AERMOD Tech Guide

Gaussian Plume Air Dispersion Model

8.10 Wind Speed & Turbulence Limits used in Model Calculations

At the time of plume rise calculations the turbulence at stack top (or whatever level the meteorology is selected from in the case of iterative plume rise) will be recalculated as

Equation (136)
Equation (137)

Dilution of the plume is determined by the wind that corresponds to average over the magnitudes of the wind vectors during a given time interval. But measurements only give us the vector averaged wind, which can be zero, even though the dilution wind is not zero. We can estimate the dilution wind by assuming that the vector wind, uv , can be expressed as

Equation (138)

wheresymbolis the mean measured wind, and the primed quantities refer to the turbulent fluctuations. Notice that, as assumedsymbol . If we assume that the measured velocity fluctuations correspond only to the angular variations of a constant vector, uv , we can write from eq. (138),

Equation (139)

In this simple model, uv , is the dilution wind. If we take u = v , the dilution wind can be written as

Equation (140)

This formulation assures that the dilution wind is not zero as long as either " or v is not zero. V Similarly, at the time of dispersion calculations, the effective turbulence and effective wind speed will be recalculated using eqs. (136), (137) & (140), where the stack top values will be replaced with w ,v & u.