Turbulence pollution transport

Nuterman R (2008) Modelling of turbulent flow and pollution transport in urban canopy. PhD Thesis, Tomsk State University, 156 p... 

The investigation of air flows, turbulence, heat exchange, and pollutant transport within residential urban areas have become important in recent years because of the ecological reasons and the elaboration of measures for a better urban comfort and countermeasures to possible terrorist s attacks. There are several scientific programs running by the USA and European Union towards a better understanding of relevant physical mechanisms and forecasts for the urban climate of some test cities, see Chapters 2 and 9. 

Aerosols Atmospheric Turbulence Cloud Physics Combustion Environmental Geochemistry Environmental Measurements Environmental Observation and Forecasting Systems Pollution Control Pollution, Environmental Soil and Groundwater Pollution Transport and Fate of Chemicals in the Environment Water Pollution... 

Early models used a value for that remained constant throughout the day. However, measurements show that the deposition velocity increases during the day as surface heating increases atmospheric turbulence and hence diffusion, and plant stomatal activity increases (50—52). More recent models take this variation of into account. In one approach, the first step is to estimate the upper limit for in terms of the transport processes alone. This value is then modified to account for surface interaction, because the earth s surface is not a perfect sink for all pollutants. This method has led to what is referred to as the resistance model (52,53) that represents as the analogue of an electrical conductance... 

Turbulent Diffusion FDmes. Laminar diffusion flames become turbulent with increasing Reynolds number (1,2). Some of the parameters that are affected by turbulence include flame speed, minimum ignition energy, flame stabilization, and rates of pollutant formation. Changes in flame stmcture are beHeved to be controlled entirely by fluid mechanics and physical transport processes (1,2,9). 

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 gradient transport model is most appropriate when the turbulence is confined to scales that are small relative to the pollutant volume. It is therefore most applicable to continuous line and area sources at ground... 

Aerosol production and transport over the oceans are of interest in studies concerning cloud physics, air pollution, atmospheric optics, and air-sea interactions. However, the contribution of sea spray droplets to the transfer of moisture and latent heat from the sea to the atmosphere is not well known. In an effort to investigate these phenomena, Edson et al.[12l used an interactive Eulerian-Lagrangian approach to simulate the generation, turbulent transport and evaporation of droplets. The k-e turbulence closure model was incorporated in the Eulerian-Lagrangian model to accurately simulate... 

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. 

These layers containing higher concentrations of pollutants provide an important mechanism for transport of ozone, particles, and their precursors to the free troposphere. In addition, in the morning when solar heating causes turbulent mixing (Fig. 2.20), these pollutants are mixed down to the surface. This not only increases the surface concentrations but also provides species that can initiate the VOC-NO chemistry that leads to more ozone formation. As a result, there is a carryover from one day to the next, leading to smog episodes in which the pollutant concentrations increase from day to day. 

As shown in Illustrative Examples 19.3 and 19.4, often it is not immediately known whether an exchange process is controlled by transport across a boundary layer or by transport in the bulk phase. In Illustrative Example 19.3 we look at the case of resuspension of particles from the polluted sediments of Boston Harbor. We are interested in the question of what fraction of the pollutants sorbed to the particles (such as polychlorinated biphenyls) can diffuse into the open water column while the particles are resuspended due to turbulence produced by tidal currents in the bay. To answer this question we need to assess the possible role of the boundary layer around the particles. 

Strictly speaking, equations 19-52 and 19-53 are valid only if the pollutant cloud is infinitely long. A more realistic situation is treated in Box 19.2 here a pollutant patch of finite length L along the x-axis is eroded on both edges due to diffusion processes (turbulence, dispersion, etc.). Again, the boundary is of the diffusive type since the transport characteristics on both sides of the boundary are assumed to be identical. 

Particle-laden multiphase flows, usually turbulent, cover a wide range of applications, such as pollution control, sediment transport, combustion processes, erosion effects in gas turbines, and so on. One of the most important aspects of particle-laden turbulent flows is the mutual interactions between particles and turbulence. PIV techniques, as a powerful tool other than numerical simulation method and theoretical analysis, have been applied to this research field of particle-laden multiphase flows. Note that, dispersed-phase particles in particle-laden... 

Transport from the atmosphere to land and water Dry deposition of particulate and gaseous pollutants Precipitation scavenging of particulate and gaseous pollutants Adsorption of gases onto particles and subsequent diy and wet deposition Transport within the atmosphere Turbulent dispersion and convection Atmospheric transformation Diffusion to the stratosphere Photochemical degradation Oxidation by free radicals and ozone Gas-to-particle conversion... 

While it is tempting, it would be premature to apply these equations and findings directly to more complex spray combustor situations. Apart from obvious differences in overall geometry, in practical sprays three effects are superimposed transients associated with oxidizer entrained in the fuel injector region, droplet-size-dependent relative motion between the fuel droplets and the surrounding gas, and oxidizer and product transport by turbulence and convection. Rather, our present QS and future transient studies of the behavior of quiescent fuel droplet clouds should be viewed as necessary first steps in the qualitative and quantitative theoretical understanding of fuel droplet sprays. Future work should be concerned not only with the conditions under which theoretical group combustion occurs in fuel sprays but also with the implications of such cooperative phenomena for combustion eflBciency in volume-limited systems, and pollutant emissions. 

The smallest spatial scale at which outdoor air pollution is of concern corresponds to the air volume affected by pollutant chemical emissions from a single point source, such as a smokestack (Fig. 4-24). Chemicals are carried downwind by advection, while turbulent transport (typically modeled as Fick-ian transport) causes the chemical concentrations to become more diluted. Typically, smokestacks produce continuous pollutant emissions, instead of single pulses of pollutants thus, steady-state analysis is often appropriate. At some distance downwind, the plume of chemical pollutants disperses sufficiently to reach the ground the point at which this occurs, and the concentrations of the chemicals at this point and elsewhere, can be estimated from solutions to the advection-dispersion-reaction equation (Section 1.5), given a knowledge of the air (wind) velocity and the magnitude of Fickian transport. 

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