Abstract
In recent years, some thermal power plants have adopted plans to limit the height of stacks for scenic considerations, cost reduction and compliance with aviation regulations. In these cases, it is important to consider the high concentration of air pollutants caused by downdrafts due to mountains located windward of stacks in the environmental impact assessment. Recently, the method of evaluating the topographical effect by means of an air flow model and a dispersion model (TOPADS) has been proposed by the authors. The validity of this method has been confirmed experimentally using wind tunnel and field tracer experiments for large-capacity elevated sources; however, the applicability of the method for evaluating atmospheric dispersion during downdraft conditions has not yet been tested. Furthermore, there has been no study on a numerical model for atmospheric dispersion, which considers downdrafts.
In this study, we examined the applicability of the atmospheric dispersion estimation method for downdrafts on the basis of meteorological observations and field tracer experiments. Turbulence and dispersion characteristics during a downdraft were studied at the base of Mt. Akagi. The TOPADS method tends to slightly overestimate the effect of topography and it is difficult to set inflow boundary conditions for turbulence statistics at a mountain area located windward of a source in the air flow model. A method of calculating dispersion, in which turbulence statistics are parameterized using the results obtained by meteorological observations (DownDraft Model: DDM), was proposed. The DDM enabled the reduction of the overestimation tendency found in certain simulation cases utilizing the TOPADS and provided reasonable estimates of atmospheric dispersion for downdraft periods. The DDM requires neither the setting of inflow boundary conditions for turbulence statistics nor calculation using an air flow model. The DDM is therefore easier to use than the TOPADS method.
