Plug-flow/dispersion model of longitudinal dispersion

A modified Fickian plug-flow/dispersion model (P/D model) is developed in this study. In P/D model, the flow process is divided into two belts, plug flow belt and dispersion belt. P/D model is very similar to Fickian model and rather perfect. The prediction by P/D model can be always consistent with experimental data in river, flume, and pond, even though the data are much skew. Therefore, P/D model is better than Fickian model and other dispersion models.     

Simulation of dispersion in moderately complex terrain—Part B. The higher order closure dispersion model

A three-dimensional higher-order closure dispersion model is presented. The model is used to simulate dispersion from point sources in complex terrain. The model uses mean and turbulence quantities simulated with the fluid dynamic model presented in Part A to simulate dispersion in a polar coordinate system with its origin in the point source. Different turbulent length scales are used for the vertical and horizontal fluxes. Simulation results are compared with data from tracer experiments performed in southern Sweden, the Vänersborg-Trollhättan region. The tracer experiments were performed during convective atmospheric conditions as well as during very stable conditions. The geographical area has terrain features that exert forcing on the meso-γ-scale. Within the area there is a relatively flat agricultural area, forested hills, a river valley and an extended lake area. The terrain height relief is typically 80 m.The simulations with the dispersion model performed in the Vänersborg-Trollhättan region show good agreement with measured data in the region for convective atmospheric conditions as well as for very stable conditions.     

A micro-scale dispersion model for motor vehicle exhaust gas in urban areas—OMG volume-source model

A micro-scale dispersion model is presented for estimating the concentration of pollutants from motor vehicle exhaust gas within an area extending 200 m from the side of the road in an urban area. The initial mixing of pollutants in a street canyon is modeled as a volume source employing an analytical solution to the Fickian diffusion equation.Parameters for the model were determined based on data from experiments performed at five locations in Osaka. In the experiments, SF₆ was released as a tracer gas. The height for wind speed measurements for use as the advection speed of the plume was determined from an analysis of the flux of SF₆. The eddy diffusivities in the vertical and lateral directions were derived from statistics of the turbulent velocity fluctuations of the air. The sensitivity analysis of the model revealed that proper characterization of the thickness of the volume source is essential for proper estimation of the concentration of pollutants.     

Random walk modeling of wake dispersion for the exhaust tower of an underground tunnel in urban area

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Wind field and dispersion in a built-up area—A comparison between field measurements and wind tunnel data

The dispersion of accidentally released gases from sources near the ground in built-up areas is determined by the complex flow conditions around the buildings. Wind tunnel experiments must be performed to obtain detailed information on the diffusion of pollutants. In order to show that the wind tunnel results are transferable to nature, parallel measurements in a wind tunnel and for prototype conditions of wind direction, wind velocity, and gas dispersion have been carried out in an industrial plant. The results show excellent agreement of model and prototype for properly scaled conditions.     

A computational model for the rise and dispersion of wind-blown,buoyancy-driven plumes—I. Neutrally stratified atmosphere

A multi-dimensional computational model for the rise and dispersion of a wind-blown, buoyancy-driven plume in a calm, neutrally stratified atmosphere is presented. Lagrangian numerical techniques, based on the extension of the vortex method to variable density flows, are used to solve the governing equations. The plume rise trajectory and the dispersion of its material in the crosswind plane are predicted. It is found that the computed trajectory agrees well with the two-thirds power law of a buoyancy-dominated plume, modified to include the effect of the initial plume size. The effect of small-scale atmospheric turbulence, modeled in terms of eddy viscosity, on the plume trajectory is found to be negligible. For all values of buoyancy Reynolds number, the plume cross-section exhibits a kidney-shaped pattern, as observed in laboratory and field experiments. This pattern is due to the formation of two counter-rotating vortices which develop as baroclinically generated vorticity rolls up on both sides of the plume cross-section. Results show that the plume rise can be described in terms of three distinct stages: a short acceleration stage, a long double-vortex stage, and a breakup stage. The induced velocity field and engulfment are dominated by the two large vortices. The effect of a flat terrain on the plume trajectory and dispersion is found to be very small. The equivalent radii of plumes with different initial cross-sectional aspect ratios increase at almost the same rate. A large aspect-ratio plume rises slower initially and then catches up with smaller aspect-ratio plumes in the breakup stage. The Boussinesq approximation is found to be valid if the ratio of the density perturation to the reference density is less than 0.1.     

Deposition velocity and the collision model of atmospheric dispersion—II. Extension to cases with variable turbulent velocity scale

The collision model of turbulent dispersion has been extended to deal with situations where the turbulent velocity scale is dependent on position, which requires that the particles of the Monte Carlo simulation accelerate between collisions.The theory has been applied to an idealized vegetative canopy in order to illustrate the differences from the predictions of eddy-diffusivity (K)-theory arising from the finite decorrelation length scale associated with vertical turbulent velocity. For cases of high deposition to canopy elements, the collision-model results do deviate from K-theory results, although the differences are not practically significant.An extension to the model enables gravitational settling to be handled in a more realistic manner than in K-theory. By this means, it is found that the simple expedient of adding the settling velocity to the deposition velocity due to other processes is more closely valid than appears from K-theory results.