Dissolved inorganic nitrogen (DIN) is considered the main factor that induces eutrophication in water, and is readily influenced by hydrodynamic activities. In this study, a 4-year field investigation of nitrogen dynamics in a turbulent river was conducted, and a laboratory study was performed in the approximately homogeneous turbulence simulation system to investigate potential mechanisms involved in DIN transformation under turbulence. The field investigation revealed that, contrary to NO⁻₃ dynamics, the NH⁺₄ concentrations in water were lower in flood seasons than in drought seasons. Further laboratory results demonstrated that limitation of dissolved oxygen (DO) caused inactive nitrification and active denitrification in static river sediment. In contrast, the increased DO levels in turbulent river intensified the mineralization of organic nitrogen in sediment; moreover, ammonification and nitrification were activated, while denitrification was first activated and then depressed. Turbulence therefore decreased NH⁺₄ and NO⁻₂ concentrations, but increased NO⁻₃ and total DIN concentrations in the overlying water, causing the total DIN to increase from 0.4 mg/L to maximum of 1.0 and 1.7 mg/L at low and high turbulence, respectively. The DIN was maintained at 0.7 and 1.0 mg/L after the 30-day incubation under low and high turbulence intensities (ε) of 3.4 × 10⁻⁴ and 7.4 × 10⁻² m²/s³, respectively. These results highlight the critical role of DO in DIN budgets under hydrodynamic turbulence, and provide new insights into the DIN transport and transformation mechanisms in turbulent rivers.

Synthetic fibers with diameters of several tens of micrometers are the most abundant type of microplastics in the marine environment, yet the most unknown regarding dynamics in the water column. Experiments proposed here are a proof-of-concept of qualitative and quantitative characteristics of fibers’ motion in still water and in the presence of thermal convection. For 12 sets of fine fibers (nylon (1.12 g/cm³) and polyester (1.35 g/cm³), 1.9–14.8 mm long, diameters 13 and 20 μm), 84 measurements of sinking velocity in still water were acquired. In still conditions, fibers settled smoothly and slowly, preserving their initial (accidental) orientation. Sinking rates of fibers with lengths <5 mm varied between 0.5 and 3.7 mm/s (the bulk mean of 1.6 mm/s). Fibers with similar properties showed 4-fold different sinking velocity, which is supposed to be the effect of their different orientation while settling: vertically oriented fibers (19% in the experiments) settled faster than those with inclined orientation (48%), and horizontally oriented fibers (33%) settled with the smallest velocities.Convective mixing of water, heated from below, principally changed the manner of sinking of fibers: their motions became unsteady and 3-dimensional. In 78 measurements for 4-mm long nylon fibers (using the “light knife” technique), only about 56% of fibers showed downward velocity component (mean 1.33 ± 0.78 mm/s), which was twice as small as in still water, however the ratio of max/min values increased up to 14. Fibers could move in different directions and follow circular motions of a convective cell. Our findings suggest two possible mechanisms retaining fibers in the water column: entrainment of some particles in horizontal and vertical motions and slowed sinking due to unsteady flow around the fiber. The retention of fibers leads to decrease in integral downward particle flux (up to 4 times in our experiments).

This study investigates the dispersion and transport of reactive pollutants in a deep urban street canyon with an aspect ratio of 2 under neutral meteorological conditions using large-eddy simulation. The spatial variation of pollutants is significant due to the existence of two unsteady vortices. The deviation of species abundance from chemical equilibrium for the upper vortex is greater than that for the lower vortex. The interplay of dynamics and chemistry is investigated using two metrics: the photostationary state defect, and the inferred ozone production rate. The latter is found to be negative at all locations within the canyon, pointing to a systematic negative offset to ozone production rates inferred by analogous approaches in environments with incomplete mixing of emissions. This study demonstrates an approach to quantify parameters for a simplified two-box model, which could support traffic management and urban planning strategies and personal exposure assessment.

Hydrodynamic fluctuations can trigger sediment suspension concomitantly with internal phosphorus release, while the interactive effect of turbulence mixing and sediment suspension on the regulation of phosphorus dynamics is in need of deep understanding. This study addressed the changes in total phosphorus (TP), phosphate (PO₄³⁻-P) and suspended sediment (SS) in the overlying water, and measured the profile of dissolved oxygen (DO), Fe(II) and soluble reactive phosphorus (SRP) across the sediment-water interface in the simulated environmental turbulence scenario, For a turbulence intensity (ε) of 3.6 × 10⁻³ m²/s³, the SRP flux increased hence PO₄³⁻-P showed a 36.36% increase relative to its initial level. Although ε of 1.3 × 10⁻² m²/s³ benefited the delivery of oxygen from the bulk aqueous phase to the upper sediment which can trigger the formation of Fe oxides and hydroxides, the turbulence-induced phosphorus diffusion from the sediment exceeded its inactivation and resulted in a large SRP flux. However, a protion of the released PO₄³⁻-P can be immobilized through SS adsorption and biotic (likely cyanobacteria) assimilation. Higher turbulence intensities (ε of 3.3 × 10⁻² and 7.4 × 10⁻² m²/s³) led to an approximately 40-fold increase in TP concentration and a significant increase in sediment suspension, which contributed to the immobilization of a majority of the phosphate through adsorption; thus, the PO₄³⁻-P concentrations in the overlying water displayed 47.75% and 41.67% decline, respectively. This study also confirmed the sequential phosphorus buffer mechanisms associated with increasing turbulence intensities. With an ε of 3.6 × 10⁻³ m²/s³, bounding to Fe ion had a significant impact on phosphorus inactivation but with an ε of 7.4 × 10⁻² m²/s³, the main immobilization mechanism is switched to phosphorus adsorption from the large quantity of suspended sediment.

hal-01150611 | International audience | Wind waves are responsible for some of the spatio-temporal gradients observed in the biotic and abiotic variables in large shallow lakes. However, their effects on the phytoplankton community composition are still largely unexplored especially in freshwater systems such as lakes. In this paper, using field observations and mesocosm bioassay experiments, we investigated the impact of turbulence generated by wind waves on the phytoplankton community composition (especially on harmful cyanobacteria) in Lake Taihu, a large, shallow eutrophic lake in China. The composition of the phytoplankton community varied with the intensity of wind waves in the different areas of the lake. During summer, when wind waves were strong in the central lake, diatoms and green algae seemed to dominate while harmful cyanobacteria dominated in the weakly influenced Meiliang Bay. Turbulence bioassays also showed that diatoms and green algae were favoured by turbulent mixing. The critical time for the shift of the phytoplankton community composition was approximately 10 days under turbulent conditions. However, short-term (6 days) turbulence is rather beneficial for the dominance of cyanobacteria. This study suggests that the duration of wind events and their associated hydrodynamics are key factors to understanding the temporal and spatial changes of phytoplankton communities.

Exposure to nanoparticles coming from road–traffic concerns a large part of urban population in both outdoor and indoor environments leading to the enhancement of short and long–term health problems. The dynamics of such small particles is very sensitive to the turbulent diffusion and Brownian motion. Hence their concentrations are dependent on the flow structure properties (length and time scales). In this paper, a wind tunnel study is conducted to assess the effect of the flow on the dispersion of nanoparticles coming out from tailpipe in the near–wake of a reduced–scale truck model. Particle number concentration (PNC) measurements are achieved at 66 positions downstream of the model. Our results point out that the interaction of the ultrafine particles (UFP) with the vortices appearing in the near-wake of a truck enhances their dispersion in both transversal and vertical directions. Increasing the inflow air velocity strengthens this spreading. Overall, we demonstrate that such wind tunnel measurements are fundamental to improve our knowledge on the existing interaction between road–traffic, turbulence and particle concentration to accurately evaluate human exposure rates to ultrafine particles and their potential consequences.

Air quality management in urban areas requires the use of advanced modeling tools, able to predict and evaluate the pollution level under different traffic and meteorological conditions. In the present paper, the Artificial Compressibility version of the Characteristic Based Split algorithm (AC–CBS) was used to assess the performance of the Spalart–Allmaras based Detached Eddy Simulation (SA–DES) model, for the calculation of incompressible turbulent flow in different urban street canyon configurations. To our knowledge, the DES version of the SA turbulence model was applied in this work for the first time for the simulation of turbulent flow in a street canyon. The proposed DES model was able to accurately reproduce the turbulent characteristics of the flow compared with results from real street canyon experiments, wind tunnel experiments, and also to that obtained with RANS simulations. These results are very similar to the ones obtained from Large Eddy Simulation (LES) of street canyons flow reported in some recent publications, but with the potential characteristic of reduced computational costs. The DES approach is very promising for the simulation of transient turbulent flows in urban areas when complex three–dimensional domains are considered. The performance of the DES model evaluated for the mean dimensionless streamwise velocity profiles was comparable to that of Reynolds–Averaged Navier–Stokes RANS approach if referred to Hit Rate (HR) validation metric, and even better if referred to Factor of two observation (FAC2) validation metric. An accurate reproduction of the turbulent flow is crucial for urban pollutant dispersion simulations, since the distribution of the pollutant concentrations could differ by order of magnitude in the different points of the street canyon. DES approach results were able to accurately predict the unsteadiness characteristic of the flow, and to reproduce some minor vortex structures, which were not observed in the RANS cases, that will lead to a more accurate reproduction of the pollutant concentrations.

Four common consumer plastic samples (polyethylene, polystyrene, polyethylene terephthalate, polyvinylchloride) were studied to investigate the impact of physical parameters such as turbulence, salinity and UV irradiance on leaching behavior of selected plastic components. Polymers were exposed to two different salinities (i.e. 0 and 35g/kg), UV radiation and turbulence. Additives (e.g. bisphenol A, phthalates, citrates, and Irgafos® 168 phosphate) and oligomers were detected in initial plastics and aqueous extracts. Identification and quantification was performed by GC–FID/MS. Bisphenol A and citrate based additives are leached easier compared to phthalates. The print highly contributed to the chemical burden of the analyzed polyethylene bag. The study underlines a positive relationship between turbulence and magnitude of leaching. Salinity had a minor impact that differs for each analyte. Global annual release of additives from assessed plastics into marine environments is estimated to be between 35 and 917tons, of which most are derived from plasticized polyvinylchloride.

The direct effect of atmospheric turbulence on plume rise in the current research work is studied through examining the turbulence intensity parameter. A hybrid unsteady Reynolds averaged Navier Stokes (RANS) and large eddy simulation (LES) numerical approach is applied with a new mixed scale sub-grid parameterization technique in the commercial ANSYS Fluent software in order to simulate the buoyant plume behavior in a turbulent crossflow. The accuracy of the simulation method is crosschecked against the wind tunnel data available in the literature. The numerical simulation results in various operating conditions are used to derive a new plume rise formula in which the direct effect of atmospheric turbulence intensity at stack height (IAir) is explicitly introduced in the plume rise formula. Furthermore, the buoyancy parameter of the flue gas is determined at some distances upstream of the stack top surface to include the whole effects of source buoyancy on the plume rise. The value of IAir at stack height is obtained by measuring the standard deviation of wind velocity at stack height. The sensitivity analysis showed that by increasing the atmospheric turbulence intensity, the final plume rise decreases because of the updraft and downdraft motions of turbulence and it has been found that there is a linear dependency between the plume rise and (IAir)−1.22. The quantile-quantile plots show that the new model can predict the simulated plume rise with a deviation factor of 1.0025 whereas the conventional models overestimate the final plume rise at least by a factor of 2.2.

The objective of this article is to present comprehensive findings and analysis of studies performed on air pollutant dispersion in urban environments. It captures India’s rising environmental pollution due to urbanization, industrialization, and population growth. Dispersion of pollutants due to the wind in the lower Atmospheric Boundary Layer (ABL) is a major concern nowadays. The dispersion field around the buildings is a critical parameter to analyze and it primarily depends on the correct simulation of the wind flow structure. Therefore, studies performed on this in past years are being reviewed. Additionally, a brief review of different air dispersion models that are integrated with the Geographic Information System (GIS) has been studied in this article to assess the exposure. The results of these studies provide the urban air dispersion model aligning to three sub-models i.e., Emission, Weather Prediction, and Dispersion models. Various factors like wind speed, wind direction, cloud cover, traffic emission, disposal of waste, transportation, and others are considered. This study also captures the problems and risks being faced while creating a model, and its possible mitigation approaches.