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R. I. Sykes and R. S. Gabruk

Abstract

Fractal analysis techniques have been applied to the concentration fields from large-eddy simulations of plume dispersion in a turbulent boundary layer. Fractal dimensions between 1.3 and 1.35 are obtained from area-perimeter and box-counting analyses for neutral and convective conditions. These values are close to previous estimates from atmospheric data. Methods for generating fractal fields with given statistical moments are examined and the simplest of these, the recursive refinement technique, is shown to be inadequate. The problem is shown to be the interpolation step of the procedure, which intrinsically reduces the variance with each refinement. Accurate statistical representation is obtained by replacing the interpolation step of the refinement technique with a sum of random pulses of appropriate width and random location. The pulse technique can easily he adapted to generate either clipped-normal or lognormal one-point probability distributions. Results from the fractal generation technique using simulated mean statistics are compared with realizations of instantaneous plume cross sections from the large-eddy simulations. The simulated probability distributions lie between the clipped normal and the lognormal, so the fractal fields cannot match the realizations precisely. Larger-scale features of the plumes are generally well represented by the fractal method, however.

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R. I. Sykes and R. S. Gabruk

Abstract

A practical model for the effect of averaging time on the turbulent dispersion of a continuous plume is presented. The model is based on a second-order turbulence closure scheme, but is applied to the integrated spatial moments of the plume to provide a Gaussian spread prediction. Velocity fluctuation variances are used directly by the closure model to predict the dispersion, and are partitioned into meandering and diffusive scales based on the instantaneous spread of the plume. Finite time averaging is represented by a simple estimate of the turbulent energy spectrum. The model is compared with short-duration atmospheric measurements for dispersing clouds.

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R. I. Sykes and D. S. Henn

Abstract

Numerical calculations of two-dimensional convection between flat plates at high Rayleigh numbers are presented. They do not show the large cell aspect ratios observed by Rothermel and Agee in their numerical calculations at high Rayleigh numbers. The present results appear to be insensitive to grid resolution, the conventional test of numerical accuracy, large amplitude random perturbations and the numerical representation of the diffusion term. We therefore suggest that the mechanism observed by Rothermel and Agee, which produces increasingly larger aspect ratio cells with increasing Rayleigh number, is strongly influenced by the numerical method used to solve the equations.

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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 and W. S. Lewellen

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A two-dimensional numerical study of breaking Kelvin-Helmholtz billows is presented. The turbulent breaking process is modeled using second-order closure methods to describe the small-wale turbulence, while the large-scale billow itself is calculated explicitly as a two-dimensional flow. The numerical results give detailed predictions of turbulence levels and time scales, and are consistent with laboratory and atmospheric observations. Two general predictions of the model are that the structure of turbulent temperature fluctuations is very different from that of the velocity fluctuations, the former being much more striated, and that the time scale of the growth and breaking process is virtually completely determined by the initial velocity shear.

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R. I. Sykes and D. S. Henn

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W. S. Lewellen and R. I. Sykes

Abstract

A probabilistic framework for incorporating uncertainty in air quality models is described. The quantitative dependence of the uncertainty in calculated air quality concentrations on the uncertainty in the input meteorological data is illustrated using a simple Second-order Closure integrated Model Plume in combination with the EPRI Plume Model Validation and Development Data Set. Evaluation of the model results demonstrate that even though individual hourly samples cannot be deterministically predicted downwind of a powerplant stack, statistical representations of the observed cumulative distribution of the sample values are quite predictable. We discuss the data needed to improve the definition of the range of meteorological uncertainty within an ensemble of flows defined by given meteorological data, and thus provide for improvements in predictability models of the type illustrated. We argue that attempts to collect the data needed to define more precisely the variance within the ensemble of compatible flows will prove more productive than attempts to eliminate meteorological uncertainties in given datasets.

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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, σc, 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 and D. S. Henn

Abstract

A series of large-eddy simulators of free and sheared convective flow between moving flat plates is presented. Results for free convection are compared with laboratory data. The ratio of friction velocity to the convective velocity sale,u */w *,is identified as an important parameter in sheared convective flow determining the formation of longitudinal rolls. Rolls are found for u */w * ≥ 0.35, with aspect ratio decreasing as this parameter increases. It is shown that, in this regime, two-dimensional simulations with a proper choice of roll orientation and turbulence length-scale can produce correct velocity variances and roll aspect ratio.

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