Page 362 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

340 11 Air Dispersion

environment. A vortex at the front side of the building does not affect the plume as
much as the roof-top and rear-side separation zones.
Mathematically, empirical equations are given by Hanna et al. [12] for predicting
the wake effect on the plume entrance. As a widely accepted safe engineering
practice, it is simple to follow the rule of thumb that stack height ðhÞ is 2.5 times the
height of the heightest building nearby.

h [ 2:5H B ð11:47Þ

where H B is the height of the building.

11.3.4 Ground Surface Reflection

We did not consider the boundaries in the analysis above. Most of the time, we are
interested in concentrations at ground level for the protection of human beings and
their properties. Although mathematically we could continue the calculation for
z\0, it is physically wrong because air pollutants cannot enter underground by
eddy dispersion. In this case, the ground acts as a wall in the computational domain
and we have to consider the wall effect.
If we ignore the deposition at the ground surface, it is commonly assumed this
wall to be reflective like a mirror. As illustrated in Fig. 11.12, the reflects at the
surface as if there was an underground mirrored source. Mathematically, the con-
2
ð
tribution of the mirror source is calculated using Eq. (11.33) by replacing z HÞ
2
with z þ HÞ (Fig. 11.12).
ð

physical source

mirror source

Fig. 11.12 Plume reflection on the ground surface