during the cold season due to a change in evaporation rates; while the open water salinity had an opposite pattern. Using salinity as a tracer, the water-age (WA) was calculated, which shows almost linear increase from the mouth to the head. The maximum WA increases from ~10 days in winter to ~30 days in summer due to the corresponding increase in longitudinal salinity gradient. Based on the calculated non-dimensional Peclet number, the diffusion process is more rapid than the advection process in this water body, especially during the cold season.     

The source and transport of bioaerosols in the air: A review

• Emission of microbe from local environments is a main source of bioaerosols. • Regional transport is another important source of the bioaerosols. • There are many factors affecting the diffusion and transport of bioaerosols. • Source identification method uncovers the contribution of sources of bioaerosols. Recent pandemic outbreak of the corona-virus disease 2019 (COVID-19) has raised widespread concerns about the importance of the bioaerosols. They are atmospheric aerosol particles of biological origins, mainly including bacteria, fungi, viruses, pollen, and cell debris. Bioaerosols can exert a substantial impact on ecosystems, climate change, air quality, and public health. Here, we review several relevant topics on bioaerosols, including sampling and detection techniques, characterization, effects on health and air quality, and control methods. However, very few studies have focused on the source apportionment and transport of bioaerosols. The knowledge of the sources and transport pathways of bioaerosols is essential for a comprehensive understanding of the role microorganisms play in the atmosphere and control the spread of epidemic diseases associated with them. Therefore, this review comprehensively summarizes the up to date progress on the source characteristics, source identification, and diffusion and transport process of bioaerosols. We intercompare three types of diffusion and transport models, with a special emphasis on a widely used mathematical model. This review also highlights the main factors affecting the source emission and transport process, such as biogeographic regions, land-use types, and environmental factors. Finally, this review outlines future perspectives on bioaerosols.     

Faster phototransformation of the formate (terrestrial) versus methanesulphonate (marine) markers of airborne particles: implications for modelling climate change

Atmospheric particulate matter is altering climate. For instance marine biogenic particles are cooling climate. Organic markers are major tools to elucidate the sources of atmospheric particulate matter. Formate is commonly used as a marker of continental aerosols, whereas methanesulphonate is used as tracer of biogenic marine aerosols. However, transformation processes during aerosol transport may modify their relative concentrations and, in turn, introduce a bias in the assessment of particle sources. Actually very little is known about the transformation of formate and methanesulphonate in aerosols. Therefore, we irradiated formate and methanesulphonate in the presence of nitrate and haematite. Nitrate and haematite are aerosol photosensitisers, producing reactive species that degrade organic compounds. The time evolution of formate and methanesulphonate was monitored by ion chromatography. Our results show that formate is transformed from 1.6 to 4.1 times faster than methanesulphonate. This trend is partly due to higher reactivity with the hydroxyl radical and partly due to additional reaction with other transients such as nitrogen dioxide. Such results strongly suggest faster formate transformation during particle transport. Therefore, when formate and methanesulphonate are used as particle tracers, an overestimation of marine biogenic versus continental particle sources is expected. This bias has major implications for climate prediction models, because marine biogenic particles have a cooling effect on climate.     

Characterization of bedload intermittency near the threshold of motion using a Lagrangian sediment transport model

At the smallest scales of sediment transport in rivers, the coherent structures of the turbulent boundary layer constitute the fundamental mechanisms of bedload transport, locally increasing the instantaneous hydrodynamic forces acting on sediment particles, and mobilizing them downstream. Near the critical threshold for initiating sediment motion, the interactions of the particles with these unsteady coherent structures and with other sediment grains, produce localized transport events with brief episodes of collective motion occurring due to the near-bed velocity fluctuations. Simulations of these flows pose a significant challenge for numerical models aimed at capturing the physical processes and complex non-linear interactions that generate highly intermittent and self-similar bedload transport fluxes. In this investigation we carry out direct numerical simulations of the flow in a rectangular flat-bed channel, at a Reynolds number equal to Re = 3632, coupled with the discrete element method to simulate the dynamics of spherical particles near the bed. We perform two-way coupled Lagrangian simulations of 48,510 sediment particles, with 4851 fixed particles to account for bed roughness. Our simulations consider a total of eight different values of the non-dimensional Shields parameter to study the evolution of transport statistics. From the trajectory and velocity of each sediment particle, we compute the changes in the probability distribution functions of velocities, bed activity, and jump lengths as the Shields number increases. For the lower shear stresses, the intermittency of the global bedload transport flux is described by computing the singularity or multifr actal spectrum of transport, which also characterizes the widespread range of transport event magnitudes. These findings can help to identify the mechanisms of sediment transport at the particle scale. The statistical analysis can also be used as an ingredient to develop larger, upscaled models for predicting mean transport rates, considering the variability of entrainment and deposition that characterizes the transport near the threshold of motion.     

Three-Dimensional Modelling of Concentration Fluctuations in Complicated Geometry

The strong fluctuating component in the measured concentration time series of a dispersing gaseous pollutant in the atmospheric boundary layer, and the hazard level associated to short-term concentration levels, demonstrate the necessity of calculating the magnitude of turbulent fluctuations of concentration using computational simulation models. Moreover the computation of concentration fluctuations in cases of dispersion in realistic situations, such as built-up areas or street canyons, is of special practical interest for hazard assessment purposes. In this paper, the formulation and evaluation of a model for concentration fluctuations, based on a transport equation, are presented. The model is applicable in cases of complex geometry. It is included in the framework of a computational code, developed for simulating the dispersion of buoyant pollutants over complex geometries. The experimental data used for the model evaluation concerned the dispersion of a passive gas in a street canyon between 4 identical rectangular buildings performed in a wind tunnel. The experimental concentration fluctuations data have been derived from measured high frequency concentrations. The concentration fluctuations model is evaluated by comparing the model's predictions with the observations in the form of scatter plots, quantile-quantile plots, contour plots and statistical indices as the fractional bias, the geometrical mean variance and the factor-of-two percentage. From the above comparisons it is concluded that the overall model performance in the present complex geometry case is satisfactory. The discrepancies between model predictions and observations are attributed to inaccuracies in prescribing the actual wind tunnel boundary conditions to the computational code.     

Scalar transport from point sources in the flow around a finite-height cylinder

This paper presents a large eddy simulation of mass transfer in the flow around a surface-mounted finite-height circular cylinder. The study was carried out for a cylinder with height-to-diameter ratio of 2.5 and a Reynolds number based on the cylinder diameter of 44000. The approach flow boundary layer had a thickness of about 10% of the cylinder height. A tracer was released at various levels upstream of the cylinder. The effect of the release position in the subsequent spreading and dilution of the plumes is analyzed. It is found that a tracer released at the top or at mid-height is entrained into the recirculation zone behind the cylinder, and therefore presents similar plume evolution in the far wake in both cases. If the tracer is released at around three-quarters of the height of the cylinder, it is not significantly entrained by the main recirculation region, leading to smaller rates of spreading of the plume. Finally, if the tracer is released near the floor, the plume is entrained by the horseshoe vortex that wraps around the cylinder, leading to a large lateral spreading of the plume, remaining always near the floor.