Page 337 - Fundamentals of Air Pollution 3E

II. Turbulence 293

One of the effects of wind speed is to dilute continuously released pollut-
ants at the point of emission. Whether a source is at the surface or elevated,
this dilution takes place in the direction of plume transport. Figure 19-2
shows this effect of wind speed for an elevated source with an emission
1
of 6 mass units per second. For a wind speed of 6 m s^ , there is 1 unit
between the vertical parallel planes 1 m apart. When the wind is slowed
1
to 2 m s" , there are 3 units between those same vertical parallel planes
1 m apart. Note that this dilution by the wind takes place at the point of
emission. Because of this, wind speeds used in estimating plume dispersion
are generally estimated at stack top.
Wind speed also affects the travel time from source to receptor; halving
of the wind speed will double the travel time. For buoyant sources, plume
rise is affected by wind speed; the stronger the wind, the lower the plume.
Specific equations for estimating plume rise are presented in Chapter 20.

II. TURBULENCE

Turbulence is highly irregular motion of the wind. The atmosphere does
not flow smoothly but has seemingly random, rapidly varying erratic mo-
tions. This uneven flow superimposed on the mean flow has swirls or

Fig. 19-2. Dilution by wind speed.