Transport wind turbulence

Deacon s intention was to separate the viscosity effect from the wind effect, so that the new model would be able to describe the change of via due to a change of water or air temperature (i.e., of viscosity) at constant wind speed. Deacon concluded that mass transfer at the interface must be controlled by the simultaneous influence of two related processes, that is, by the transport of chemicals (described by molecular diffusivity Dia), and by the transport of turbulence (described by the coefficient of kinematic viscosity va). Note that v has the same dimension as Dia. Thus, the ratio between the two quantities is nondimensional. It is called the Schmidt Number, Scia ... 

Figure 16.1 shows a typical record of one wind component that might be obtained by an anemometer. From such a record a mean velocity u, can be obtained. Wind turbulence refers to the properties of the fluctuations about the mean in a record such as the one shown in Figure 16.1. These fluctuations are primarily responsible for the spreading of a cloud of pollutants in the atmosphere. It is important to measure all three components of the wind velocity, since the vertical fluctuations are often the most important in transporting pollutants. The vertical velocity fluctuations provide a direct measure of the stability of the atmosphere. In order to obtain a record such as the one shown in Figure 16.1 a sensitive, rapidly responding wind vane and anemometer are required. Vertical fluctuations can be measured by ...

In its simplest form, a model requires two types of data inputs information on the source or sources including pollutant emission rate, and meteorological data such as wind velocity and turbulence. The model then simulates mathematically the pollutant s transport and dispersion, and perhaps its chemical and physical transformations and removal processes. The model output is air pollutant concentration for a particular time period, usually at specific receptor locations. 

The model contains a surface energy method for parameterizing winds and turbulence near the ground. Its chemical database library has physical properties (seven types, three temperature dependent) for 190 chemical compounds obtained from the DIPPR" database. Physical property data for any of the over 900 chemicals in DIPPR can be incorporated into the model, as needed. The model computes hazard zones and related health consequences. An option is provided to account for the accident frequency and chemical release probability from transportation of hazardous material containers. When coupled with preprocessed historical meteorology and population den.sitie.s, it provides quantitative risk estimates. The model is not capable of simulating dense-gas behavior. 

HOTM AC/RAPTAD contains individual codes HOTMAC (Higher Order Turbulence Model for Atmospheric Circulation), RAPTAD (Random Particle Transport and Diffusion), and computer modules HOTPLT, RAPLOT, and CONPLT for displaying the results of the ctdculalinns. HOTMAC uses 3-dimensional, time-dependent conservation equations to describe wind, lempcrature, moisture, turbulence length, and turbulent kinetic energy. 

Transport of outdoor contaminants is controlled by both the mean motion of winds and dispersal by turbulence. Since airflow around buildings has... 

In gridpoint models, transport processes such as speed and direction of wind and ocean currents, and turbulent diffusivities (see Section 4.8.1) normally have to be prescribed. Information on these physical quantities may come from observations or from other (dynamic) models, which calculate the flow patterns from basic hydrodynamic equations. Tracer transport models, in which the transport processes are prescribed in this way, are often referred to as off-line models. An on-line model, on the other hand, is one where the tracers have been incorporated directly into a d3mamic model such that the tracer concentrations and the motions are calculated simultaneously. A major advantage of an on-line model is that feedbacks of the tracer on the energy balance can be described... 

Dispersion models describe the airborne transport of toxic materials away from the accident site and into the plant and community. After a release the airborne toxic material is carried away by the wind in a characteristic plume, as shown in Figure 5-1, or a puff, as shown in Figure 5-2. The maximum concentration of toxic material occurs at the release point (which may not be at ground level). Concentrations downwind are less, because of turbulent mixing and dispersion of the toxic substance with air. 

In order to make the transport model adaptable to measurement results some simplifications are used. Vertical and lateral components of wind are neglible, the mean transport velocity U in x-direction is steady the pollutant transfer by advection in the drift direction is greater than by turbulent diffusion at the ground total reflection is assumed. For the case that the concentration at any point in space is independent of t and that the diffusivities are independent of x, y and z the simplified diffusion equation of the K-therory /8/ becomes... 

Contaminant volatilization from subsurface solid and aqueous phases may lead, on the one hand, to pollution of the atmosphere and, on the other hand, to contamination (by vapor transport) of the vadose zone and groundwater. Potential volatihty of a contaminant is related to its inherent vapor pressure, but actual vaporization rates depend on the environmental conditions and other factors that control behavior of chemicals at the solid-gas-water interface. For surface deposits, the actual rate of loss, or the pro-portionahty constant relating vapor pressure to volatilization rates, depends on external conditions (such as turbulence, surface roughness, and wind speed) that affect movement away from the evaporating surface. Close to the evaporating surface, there is relatively little movement of air and the vaporized substance is transported from the surface through the stagnant air layer only by molecular diffusion. The rate of contaminant volatilization from the subsurface is a function of the equilibrium distribution between the gas, water, and solid phases, as related to vapor pressure solubility and adsorption, as well as of the rate of contaminant movement to the soil surface. 

A different situation is encountered at the bottom of a water body. The sediment-water interface is characterized by, on one side, a water column which is mostly turbulent (although usually less intensive than at the water surface), and, on the other side, by the pore space of the sediment column in which transport occurs by molecular diffusion. Thus, the turbulent water body meets a wall into which transport is slow, hence the term wall boundary (Fig. 19.3b). A wall boundary is like a one-sided bottleneck boundary, that is, like a freeway leading into a narrow winding road. 

Vertical transport and gravitational circulation in estuaries are influenced by both stratification and turbulent mixing, which result from the destabilizing forces of tides and/or wind processes. Under highly stratified conditions, two-layered flow is largely determined by the interaction of baroclinic and barotropic forcing. 

As characterized in Fig. 6.7, once a chemical is released into the atmosphere, it is rapidly transported by the average wind and subjected to dispersion, defined as spreading as a result of thermal or density gradients and/or turbulence, and advection, defined as movement as... 

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