Concentration fields, plumes

The length scales for the turbulent concentration field range from the plume width to the scale at which molecular diffusion acts to homogenize the distribution (or dissipate the variance of the scalar fluctuations). The smallest length scale is referred to as the Batchelor scale and is estimated as... 

Figure 5.2 See color plates. Sample instantaneous concentration field near the plume source. H is the channel depth and equals 20 cm. The contour values are normalized by the source concentration. (Figure adapted from data in Webster et al. [3].)...

As described above, the plume becomes wider and more dilute as it evolves in the streamwise direction, thus ccenteriine and a are changing with x. The decrease of the time-averaged concentration along the centerline of the plume follows a v 1 profile for x/H > 2 (Fig. 5.8). This power law decrease agrees well with the time-averaged concentration field predicted by modeling efforts that assume... 

Most complete is the recently published work by Shwab [2] which was carried out in Leningrad as early as 1940. In it the turbulent plume of a flame is analyzed in detail for both the case when a pure combustible gas is supplied and for the case of a gas mixed with an insufficient amount of air, which we do not consider. Shwab finds the relations between the concentration fields of the gas, oxygen, and combustion products and of temperature, and the gas velocity field, also not considered by us. A number of the results obtained by Shwab (in particular, the constancy of the concentration of the combustion products and temperature) on the flame surface are reproduced in our article for the sake of completeness. 

This simple form shows how the concentration field varies markedly as the height of the source varies near an obstacle. The results are consistent with wind tunnel concentration measurements of steady plumes in and near obstacle wakes (Robins and Apsley, 2000 [539]). 

SLAB calculates chemical concentrations at positions downwind and heights above the ground. Tlic plume may be denser-than-air, neutrally-buoyant, or less dense than air. Thermodynamics effeci.s are accounted for, including latent heat exchanges due to the condensation or evaporation ot liquids, Time averaged results may be calculated. SLAB is the easiest of the publicly-available dense gas models to set up and mn. It has been extensively validated against large-scale field data. 

Important issues in groundwater model validation are the estimation of the aquifer physical properties, the estimation of the pollutant diffusion and decay coefficient. The aquifer properties are obtained via flow model calibration (i.e., parameter estimation see Bear, 20), and by employing various mathematical techniques such as kriging. The other parameters are obtained by comparing model output (i.e., predicted concentrations) to field measurements a quite difficult task, because clear contaminant plume shapes do not always exist in real life. 

Other measurements such as gas species and soot all have importance in fire plumes but will not be discussed here. As we have seen for simple diffusion flames, the mixture fraction plays a role in generalizing these spatial distributions. Thus, if the mixture fraction is determined for the flow field, the prospect of establishing the primary species concentration profiles is possible. 

A field study was conducted by Larson et al. [31] to characterise the impact of effluent discharges on a sandy soil about 0.5 m below the surface. A 2.5 m thick unsaturated zone and a 3—4 m thick unconfined sand/gravel aquifer underlaid the tile field. LAS concentrations in the effluent plume decreased over a distance of 10 m from 10 000 to 30 xg L-1. A further object of study was a laundromat pond exposed to LAS-containing sewage for more than 25 years. A clay layer separated the natural pond from the vadose zone made up of porous sand. Measurements of LAS levels as a function of soil depth beneath the pond showed a rapid decrease from about 220 mg kg-1 at 30 cm to... 

Descriptions of field studies of power-plant stack plumes were given by Davis et al. The ozone concentration appears to be lower in regions of high sulfur dioxide content. At 32 km downwind from the stacks, it was claimed that the ozone concentration in the plume (now 11 km wide) is higher than the ambient concentration ( 0.08 ppm) by approximately 0.02 ppm. Simultaneous measurements of nitric oxide and nitrogen dioxide were integrated across the plume. Values of the ratio of nitric... 

Field measurements of a thermal plume downstream of a thermal power plant, illustrated in Figure E4.6.1, are required to determine the impact of the heated water on the river biota. These are near field because they occur before the river is mixed across its width and depth. Since field measurements are expensive and time consuming, it is desirable to select the measurements on which to concentrate the effort. 

Uncontrolled open fires in tyre dumps may burn for several months (e.g., Rhinehard tyre fire in Winchester, Virginia), generating many hazardous products of incomplete combustion, which are released directly into the atmosphere (EPA 1997). Owing to safety concerns and other factors (e.g., meteorological conditions, fire-fighting activities), it is difficult to collect air samples from the smoke plumes of these fires. The available field data revealed potentially hazardous levels of several PAHs and CO, as well as relatively high concentrations of Pb and Zn in such plumes (Ohio Air Quality Development Authority 1991 EPA 1997). 

---