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  • Author or Editor: W. S. Lewellen x
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W. S. Lewellen
and
R. I. Sykes

Abstract

A series of nearly instantaneous vertical cross sections of power-plant plume concentrations obtained by both airborne and ground-based lidar systems for the Electric Power Research Institute (EPRI) Plume Model Validation and Development Project have been analyzed. By statistically resampling the data, values of the ratio of the ensemble rms concentration fluctuation, σ c , to the ensemble mean concentration, , near the center of the plumes are found to vary from 0.2 to 4. More importantly, it is found that the normalized probability distribution function can be well represented as that resulting from a Gaussian distribution with any nonrealizable negative tail replaced by a delta function, representing intermittency at zero.

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R. I. Sykes
,
W. S. Lewellen
, and
S. F. Parker

Abstract

A practical model of atmospheric dispersion of a passive tracer based on systematic reduction of the second-order closure transport equations using Gaussian shape assumptions is presented. The model is comparable with conventional Gaussian plume models in complexity, but still maintains the capability to also predict concentration fluctuation variance and to utilize direct measurements of turbulent velocity variances in a consistent manner. Comparison with laboratory data demonstrates the model's ability to produce reasonable predictions for the concentration field.

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R. I. Sykes
,
S. F. Parker
,
D. S. Henn
, and
W. S. Lewellen

Abstract

Detailed statistics of the fluctuating concentration field produced by large-eddy simulations (LES) of the chemically reactive mixing of two species in a convectively driven mixed layer are presented. The effect of the turbulent mixing on the effective reaction rate between the species is analysed. The segregation between the species is shown to be significant for fast reactions, and therefore correct model predictions of the evolution of the species concentration requires an estimate of the segregation coefficient. Some simple modeling concepts for one-point second-order turbulence closure schemes are examined and compared with the LES results. The results are a promising indication that second-order closure schemes can be extended to provide a practical calculation of the turbulent mixing effects on fast chemical reactions.

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R. I. Sykes
,
S. F. Parker
,
D. S. Henn
, and
W. S. Lewellen

Abstract

A long-range transport model based on turbulence closure concepts is described. The model extends the description of planetary boundary layer turbulent diffusion to the larger scales and uses statistical wind information to predict contaminant dispersion. The model also contains a prediction of the statistical fluctuations in the tracer concentration resulting from the unresolved velocity fluctuations. The dispersion calculation is made by means of a Lagrangian puff representation, allowing the use of time-dependent three-dimensional flow fields. Predictions of the ANATEX (Across North America Tracer Experiment) releases are compared with observations. Both 24-h average surface and short-term aircraft sampler concentrations are calculated using the high-resolution wind fields from the NMC Nested Grid Model. The statistical prediction is also tested using long-term average wind data.

Statistical uncertainty in the predictions, due to the unresolved wind fluctuations, is found to be small for the 24-h average surface concentrations obtained with the high-resolution winds but is very significant for the short-term aircraft sampler concentrations. A clipped normal probability distribution provides a reasonably good description of the overall cumulative distribution of the aircraft sampler concentrations. A reasonably good description of the 24-h surface concentrations is also obtained using only the long-term average wind statistics and a lognormal probability distribution for the concentration values.

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