共查询到20条相似文献，搜索用时 421 毫秒
1.
Thomas W. Horst 《Boundary-Layer Meteorology》1984,30(1-4):413-430
The assumptions and predictions of four diffusion-deposition models are compared, and two simple plume depletion models are recommended. One model applies an analytical, constant eddy-diffusivity solution of the advection-diffusion equation as a deposition correction to the general Gaussian plume model. Predictions of this model compare moderately well with those of the surface depletion model, an exact treatment of plume depletion, and it is particularly useful for estimating the transport and deposition of settling particles. The second model is a correction to the simple source depletion model that also accounts for the change in the vertical concentration profile caused by deposition. The computational requirements of this model are similar to those of the unmodified source depletion model, while its predictions near the surface are very close to those of the surface depletion model. 相似文献
2.
Measurements of concentration fluctuation intensity, intermittency factor, and integral time scale were made in a water channel
for a plume dispersing in a well-developed, rough surface, neutrally stable, boundary layer, and in grid-generated turbulence
with no mean velocity shear. The water-channel simulations apply to full-scale atmospheric plumes with very short averaging
times, on the order of 1–4 min, because plume meandering was suppressed by the water-channel side walls. High spatial and
temporal resolution vertical and crosswind profiles of fluctuations in the plume were obtained using a linescan camera laser-induced
dye tracer fluorescence technique. A semi-empirical algebraic mean velocity shear history model was developed to predict these
concentration statistics. This shear history concentration fluctuation model requires only a minimal set of parameters to
be known: atmospheric stability, surface roughness, vertical velocity profile, and vertical and crosswind plume spreads. The
universal shear history parameter used was the mean velocity shear normalized by surface friction velocity, plume travel time,
and local mean wind speed. The reference height at which this non-dimensional shear history was calculated was important,
because both the source and the receptor positions influence the history of particles passing through the receptor position. 相似文献
3.
Jeffrey C. Weil Peter P. Sullivan Edward G. Patton Chin-Hoh Moeng 《Boundary-Layer Meteorology》2012,145(1):185-210
A Lagrangian particle dispersion model (LPDM) driven by velocity fields from large-eddy simulations (LESs) is used to determine the mean and variability of plume dispersion in a highly convective planetary boundary layer (PBL). The total velocity of a “particle” is divided into resolved and unresolved or random (subfilter scale, SFS) velocities with the resolved component obtained from the LES and the SFS velocity from a Lagrangian stochastic model. This LPDM-LES model is used to obtain an ensemble of dispersion realizations for calculating the mean, root-mean-square (r.m.s.) deviation, and fluctuating fields of dispersion quantities. An ensemble of 30 realizations is generated for each of three source heights: surface, near-surface, and elevated. We compare the LPDM calculations with convection tank experiments and field observations to assess the realism of the results. The overall conclusion is that the LPDM-LES model produces a realistic range of dispersion realizations and statistical variability (i.e., r.m.s. deviations) that match observations in this highly convective PBL, while also matching the ensemble-mean properties. This is true for the plume height or trajectory, vertical dispersion, and the surface values of the crosswind-integrated concentration (CWIC), and their dependence on downstream distance. One exception is the crosswind dispersion for an elevated source, which is underestimated by the model. Other analyses that highlight important LPDM results include: (1) the plume meander and CWIC fluctuation intensity at the surface, (2) the applicability of a similarity theory for plume height from a surface source to only the very strong updraft plumes—not the mean height, and (3) the appropriate variation with distance of the mean surface CWIC and the lower bound of the CWIC realizations for a surface source. 相似文献
4.
Water-tunnel measurements of velocity, turbulence and scalar concentration for three model urban canopies with aspect ratios A r of building height-to-width of 0.25, 1 and 3 are presented. The measurements for the canopies with A r = 1 and 3 are new, while the measurements for A r = 0.25 were previously published. A passive scalar was continuously released from a near-ground point source, and the concentration was measured at several distances from the source and at different heights above the ground. Plume spreads, concentration and distance from the source were non-dimensionalized using length, time and velocity scales reflecting the geometry of the buildings. The scaling collapses the data for all aspect ratios and is valid when the vertical extent of the plume is smaller than the canopy height. The observed plume spreads are compared with analytical relations, which predict linear growth in both transverse and vertical directions. The observed mean concentration is compared with a Gaussian dispersion model that predicts a ?2 power-law decay with distance from the source. 相似文献
5.
We present a semi-analytical model for the dispersion of passive scalars from continuous ground sources up to distances of a few hundred metres. We attempt to cope with problems typical of analytical models, such as the correct representation of the near-ground concentration and lateral dispersion, while avoiding the use of any empirical parameters. A previous analysis of Prairie Grass Project (PGP) data has shown that the near-ground, cross-wind integrated concentration decreases as some power of the distance from the source that is, itself, distance dependent. As the conventional power-law model is incapable of reproducing this behaviour, we propose a model in which the vertical diffusivity depends on both the height as a power law, and on the distance from the source. For this equation, we construct an infinite-series formal solution, with the first term used as an approximation. A set of equations based on this approximation and on Monin–Obukhov similarity theory is proposed for the the vertical diffusivity, from which the cross-wind integrated concentration is derived analytically. We further construct a simple empirical model for the distance-dependent vertical diffusivity. For the plume lateral width, a Langevin stochastic model depending on the plume height is proposed, whose formal analytical solution is used to derive a set of equations for the cloud width, which are easily solved numerically, with the results validated against PGP data. We apply four statistical measures to evaluate the performance of the model, including the computation of the 95% confidence intervals, for which we find very good agreement. Implementation of this model is extremely simple and computationally efficient. 相似文献
6.
The effects of a pressure jump and a following internal gravity wave on turbulence and plume diffusion in the stable planetary
boundary layer are examined. The pressure jump was accompanied by a sudden increase in turbulence and plume dispersion. The
effects of wave perturbations on turbulence statistics are analysed by calculating fluxes and variances with and without the
wave signal for averaging times ranging from 1 to 30 min. The wave signals are obtained using a band-pass filter. It is shown
that second-order turbulence quantities calculated without first subtracting the wave perturbations from the time are greater
than those calculated when the wave signal is separated from the turbulence. Estimates of the vertical dispersion of an elevated
tracer plume in the stable boundary layer are made using an elastic backscatter lidar. Plume dispersion observed 25 m downwind
of the source increases rapidly with the arrival of the flow disturbances. Measured plume dispersion and plume centreline
height correlate with the standard deviation of the vertical velocity but not with the wave signal. 相似文献
7.
Ashok K. Luhar 《Boundary-Layer Meteorology》2002,102(1):1-38
The mean concentration distributionwithin a plume released from a point source in the atmosphericboundary layer can be greatly influenced by the systematic turningof wind with height (i.e. vertical wind direction shear). Such aninfluence includes a deflection of the plume centroid, with anassociated shearing of the vertical plume cross-section, and anenhancement of dispersion, in the horizontal plane. Wind directionshear is normally not accounted for in coastal fumigation models,although dispersion observations with shear acting as acontrolling parameter are not uncommon. A three-dimensionalLagrangian stochastic model is used to investigate the influenceof uniform wind direction shear on the diffusion of a point-sourceplume within the horizontally homogeneous convective boundarylayer, with the source located at the top of the boundary layer.Parameterisations are developed for the plume deflection andenhanced dispersion due to shear within the framework of aprobability density function (PDF) approach, and compared with theLagrangian model results. These parameterisations are thenincorporated into two applied coastal fumigation models: a PDFmodel, and a commonly used model that assumes uniform andinstantaneous mixing in the vertical direction. The PDF modelrepresents the vertical mixing process more realistically. A moreefficient version of the PDF model, which assumes a well-mixedconcentration distribution in the vertical at large times, isapplied to simulate sulfur dioxide data from the Kwinana CoastalFumigation Study. A comparison between the model results and thedata show that the model performs much better when the wind-sheareffects are included. 相似文献
8.
Analytical solution for two-dimensional thermal plume updraft velocity is obtained under the assumption of a uniform temperature excess inside the plume. In this way, the thermal plume motion is modeled in both mixed layer and entrainment zone. Also, a semi-analytical solution is obtained using an empirical model for the plume temperature excess in the mixed layer. In addition, an analytical model for entrainment zone thickness is obtained by computing the overshoot distance of the modeled plumes, and a semi-analytical model by using the empirical model for plume temperature. By using a nonlinear profile for the lapse rate in the surface layer based on the Monin–Obukhov similarity theory, our model predicts that the characteristics of the surface layer plays an important role in the structure of the entrainment zone. Finally, our solutions for plume velocity allow us to consider the effect of the lateral entrainment on the plume excess temperature and velocity in the convective boundary layer. It is shown that the lateral entrainment has an important role on the plume dynamics and the solutions in the zero entrainment limit offer large overestimated values for the plume velocity, which also result in overestimated values of the entrainment zone thickness. 相似文献
9.
Resolved Versus Parametrized Boundary-Layer Plumes. Part II: Continuous Formulations of Mixing Rates for Mass-Flux Schemes 总被引：1，自引：0，他引：1
The conditional sampling of coherent structures in large-eddy simulations of the convective boundary layer (Couvreux et al.
Boundary-layer Meteorol 134:441–458, 2010) is used to propose and evaluate formulations of fractional entrainment and detrainment
rates for mass-flux schemes. The proposed formulations are physically-based and continuous from the surface to the top of
clouds. Entrainment is related to the updraft vertical velocity divergence, while detrainment depends on the thermal vertical
velocity, on buoyancy and on the moisture contrast between the mean plume and its environment. The proposed formulations are
first directly evaluated in simulations of shallow clouds. They are then tested in single-column simulations with the thermal
plume model, a mass-flux representation of boundary-layer thermals. 相似文献
10.
Summary In the present paper, a model for the diffusion of material from a point source in an urban atmosphere is incorporated. The
plume is assumed to have a well-defined edge at which the concentration falls to zero. The vertical wind shear is estimated
using logarithmic law, by employing most of the available techniques of stability categories. The concentrations estimated
from the model were compared favorably with the field observations of other investigators. 相似文献
11.
A wind-tunnel study was conducted to investigate ventilation of scalars from urban-like geometries at neighbourhood scale
by exploring two different geometries a uniform height roughness and a non-uniform height roughness, both with an equal plan
and frontal density of λ
p
= λ
f
= 25%. In both configurations a sub-unit of the idealized urban surface was coated with a thin layer of naphthalene to represent
area sources. The naphthalene sublimation method was used to measure directly total area-averaged transport of scalars out
of the complex geometries. At the same time, naphthalene vapour concentrations controlled by the turbulent fluxes were detected
using a fast Flame Ionisation Detection (FID) technique. This paper describes the novel use of a naphthalene coated surface
as an area source in dispersion studies. Particular emphasis was also given to testing whether the concentration measurements
were independent of Reynolds number. For low wind speeds, transfer from the naphthalene surface is determined by a combination
of forced and natural convection. Compared with a propane point source release, a 25% higher free stream velocity was needed
for the naphthalene area source to yield Reynolds-number-independent concentration fields. Ventilation transfer coefficients
w
T
/U derived from the naphthalene sublimation method showed that, whilst there was enhanced vertical momentum exchange due to
obstacle height variability, advection was reduced and dispersion from the source area was not enhanced. Thus, the height
variability of a canopy is an important parameter when generalising urban dispersion. Fine resolution concentration measurements
in the canopy showed the effect of height variability on dispersion at street scale. Rapid vertical transport in the wake
of individual high-rise obstacles was found to generate elevated point-like sources. A Gaussian plume model was used to analyse
differences in the downstream plumes. Intensified lateral and vertical plume spread and plume dilution with height was found
for the non-uniform height roughness. 相似文献
12.
Dispersion of Heavy Particles Emitted from Area Sources in the Unstable Atmospheric Boundary Layer 总被引：2，自引：2，他引：0
Reliable predictions of the daytime dispersal of heavy particles in the unstable atmospheric boundary layer are important in a variety of disciplines. For many applications, particles disperse from area sources near the ground, and corresponding theoretical solutions are desired to reveal insight into the physical processes. Here, theoretical solutions recently developed for neutral conditions are modified to include the effects of atmospheric instability. The Obukhov length L O and convection velocity w ? are introduced to characterize the patterns of particle dispersion, in additional to friction velocity u ? and settling velocity w s used in the neutral case. The major effects of atmospheric instability are accounted for by modifying the vertical velocity variance profile and considering the ratio of velocity scales w ?/u ?. Theoretical predictions including the mean concentration profile, plume height, and horizontal transport above the source, and ground deposition flux downwind from the source agree well with large-eddy simulation results while the particle plume is within the atmospheric surface layer. The deposition curve is characterized by a power-law decay whose exponent depends on u ?, w s, and w ?. A second steeper power-law develops once the plume extends into the mixed layer. This effect is enhanced with increasing atmospheric instability, implying that particles disperse farther from the source. 相似文献
13.
Pietro Salizzoni Raphaël Van Liefferinge Patrick Mejean Lionel Soulhac Richard J. Perkins 《Boundary-Layer Meteorology》2010,135(3):455-467
The influence of surface roughness on the dispersion of a passive scalar in a rough wall turbulent boundary layer has been
studied using wind-tunnel experiments. The surface roughness was varied using different sizes of roughness elements, and different
spacings between the elements. Vertical profiles of average concentration were measured at different distances downwind of
the source, and the vertical spread of the plume was computed by fitting a double Gaussian profile to the data. An estimate
of the integral length scale is derived from the turbulence characteristics of the boundary layer and is then used to scale
the measured values of plume spread. This scaling reduces the variability in the data, confirming the validity of the model
for the Lagrangian integral time scale, but does not remove it entirely. The scaled plume spreading shows significant differences
from predictions of theoretical models both in the near and in the far field. In the region immediately downwind of the source
this is due to the influence of the wake of the injector for which we have developed a simple model. In the far field we explain
that the differences are mainly due to the absence of large-scale motions. Finally, further downwind of the source the scaled
values of plume spread fall into two distinct groups. It is suggested that the difference between the two groups may be related
to the lack of dynamical similarity between the boundary-layer flows for varying surface roughness or to biased estimates
of the plume spread. 相似文献
14.
An ocean carbon cycle model driven by a constant flow field produced by a two-dimensional thermohaline circu-lation model is developed. Assuming that the biogenic carbon in the oceans is in a dynamic equilibrium, the inorganic carbon cycle is investigated. Before the oceanic uptake of CO2 is carried out, the investigation of 14C distributions in the oceans, including natural and bomb-produced 14C, is conducted by using different values of the exchange coefficient of CO2 for different flow fields (different vertical diffusivities) to test the performance of the model. The suitable values of the exchange coefficient and vertical diffusivities are chosen for the carbon cycle model. Under the forcing of given preindustrial atmospheric CO2 concentration of 280 ppmv, the carbon cycle model is integrated for seven thousand years to reach a steady state. For the human perturbation, two methods including the prescribed at-mospheric pCO2 and prescribed industrial emissions are used in this work. The results from the prescribed atmospher-ic pCO2 show that the oceans take up 36% of carbon dioxide released by human activities for the period of 1980-1989, while the results from the prescribed industrial emission rates show that the oceans take up 34% of car-bon dioxide emitted by industrial sources for the same period. By using the simple method of subtracting industrial emission rate from the total atmosphere+ocean accumulating rate, it can be deduced that before industrial revolution a non-industrial source exists, while after 1940 an extra sink is needed, and that a total non-industrial source of 45 GtC is obtained for the period of 1790-1990. 相似文献
15.
B. Henderson-Sellers 《Boundary-Layer Meteorology》1980,19(4):495-499
The three-phase plume model (for predicting plume rise) is linked to the Gaussian plume model (for predicting dispersion from a non-buoyant source) by demonstration that the entrainment velocity associated with the third phase of the plume rise is of identical functional form to that derived for Fickian diffusion. 相似文献
16.
By D. MÜLLER B. HEINOLD M. TESCHE I. TEGEN D. ALTHAUSEN L. ALADOS ARBOLEDAS V. AMIRIDIS A. AMODEO A. ANSMANN D. BALIS A. COMERON G. D'AMICO E. GERASOPOULOS J. L. GUERRERO-RASCADO V. FREUDENTHALER E. GIANNAKAKI B. HEESE M. IARLORI P. KNIPPERTZ R. E. MAMOURI L. MONA A. PAPAYANNIS G. PAPPALARDO R.-M. PERRONE G. PISANI V. RIZI M. SICARD N. SPINELLI A. TAFURO M. WIEGNER 《Tellus. Series B, Chemical and physical meteorology》2009,61(1):325-339
We observed a long-range transport event of mineral dust from North Africa to South Europe during the Saharan Mineral Dust Experiment (SAMUM) 2006. Geometrical and optical properties of that dust plume were determined with Sun photometer of the Aerosol Robotic Network (AERONET) and Raman lidar near the North African source region, and with Sun photometers of AERONET and lidars of the European Aerosol Research Lidar Network (EARLINET) in the far field in Europe. Extinction-to-backscatter ratios of the dust plume over Morocco and Southern Europe do not differ. Ångström exponents increase with distance from Morocco. We simulated the transport, and geometrical and optical properties of the dust plume with a dust transport model. The model results and the experimental data show similar times regarding the appearance of the dust plume over each EARLINET site. Dust optical depth from the model agrees in most cases to particle optical depth measured with the Sun photometers. The vertical distribution of the mineral dust could be satisfactorily reproduced, if we use as benchmark the extinction profiles measured with lidar. In some cases we find differences. We assume that insufficient vertical resolution of the dust plume in the model calculations is one reason for these deviations. 相似文献
17.
The effects of source size on plume behaviour have been examined in a 1.2 m wind tunnel boundary layer for isokinetic sources with diameters from 3 to 35 mm at source heights of 230 mm and at ground level. Experimental measurements of mean concentration and the variance, intermittency and probability density functions of the concentration fluctuations were obtained. In addition, a fluctuating Gaussian plume model is presented which reproduces many of the observed features of the elevated emission. The mean plume width becomes independent of source size much more rapidly than the instantaneous plume width. Since it is the meandering of the instantaneous plume which generates most of the concentration fluctuations near the source, these are also dependent on source size. The flux of variance in the plume reaches a maximum, whose value is greatest for the smallest source size, close to the source and thereafter is monotonically decreasing. The intermittency factor reaches a minimum, whose value is lowest for the smallest source, and increases back towards one. Concentration fluctuations for the ground-level source are much less dependent on source size due to the effects of the surface. 相似文献
18.
The vertical velocity field and the convective plumes in the atmospheric boundary layer have been observed during morning hours with the acoustic Doppler sounder of the C.R.P.E. A method for plume determination using acoustic soundings in the well-mixed layer is presented. Using Telford's 1970 and Manton's 1975 models, a comparison is made between the predictions of the models and the plume properties as observed by the Doppler sodar. The mean plume velocity is found to be parabolic. It is shown, restricting Monin and Obukhov similarity to conditions inside plumes and using only vertical velocity within plumes, that the observed convective plumes carry nearly sixty percent of the sensible heat flux at the top of the surface layer. 相似文献
19.
20.
Eugene Yee R. Chan P. R. Kosteniuk G. M. Chandler C. A. Biltoft J. F. Bowers 《Boundary-Layer Meteorology》1995,76(1-2):41-67
This paper describes a study of the vertical structure of concentration fluctuations in a neutrally buoyant plume from an elevated point source in slightly convective to moderately stable meteorological conditions at ranges of between 12.5 and 100 m for a range of source heights between 1 and 5 m. Observations were made of concentration fluctuations in a dispersing plume using a vertical array of sixteen very fast-response photoionization detectors placed at heights between 0.5 and 16 m. Vertical profiles of a number of concentration statistics were extracted, namely, mean concentration, fluctuation intensity, intermittency factor, peak-to-mean concentration ratio, mean dissipation rate of concentration variance, and various concentration time and length scales of dominant motions in the plume (e.g., integral macro-scale, in-plume mid-scale and Taylor micro-scale). The profiles revealed a similarity to corresponding crosswind profiles for a fully elevated plume, but showed greater and greater departure from the latter shapes once the plume had grown in the vertical so that its lower dege began to interact progressively more strongly with the ground. The evolution of the concentration probability density function at a fixed range, but with decreasing height from the ground, is similar to that obtained at a fixed height but with increasing distance from the source. Concentration power spectra obtained at different heights all had an extensive inertial-convective subrange spanning at least two decades in frequency, but spectra measured near the ground had a greater proportion of the total concentration variance in the lower frequencies (energetic subrange), with a correspondingly smaller proportion in the higher frequencies (inertial-convective subrange). It is believed that these effects result from the increased mean shear near the surface, and blocking by the surface. The effect of enhanced shear-induced molecular diffusion on concentration fluctuations is examined. 相似文献