All large structures distort the atmosphere and interfere with wind flow to some extent. These atmospheric distortions usually take the form of a wake, which consists of a pocket of slower, more turbulent air. If a plume is emitted near a wake, it is usually pulled down because of the lower pressure in the wake region. This effect is termed downwash. When downwash occurs, the plume is brought down to the ground near the emission source more quickly.
A wake that causes downwash usually occurs as the result of one of three physical conditions: 1) the stack, referred to as stack-tip downwash, 2) local topography, or 3) nearby large structures or building downwash. Figure 5.8.10 shows examples of each of these conditions.
Stack-tip downwash occurs when the ambient wind speed is high enough relative to the exit velocity of the plume so that some or all of the plume is pulled into the wake directly downwind of the stack, as shown in Figure 5.8.11. This downwash has two effects on plume rise. First, the pollutants drawn into the stack wake leave the stack region at a lower height than that of the stack and with a lower upward velocity. Second, the downwash increases the plume cross section, which decreases the concentration.
To avoid stack-tip downwash, environmental engineers should consider the ratio of emission velocity (vs) to wind speed velocity at the stack height (us) in the stack design. If vs < 1.5 us, then the physical stack height should be adjusted by the following equation:
If vs/us > 1.5, then stack-tip downwash is avoided since the exhaust gas is emitted from the stack at sufficient velocity to clear the downwash area on the downwind side of the stack. If vs/us < 1.0, downwash will probably occur, possibly seriously. For intermediate values of vs/us, rvi p-t'l
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