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1.
An analysis of concentration time series measured in a boundary-layer wind tunnel at the University of Hamburg is presented.
The measurements were conducted with a detailed aerodynamic model of the Oklahoma City (OKC) central business district (CBD)
at the scale of 1:300 and were part of the Joint Urban 2003 (JU2003) project. Concentration statistics, as well as concentration
probability density (PDF) and exceedance probability (EDF) functions were computed for street- and roof-level sites for three
different wind directions. Taking into account the different length scales and wind speeds in the wind-tunnel (WT) and full-scale
experiments, dimensionless concentrations and a dimensionless time scale are computed for the comparison with data from the
JU2003 full-scale tracer experiments, conducted in OKC in 2003. Using such dimensionless time, the WT time series cover a
~20 times longer time span than the JU2003 full-scale time series, which are analysed in detail in an accompanying, first
part of this paper. The WT time series are thus divided into 20 consecutive blocks of equal length and the statistical significance
of parameters based on relatively short records is assessed by studying the variability of the concentration statistics and
probability functions for the different blocks. In particular at sites closer to the plume edge, the results for the individual
blocks vary significantly and at such sites statistics from short records are not very representative. While the location
of three sampling sites in the WT closely matched the sites during the full-scale experiments, the prevailing wind directions
during the JU2003 releases were not exactly matched. The comparison between full-scale and WT concentration parameters should
thus primarily be interpreted in a qualitative rather than direct quantitative sense. Given the differences in mean wind directions
and concerns about the representativeness of full-scale concentration statistics, the WT and full-scale results compared well.
The 98 percentile concentrations for almost all full-scale releases analyzed are within the scatter of the percentiles observed
in the block analysis of the WT time series. Furthermore, the concentration percentiles appear linearly correlated with the
fluctuation intensities and the linear relationships determined in the wind tunnel agree well with full-scale results. 相似文献
2.
Eugene Yee 《Environmental Fluid Mechanics》2009,9(4):389-407
The relationships between various normalized higher-order concentration moments in plumes dispersing in a built-up (urban)
environment have been investigated using a large concentration data set obtained in a boundary-layer water channel. This data
set consists of measurements of plume dispersion in a number of idealized obstacle arrays (e.g., cubical and non-cubical obstacles
in aligned and staggered arrangements with uniform, random and alternating heights). A remarkably robust feature of all the
concentration data was the observed collapse of the third- and fourth-order normalized concentration moments on the second-order
normalized concentration moment. The data are shown to collapse to a series of universal curves (independent of the geometry
of the obstacle array) and these curves were found to be identical to those observed previously for open-terrain plumes. The
results imply that the probability law of concentration in a plume dispersing in either a built-up environment or open terrain
has a universal form that can be specified by at most two independent parameters. The universal functions representing the
relationships between the normalized concentration moments were found to be well modeled (approximated) using a two-parameter
clipped-gamma probability law for the concentration. Finally, the clipped-gamma distribution was found to be in very good
conformance with the measured probability distribution of concentration for plumes dispersing in a built-up environment. 相似文献
3.
The peak values observed in a measured concentration time series of a dispersing gaseous pollutant released continuously from a point source in urban environments, and the hazard level associated with them, demonstrate the necessity of predicting the upper tail of concentration distributions. For the prediction of concentration distributions statistical models are preferably employed which provide information about the probability of occurrence. In this paper a concentration database pertaining to a field experiment is used for the selection of the statistical distribution. The inverses of the gamma cumulative distribution function (cdf) for 75th–99th percentiles of concentration are found to be more consistent with the experimental data than those of the log-normal distribution. The experimental values have been derived from measured high frequency time series by sorting first the concentrations and then finding the concentration which corresponds to each probability. Then the concentration mean and variance that are predicted with Computational Fluid Dynamics-Reynolds Averaged Navier–Stokes (RANS) methodology are used to construct the gamma distribution. The proposed model (“RANS-gamma”) is included in the framework of a computational code (ADREA-HF) suitable for simulating the dispersion of airborne pollutants over complex geometries. The methodology is validated by comparing the inverses of the model cdfs with the observed ones from two wind tunnel experiments. The evaluation is performed in the form of validation metrics such as the fractional bias, the normalized mean square error and the factor-of-two percentage. From the above comparisons it is concluded that the overall model performance for the present cases is satisfactory. 相似文献
4.
The statistics of the fluctuating concentration field within a plume is important in the analysis of atmospheric dispersion of toxic, inflammable and odorous gases. Previous work has tended to focus on concentration fluctuations in single plumes released in the surface layer or at ground level and there is a general lack of information about the mixing of two adjacent plumes and how the statistical properties of the concentration fluctuations are modified in these circumstances. In this work, data from wind tunnel experiments are used to analyse the variance, skewness, kurtosis, intermittency, probability density function and power spectrum of the concentration field during the mixing of two identical plumes and results are compared with those obtained for an equivalent single plume. The normalised variance, skewness and kurtosis on the centre-lines of the combined plume increase with distance downwind of the stack and, in the two-source configuration, takes lower values than those found in the single plumes. The results reflect the merging process at short range, which is least protracted for cases in which the sources are in-line or up to 30 \(^{\circ }\) off-line. At angles of 45 \(^{\circ }\) and more, the plumes are effectively side-by-side during the merging process and the interaction between the vortex pairs in each plume is strong. Vertical asymmetry is observed between the upper and the lower parts of the plumes, with the upper part having greater intermittency (i.e. the probability that no plume material is present) and a more pronounced tail to the concentration probability distribution. This asymmetry tends to diminish at greater distances from the source but occurs in both buoyant and neutral plumes and is believed to be associated with the ‘bending-over’ of the emission in the cross-flow and the vortex pair that this generates. The results allowed us to identify three phases in plume development. The first, very near the stack, is dominated by turbulence generated within the plume and characterised by concentration spectra with distinct peaks corresponding to scales comparable with those of the counter-rotating vortex pair. A second phase follows at somewhat greater distances downwind, in which there are significant contributions to the concentration fluctuations from both the turbulence internal to the plume and the external turbulence. The third phase is one in which the concentration fluctuations appear to be controlled by the external turbulence present in the ambient flow. 相似文献
5.
Justus Kristine A. Murlis John Jones Chris Cardé Ring T. 《Environmental Fluid Mechanics》2002,2(1-2):115-142
The patterns of stimulus available to moths flying along pheromone plumes in a 3-m-long wind tunnel were characterized using a high frequency photoionization detector in conjunction with an inert tracer gas. Four contrasting flow regimes and source conditions were produced: odor released in pulses from a vertical and horizontal array of four sources, odor released continuously from a point source, and odor released continuously from a point source into an oscillatory wake. Although the four flow regimes produced plumes of intermittent and fluctuating concentration, there were considerable differences in the structure of the signal presented to the sensor. Pulses of tracer gas released at 10 Hz retained most of their longitudinal and lateral separation. The plume growing in the disturbed flow (`oscillatory'), was broader in its lateral extent than the plume growing in an undisturbed flow (`continuous'), and the concentrations in the former were lower at each downstream position. The signal recorded in the disturbed flow had higher intermittency, but the ratio between the peak concentration and the signal mean was lower than in the continuous plume. Time scales were typically longer in the tunnel than in a field setting, but length scales and the main features of intermittency and fluctuation were similar. Moths flying along plumes of pheromone in this and similar wind tunnels typically slow their velocity and narrow the lateral excursions of their flight track as they approach a pheromone source. Which features of the plumes measured in this study account for these behavioral reactions remains to be determined. 相似文献
6.
Time-dependent three-dimensional numerical simulations such as large-eddy simulation (LES) play an important role in fundamental research and practical applications in meteorology and wind engineering. Whether these simulations provide a sufficiently accurate picture of the time-dependent structure of the flow, however, is often not determined in enough detail. We propose an application-specific validation procedure for LES that focuses on the time dependent nature of mechanically induced shear-layer turbulence to derive information about strengths and limitations of the model. The validation procedure is tested for LES of turbulent flow in a complex city, for which reference data from wind-tunnel experiments are available. An initial comparison of mean flow statistics and frequency distributions was presented in part I. Part II focuses on comparing eddy statistics and flow structures. Analyses of integral time scales and auto-spectral energy densities show that the tested LES reproduces the temporal characteristics of energy-dominant and flux-carrying eddies accurately. Quadrant analysis of the vertical turbulent momentum flux reveals strong similarities between instantaneous ejection-sweep patterns in the LES and the laboratory flow, also showing comparable occurrence statistics of rare but strong flux events. A further comparison of wavelet-coefficient frequency distributions and associated high-order statistics reveals a strong agreement of location-dependent intermittency patterns induced by resolved eddies in the energy-production range. The validation concept enables wide-ranging conclusions to be drawn about the skill of turbulence-resolving simulations than the traditional approach of comparing only mean flow and turbulence statistics. Based on the accuracy levels determined, it can be stated that the tested LES is sufficiently accurate for its purpose of generating realistic urban wind fields that can be used to drive simpler dispersion models. 相似文献
7.
Experimental Study of the Criteria of Flow Laminarization in Two-Dimensional Dense Gas Plumes 总被引:1,自引:0,他引:1
Laminarization of flow in a two-dimensional dense gas plume was experimentally investigated in this study. The plume was created by releasing CO2 through a ground-level line source into a simulated turbulent boundary layer over an aerodynamically rough surface in a meteorological wind tunnel. The bulk Richardson number (Ri*), based on negative plume buoyancy, plume thickness, and friction velocity, was varied over a wide range so that the effects of stable stratification on plume laminarization could be observed. A variety of ambient wind speeds as well as three different sizes of roughness arrays were used so that possible effects of roughness Reynolds number (Re*) on plume laminarization could also be identified. Both flow visualization methods and quantitative measurements of velocity and intermittency of turbulence were used to provide quantitative assessments of plume laminarization.Flow visualization provided an overall picture of how the plume was affected by the negative buoyancy. With increasing Ri*, both the plume depth and the vertical mixing were significantly suppressed, while upstream propagation of the plume from the source was enhanced. The most important feature of the flow revealed by visualization was the laminarization of flow in the lower part of the plume, which appeared to be closely related to both Ri* and Re*.Measurements within the simulated dense gas plumes revealed the influence of the stable stratification on mean velocity and turbulence intensity profiles. Both the mean velocity and turbulence intensity were significantly reduced near the surface; and these reductions systematically depended on Ri*. The roughness Reynolds number also had considerable influence on the mean flow and turbulence structure of the dense gas plumes.An intermittency analysis technique was developed and applied to the digitized instantaneous velocity signals. It not only confirmed the general flow picture within the dense plume indicated by the flow visualization, but also clearly demonstrated the changes of flow regime with variations in Ri* and Re*. Most importantly, based on this intermittency analysis, simple criteria for characterizing different flow regimes are formulated; these may be useful in predicting when plume laminarization might occur. 相似文献
8.
基于TM影像的兰州市热岛效应及预测分析 总被引:1,自引:0,他引:1
近些年来随着城市建设的高速发展,热岛效应越来越被人们所重视。采用2007年的TM遥感影像,分析了兰州市热岛效应热场强度的分布规律,并应用灰色模型对其未来发展趋势进行了预测。研究发现:兰州市热岛效应明显且稳定存在,热岛中心主要分布在城关区、七里河区人口密度大以及西固区工业聚集的地方;植被和水体有减弱热岛效应的作用。灰色关联度分析表明:人为因素对兰州城市热岛强度效应的影响相对突出;其次,在气象因子中,风速对城市热岛效应的形成和加重也起到重要的作用。灰色预测结果表明:兰州市热岛强度将会持续波动上升,2012年将达到3.826℃。 相似文献
9.
10.
Comparative study of the fate and mobility of metals discharged in mining and urban effluents using sequential extractions on suspended solids 总被引:1,自引:0,他引:1
Christian Gagnon Patrice Turcotte Bernard Vigneault 《Environmental geochemistry and health》2009,31(6):657-671
The fate, bioavailability and environmental impacts of metals discharged in municipal and mining wastewater discharge will
depend to a large extent on chemical speciation and distribution. Previous studies on metal bioaccumulation have shown that
total metal concentrations are not a good predictor of bioavailability in the dispersion plumes of municipal effluents. The
objective of this study was to determine the solid phase speciation of metals in surface waters receiving urban and mining
effluents in order to assess their fate and relative mobility in the receiving environment. Suspended particulate matter was
sampled using sediment traps at several sites downstream of effluent outfall plumes as well as at reference upstream sites.
Particulate metal in operationally defined fractions—exchangeable/carbonates, reducible, oxidisable and residual—were determined
in suspended particulate matter with a series of selective chemical extractions. Metal enrichment in suspended particles was
generally observed in both mining and urban effluent discharges. When compared to its receiving environment, the mining effluent
appeared to release more particulate metals (Cu, Fe, Zn) in the most reactive fractions (i.e. exchangeable/carbonates + reducible
forms, 23–43%), while other released metals, such as Cd and Mn, were predominantly in the least reactive forms (i.e., oxidisable + residual,
73–97%). In contrast, the reactivity of all particulate metals, with the exception of Mn, from the urban effluent was much
higher, with up to 65, 42, 30 and 43% for Cd, Cu, Fe and Zn, respectively, in the two most reactive fractions. As expected
in effluent dispersion plumes, parameters such as the organic carbon, Fe oxide and carbonate contents have specific effects
on the partitioning of several trace metals, particularly Cd, Cu and Zn. Our results indicated that the relative distributions
of metals among geochemical fractions varied in the effluent receiving waters where organic carbon and Fe oxides appeared
as the most important parameters. This could therefore decrease the exposure for aquatic organisms that are exposed to those
contaminated sediments as well as the risk to human health. 相似文献
11.
Farrell Jay A. Murlis John Long Xuezhu Li Wei Cardé Ring T. 《Environmental Fluid Mechanics》2002,2(1-2):143-169
This article presents the theoretical motivation, implementation approach, and example validation results for a computationally efficient plume simulation model, designed to replicate both the short-term time signature and long-term exposure statistics of a chemical plume evolving in a turbulent flow. Within the resulting plume, the odor concentration is intermittent with rapidly changing spatial gradient. The model includes a wind field defined over the region of interest that is continuous, but which varies with location and time in both magnitude and direction. The plume shape takes a time varying sinuous form that is determined by the integrated effect of the wind field. Simulated and field data are compared. The motivation for the development of such a simulation model was the desire to evaluate various strategies for tracing odor plumes to their source, under identical conditions. The performance of such strategies depends in part on the instantaneous response of target receptors; therefore, the sequence of events is of considerable consequence and individual exemplar plume realizations are required. Due to the high number of required simulations, computational efficiency was critically important. 相似文献
12.
Carlo Grillenzoni 《Environmental and Ecological Statistics》2014,21(1):95-111
The paper compares recursive methods for detecting change points in environmental time series. Timely identification of peaks and troughs is important for planning defense actions and preventing risks. We consider linear nonparametric methods, such as time-varying coefficients, double exponential smoothers and prediction error statistics. These methods are often used in surveillance, forecasting and control, and their common features are sequential computation and exponential weighting of data. The new approach proposed here is to select their coefficients by maximizing the difference between subsequent peaks and troughs detected on past data. We compare the methods with applications to meteorological, astronomical and ecological data, and Monte-Carlo simulations. 相似文献
13.
14.
冬季降雪过程对城市大气气态汞污染的影响 总被引:3,自引:0,他引:3
2009年降雪和非降雪期间对北京西北城区的气态总汞浓度进行了连续采样,比较了降雪期间、非降雪期间的气态总汞浓度日变化过程;降雪期间气态总汞浓度的降低和恢复过程。结果表明,降雪和非降雪期间大气气态总汞浓度的日均值有显著差异,降雪期间气态总汞的平均浓度为5.64ng·m^-3,非降雪期间的平均浓度为7.43ng·m^-3,前者约为后者的70%。降雪后约7h气态总汞浓度恢复到降雪前水平。研究中分析了气象因素(气压、风速、阵风速度、气温和相对湿度)对于气态总汞浓度的影响,结果表明:降雪期间主要受到风速(r=-0.527)和阵风速度(r=-0.574)的影响;非降雪期间主要受到风速(r=-0.691),阵风速度(r=-0.726)和相对湿度(r=0.692)的影响,并且相对湿度的影响与风速的影响相近。降雪和非降雪期间气态总汞的日变化有所差异:非降雪期间气态总汞浓度在午夜和清晨较高,日变化趋势与相对湿度一致;降雪期间气态总汞的日变化没有明显规律。 相似文献
15.
Ruo-Qian Wang Adrian Wing-Keung Law E. Eric Adams Oliver B. Fringer 《Environmental Fluid Mechanics》2011,11(6):591-609
A series of Large Eddy Simulations (LES) are performed to investigate the penetration of starting buoyant jets. The LES code
is first validated by comparing simulation results with existing experimental data for both steady and starting pure jets
and lazy plumes. The centerline decay and the growth rate of the velocity and concentration fields for steady jets and plumes,
as well as the simulated transient penetration rate of a starting pure jet and a starting lazy plume, are found to compare
well with the experiments. After validation, the LES code is used to study the penetration of starting buoyant jets with three
different Reynolds numbers from 2000 to 3000, and with a wide range of buoyancy fluxes from pure jets to lazy plumes. The
penetration rate is found to increase with an increasing buoyancy flux. It is also observed that, in the initial Period of
Flow Development, the two penetrative mechanisms driven by the initial buoyancy and momentum fluxes are uncoupled; therefore
the total penetration rate can be resolved as the linear addition of these two effects. A fitting equation is proposed to
predict the penetration rate by combining the two independent mechanisms. 相似文献
16.
Panayotis C. Yannopoulos 《Environmental Fluid Mechanics》2010,10(4):415-450
An integral model that combines all advantages of Superposition Method (SM), Entrainment Restriction Approach (ERA) and Second
Order Approach (SOA) is proposed to predict the mean axial velocity and concentration fields of a group of N interacting vertical round turbulent buoyant jets. SM is successful in predicting the fields of mean axial velocity and mean
concentration for a group of N interacting jets or plumes and ERA is advantageous in predicting the above fields for either two or large number (N → ∞) of interacting buoyant jets in the whole range of buoyancy. SOA takes into consideration in a dynamic way the turbulent
contribution to the momentum and buoyancy fluxes and provides better accuracy than the usual procedures. A novelty of the
proposed model is the production and utilisation of advanced profile distributions, convenient for the mean axial velocities
and concentrations in a cross-section of the entire group of buoyant jets. These profiles are developed on the basis of flux
conservation of momentum, buoyancy and kinetic energy for the mean motion. They enhance dynamic adaptation of the individual
buoyant jet axes to the group centreline. Due to these profile distributions, the present model owns generality of application
and better accuracy of predictions compared to usual integral models using simple Gaussian or top-hat profiles; thus it conferred
the name Advanced Integral Model (AIM). AIM is herein applied to predict the mean flow properties of two different arrangement
types of any number of buoyant jets: (a) linear diffusers and (b) rosette-type risers. Present results are compared to available
experimental data and traditional solutions based on Gaussian profiles. Findings may be useful for design purposes and environmental
impact assessment. 相似文献
17.
Sea surface colour data, derived from the Coastal Zone Colour Scanner (CZCS) archive, have been used to assess the space/time
variability of coastal plumes and run-off in the Mediterranean Sea. A time series of 2645 scenes, collected by the CZCS from
1979 to 1985, was processed to apply sensor calibration algorithms, correct for atmospheric contamination, and derive chlorophyll-like
pigment concentration. Individual images, remapped on a 1-km2 pixel grid, were generated for each available day, and then mean values calculated pixel by pixel to form monthly, seasonal
and annual composites. The results obtained must be taken with caution, due to the CZCS limitations in the quantitative assessment
of bio-optical pigments when high concentrations of dissolved organics or suspended sediments are present, e.g. along littorals
or within plumes. Marked differences appear in the distribution of water constituents between coastal zones and open sea,
northern and southern near-coastal areas, western and eastern sub-basins. The oligotrophic character of the basin contrasts
with areas of high concentration related to river plumes—Ebro (Ebre), Po, Rhone, Nile—, coastal run-off patterns, and persistent
mesoscale features (e.g. coastal filaments and eddies). Seasonal variability appears to be high, with higher concentrations
occurring over most of the basin in the cold season, when climatic conditions are favourable to coastal run-off and vertical
mixing. Atmospheric forcing (wind and rainfall over continental margins) could play an important role in establishing the
observed space/time distribution of water constituents. The impact of continental interactions (fluvial and coastal run-off),
or that of exchanges between coastal zone and open sea, could have paramount influence on the biogeochemical fluxes in the
entire basin. 相似文献
18.
A multimedia model for the temporal evolution of the concentration of chemical species in a water basin and its bottom sediment layer has been tested with time-dependent air-water transfer rates varying with the meteorological conditions over the basin. The multimedia model uses the chemical fugacity approach in a system of two ordinary differential equations for the chemical species partition in the two mentioned media, with transfer through the sediment-water and the air-water interfaces. The air-water transfer rates are estimated by a micrometeorological preprocessor. A scenario in which known concentrations in air of a soluble pollutant (benzene) induce water and sediment pollution is tested on both synthetic (MonteCarlo-simulated) and real series of meteorological data. It is found that the use of time-dependent transfer coefficients affects not only the relaxation time, but also the long term concentration of the pollutant in water and sediment, that differs between 10 and 40% in the examined cases when compared with the same multimedia model using average constant transfer rates as usual. This is shown to be due to the statistical correlations between meteorological parameters and air pollutant concentrations, which stresses the advantages of a time-dependent estimation of the transfer coefficients. Correction terms are proposed to take into account the correlation effects when a constant parameters multimedia model is used. 相似文献
19.
Alessandro José Queiroz Sarnaglia Nátaly Adriana Jiménez Monroy Arthur Gomes da Vitória 《Environmental and Ecological Statistics》2018,25(4):443-469
This paper considers the modeling and forecasting of daily maximum hourly ozone concentrations in Laranjeiras, Serra, Brazil, through dynamic regression models. In order to take into account the natural skewness and heavy-tailness of the data, a linear regression model with autoregressive errors and innovations following a member of the family of scale mixture of skew-normal distributions was considered. Pollutants and meteorological variables were considered as predictors, along with some deterministic factors, namely week-days and seasons. The Oceanic Niño Index was also considered as a predictor. The estimated model was able to explain satisfactorily well the correlation structure of the ozone time series. An out-of-sample forecast study was also performed. The skew-normal and skew-t models displayed quite competitive point forecasts compared to the similar model with gaussian innovations. On the other hand, in terms of forecast intervals, the skewed models presented much better performance with more accurate prediction intervals. These findings were empirically corroborated by a forecast Monte Carlo experiment. 相似文献
20.
Several reaction schemes, based on the conserved scalar theory, are implemented within a stochastic Lagrangian micromixing model to simulate the dispersion of reactive scalars in turbulent flows. In particular, the formulation of the reaction-dominated limit (RDL) reaction scheme is here extended to improve the model performance under non-homogeneous conditions (NHRDL scheme). The validation of the stochastic model is obtained by comparison with the available measurements of reactive pollutant concentrations in a grid-generated turbulent flow. This test case describes the dispersion of two atmospheric reactant species (NO and O3) and their reaction product (NO2) in an unbounded turbulent flow. Model inter-comparisons are also assessed, by considering the results of state-of-the-art models for pollutant dispersion. The present validation shows that RDL reaction scheme provides a systematic overestimation (relative error of ca. 85% around the centreline) in computing the local reactant consumption/production rate, whereas the NHRDL scheme drastically reduces this gap (relative error lower than 5% around the centreline). In terms of NO2 production (or reactant consumption), neglecting concentration fluctuations determines overestimations of the product mean of around 100% and a NO2 local production of one order of magnitude higher than the reference simulation. In terms of standard deviations, the concentration fluctuations of both the passive and reactive scalars are generally of the same order of magnitude or up to 1 or 2 orders of magnitudes higher than the corresponding ensemble mean values, except for the background reactant close to the plume edges. The study highlights the importance of modelling pollutant reactions depending on the instantaneous instead of the mean concentrations of the reactants, thus quantifying the role of the turbulent fluctuations of concentration, in terms of scalar statistics (mean, standard deviation, intensity of fluctuations, skewness and kurtosis of concentration, segregation coefficient, simulated reaction rate). This stochastic particle method represents an efficient numerical technique to solve the convection–diffusion equation for reactive scalars and involves several application fields: micro-scale air quality (urban and street-canyon scales), accidental releases, impact of odours, water quality and fluid flow industrial processes (e.g. combustion). 相似文献