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Journal ArticleDOI

Concentration fluctuations and fluxes in plumes from point sources in a turbulent boundary layer

01 Apr 1982-Journal of Fluid Mechanics (Cambridge University Press)-Vol. 117, Iss: -1, pp 1-26
TL;DR: In this paper, measurements have been made of concentration fluctuations and turbulent fluxes for two passive plumes from an elevated and a ground-level source in a turbulent boundary layer, and the balance of terms in the variance transport equation is examined, as is the overall level of fluctuations along the plume.
Abstract: Measurements have been made of concentration fluctuations and turbulent fluxes for two passive plumes from an elevated and a ground-level source in a turbulent boundary layer. For the concentration fluctuations, results are presented for the variance, the intermittency, peak values of concentration, probability-density functions and spectra. The balance of terms in the variance transport equation is examined, as is the overall level of fluctuations along the plume. It is shown that most of the production of fluctuations occurs very near the source. Then, the level of fluctuation decays, roughly in accordance with a balance between advection and dissipation. For the turbulent fluxes of concentration, results are presented for the vertical and lateral fluxes, with the associated behaviour of the vertical and lateral eddy diffusivities. The balance of terms in the transport equations for the fluxes is examined. The essential differences between vertical diffusion from ground-level and elevated sources and between near-field and far-field behaviour are shown to be due to the relative importance of the advection and diffusion terms in these equations.
Citations
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Journal ArticleDOI
TL;DR: In this article, the complex morphology of the scalar field is reviewed, and they are related to the intermittency problem and other aspects of passive scalar behavior such as spectrum, probability density function, flux, and variance are also addressed.
Abstract: ▪ Abstract Passive scalar behavior is important in turbulent mixing, combustion, and pollution and provides impetus for the study of turbulence itself. The conceptual framework of the subject, strongly influenced by the Kolmogorov cascade phenomenology, is undergoing a drastic reinterpretation as empirical evidence shows that local isotropy, both at the inertial and dissipation scales, is violated. New results of the complex morphology of the scalar field are reviewed, and they are related to the intermittency problem. Recent work on other aspects of passive scalar behavior—its spectrum, probability density function, flux, and variance—is also addressed.

941 citations

Journal ArticleDOI
TL;DR: Odor plumes and how insects use them, Odor plume and how Insects use them and how animals and birds use them are discussed.
Abstract: Odor plumes and how insects use them , Odor plumes and how insects use them , مرکز فناوری اطلاعات و اطلاع رسانی کشاورزی

681 citations

Journal ArticleDOI
TL;DR: In this paper, the dispersion of trace heat from elevated line and plane sources within a model plant canopy in a wind tunnel was investigated, and it was found that the dispersive flux due to spatial correlations between time-averaged streamwise and vertical velocity components (the dispersion flux) was negligible, at heights near and above the top of the canopy.
Abstract: This is the first of a series of three papers describing experiments on the dispersion of trace heat from elevated line and plane sources within a model plant canopy in a wind tunnel. Here we consider the wind field and turbulence structure. The model canopy consisted of bluff elements 60 mm high and 10 mm wide in a diamond array with frontal area index 0.23; streamwise and vertical velocity components were measured with a special three-hot-wire anemometer designed for optimum performance in flows of high turbulence intensity. We found that: (i) The momentum flux due to spatial correlations between time-averaged streamwise and vertical velocity components (the dispersive flux) was negligible, at heights near and above the top of the canopy. (ii) In the turbulent energy budget, turbulent transport was a major loss (of about one-third of local production) near the top of the canopy, and was the principal gain mechanism lower down. Wake production was greater than shear production throughout the canopy. Pressure transport just above the canopy, inferred by difference, appeared to be a gain in approximate balance with the turbulent transport loss. (iii) In the shear stress budget, wake production was negligible. The role of turbulent transport was equivalent to that in the turbulent energy budget, though smaller. (iv) Velocity spectra above and within the canopy showed the dominance of large eddies occupying much of the boundary layer and moving downstream with a height-independent convection velocity. Within the canopy, much of the vertical but relatively little of the streamwise variance occurred at frequencies characteristic of wake turbulence. (v) Quadrant analysis of the shear stress showed only a slight excess of sweeps over ejections near the top of the canopy, in contrast with previous studies. This is a result of improved measurement techniques; it suggests some reappraisal of inferences previously drawn from quadrant analysis.

349 citations

Journal ArticleDOI
David J. Thomson1
Abstract: A new stochastic model for the motion of particle pairs in isotropic high-Reynolds-number turbulence is proposed. The model is three-dimensional and its formulation takes account of recent improvements in the understanding of one-particle models. In particular the model is designed so that if the particle pairs are initially well mixed in the fluid, they will remain so. In contrast to previous models, the new model leads to a prediction for the particle separation probability density function which is in qualitative agreement with inertial subrange theory. The values of concentration variance from the model show encouraging agreement with experimental data. The model results suggest that, at large times, the intensity of concentration fluctuations (i.e. standard deviation of concentration divided by mean concentration) tends to zero in stationary conditions and to a constant in decaying turbulence.

250 citations

Journal ArticleDOI
TL;DR: In this article, large-eddy simulation has been applied to calculate the turbulent flow over staggered wall-mounted cubes and staggered random arrays of obstacles with area density 25%, at Reynolds numbers between 5 × 103 and 5 106, based on the free stream velocity and the obstacle height.
Abstract: Large-eddy simulation (LES) has been applied to calculate the turbulent flow over staggered wall-mounted cubes and staggered random arrays of obstacles with area density 25%, at Reynolds numbers between 5 × 103 and 5 106, based on the free stream velocity and the obstacle height. Re = 5 × 103 data were intensively validated against direct numerical simulation (DNS) results at the same Re and experimental data obtained in a boundary layer developing over an identical roughness and at a rather higher Re. The results collectively confirm that Reynolds number dependency is very weak, principally because the surface drag is predominantly form drag and the turbulence production process is at scales comparable to the roughness element sizes. LES is thus able to simulate turbulent flow over the urban-like obstacles at high Re with grids that would be far too coarse for adequate computation of corresponding smooth-wall flows. Comparison between LES and steady Reynolds-averaged Navier-Stokes (RANS) results are included, emphasising that the latter are inadequate, especially within the canopy region.

237 citations

References
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Journal ArticleDOI
TL;DR: In this paper, an experimental assessment of various types of turbulence generators has led to the adoption of a system of "elliptic wedge" generators and a castellated barrier to produce a simulated rough wall boundary layer.

304 citations

Book ChapterDOI
TL;DR: In this article, a mathematical model of turbulent dispersion has been presented and rationalized, by separating the total plume dispersion into two components, spreading and meandering, and various properties of the resulting material concentration distribution are then deduced.
Abstract: A mathematical model of continuous source, turbulent dispersion has been presented and rationalized. This differs from those now in use in that provision is made for fluctuations of the plume to occur, by separating the total plume dispersion into two components, spreading and meandering. Various properties of the resulting material concentration distribution are then deduced. The mean concentration distribution derived from the fluctuating plume model is identical with the material distribution in present, steady plume models. Other important properties can also be derived, particularly the variance of point concentrations, and the frequency distribution of point concentrations, that do not follow from present plume models. The fluctuating plume model, in this way, provides a rational approach to practical air pollution problems that depend on concentration properties other than the mean level of concentration. Two independent dispersion parameters are involved; that describing the instantaneous dispersion, or spreading, of a plume element and that describing the overall dispersion, or meandering, of the plume centre relative to the fixed x-axis. These are identified with dispersion functions discussed theoretically by Batchelor, Brier, and Taylor. As far as the mean concentration distribution is concerned, dispersion coefficients of the fluctuating plume model have an immediate interpretation in terms of the steady plume models of Sutton, and Frenkiel (or of any other, essentially Gaussian plume dispersion model). Because of the variety of predictions concerning various statistical properties of the fluctuating plume model, many observed characteristics of concentration statistics, so far largely ignored in practical pollution studies, may now be related to dispersion. These include the form, and location and intensity of the maximum point, of the concentration frequency distribution, the variance of the distribution, and so on. A small amount of detailed concentration data is available for comparison with the fluctuating plume model. These permit verification of certain of the predicted features, particularly the form of the concentration frequency distribution, and the behaviour of its maximum point as a function of atmospheric stability and dispersion time. The form of the ratio of peak to average concentration values, as a function of dispersion time, also follows from the fluctuating plume model and is verified by comparison with concentration data.

241 citations

Book ChapterDOI
J.S. Hay1, F. Pasquill1
TL;DR: In this paper, the authors derived the turbulent spread of particles directly from wind fluctuation records, assuming that the Lagrangian and Eulerian correlograms have similar shapes, but different scales (ratio β:l).
Abstract: Recent observations of the crosswind spread of particles at 100 m. from a continuous ground-level source are analysed assuming that the Lagrangian and Eulerian correlograms have similar shapes, but different “scales”(ratio β:l). This leads to a simple method of deriving the turbulent spread of particles directly from wind fluctuation records. The implied values of β have considerable scatter, but an average of four, independent of wind speed and stability, is suggested. Previous U.K. and recent U.S. results on diffusion over a few hundreds of metres provide further support. Data on wind tunnel diffusion and the spread of geostrophic trajectories indicate that j8 is insensitive to scale. Similarity in the power law indices representing the dependence of spread on distance over very different ranges is a coincidence due to these ranges being proportional to the Lagrangian scales.

177 citations

Journal ArticleDOI
J. C. R. Hunt, A. H. Weber1
TL;DR: In this article, a Lagrangian statistical analysis is proposed for the motions of a single particle and the growth of a small cloud released from the ground in a turbulent boundary layer.
Abstract: SUMMARY A Lagrangian statistical analysis is proposed for the motions of a single particle and the growth of a small cloud released from the ground in a turbulent boundary layer, essentially by considering the fluctuating as well as the mean vertical Lagrangian velocity, by noting that its integral time scale is much smaller than the travel time of a particle from the source and by estimating (quite conventionally) the Lagrangian statistics in terms of the measured Eulerian statistics of atmospheric surface layer turbulence, in neutrally stable conditions. Comparing the results with a 1968 analysis by P. C. Chatwin based on a diffusion equation, the same value is obtained for the dispersion of the vertical displacement of a particle as a function of its mean travel time; however, a small but physically significant difference is predicted for the growth of a cloud released at ground level. The validated diffusion equation is then used to relate the concentration of a cloud to that of a continuous plume. It is shown that the index, s, used in the exponential formula exp( --Azs) to describe observations of the concentration in a plume, varies slowly with distance downwind of a source. Diffusion measurements in the atmospheric boundary layer and in wind tunnels are shown by the theory to be consistent with turbulence measurements. The theory suggests the kind of further turbulence and diffusion measurements that ought to be made.

83 citations

Journal ArticleDOI
F. B. Smith1, J. S. Hay1
TL;DR: In this article, the problem of the spatial expansion of a cluster of particles under the action of turbulence is investigated and a simple working approximation based on the assumption that the Lagrangian and Eulerian time-correlograms are functionally similar is proposed.
Abstract: In this paper, the problem of the spatial expansion of a cluster of particles under the action of turbulence is investigated. The basic difficulty, that this form of diffusion depends on the Lagrangian properties of the turbulent field, is met by the assumption that the Lagrangian and Eulerian time-correlograms are functionally similar. In consequence, it is possible to solve the equation governing the cluster's expansion to give prediction laws expressed only in terms of easily measured parameters of the turbulence. The application of the full solution is lengthy but a simple working approximation follows from the fact that over an important part of the expansion at which the size of the cluster is of the same order of magnitude as the Eulerian length-scale, the rate of expansion is approximately constant and is proportional to the square of the intensity of turbulence whilst independent of the length scale itself. Experiments to test these formulae have been carried out on the diffusion of clusters of Lycopodium spores over a few hundred metres downwind travel near the ground, and the diffusion of long crosswind clouds of aircraft-released fluorescent particles, sampled along the vertical, after several kilometres travel in the free atmosphere. Whilst the experiments are few in number and the conditions often did not strictly conform to the conditions of isotropy and homogeneity required by the theory, the results are satisfactorily consistent with the prediction formulae and imply an effectively constant ratio between the scales of the Lagrangian and Eulerian time-correlograms.

80 citations