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  • Author or Editor: D. S. Henn x
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R. I. Sykes and D. S. Henn

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R. I. Sykes and D. S. Henn

Abstract

The Gaussian puff model framework is extended to provide a description of velocity shear distortion effects. An efficient splitting-merging algorithm is presented so that a maximum puff size can be specified for a calculation. This localizes the Gaussian puffs so that they represent only a limited region of the flow and the accuracy of the representation is therefore controlled. The model is shown to perform well on the deformational flow of Smolarkiewicz, providing an accurate calculation of the highly distorted solution. The extended puff methodology allows practical applications of an efficient Lagrangian dispersion technique in complex flow fields.

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R. I. Sykes, R. S. Gabruk, and D. S. Henn

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An improved method for representing the small-scale structure of a turbulent scalar field using fractal recursion techniques is described. The model generalizes the fractal successive refinement method described by Sykes and Gabruk to include a more realistic description of the pseudodissipation field. that is, the square of the scalar gradient. Turbulent dissipation fields are known to be multifractal, so a multifractal generation technique has been incorporated into the fractal refinement model to yield a scalar field with isosurfaces but with a multifractal pseudodissipation field.

The model fields are compared with realizations from large-eddy simulations of turbulent scalar dispersion and shown to provide improved agreement with the small-scale structure. The simple combination of fractal and multifractal properties employed in the model also provides insight into the structure of the random scalar field. Finally, the generation technique is completely localized in physical space and is therefore applicable to inhomogeneous fields.

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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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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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