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Air Pollution and Turbulence: Modeling and Applications
9.3.6.2 Dispersion from Near-Surface Sources Convective Dispersion
The CBL is characterized by large-scale fl ow motions, which consist of strong
updrafts of hot air (i.e., thermals) and weaker downdrafts surrounding these thermals.
One of the most important features of the CBL is the difference in strength between
the updrafts and downdrafts, which causes an asymmetry of the turbulent fl ow fi eld.
This asymmetry makes the dispersion characteristics depend on the location of the
source. As a result, the dispersion from a source at the surface, primarily caused
by positive velocity fl uctuations, is different from the dispersion from an elevated
source. Moreover, the analysis of both release heights (near-surface and elevated)
for the CBL allows us to compare our results with those from the well-established
laboratory experiments of Willis and Deardorff (1976, 1978, 1981). In fact, their
classical investigations demonstrated that for an elevated source, the plume descends
within a short distance from the source until it reaches the ground. In contrast, the
average plume centerline from a near-surface source ascends after a short downwind
distance (Sorbjan and Uliasz, 1999).
To investigate convective plume dispersion from a surface source, we injected
the contaminant from a point source situated close to the ground (h/H S = 0.25) in the
CBL described in the previous section. To get proper plume behavior, the contaminant was emitted throughout the simulation period (i.e., the release time, 2000Δt),
while the concentration statistics were determined only in the second half period
(i.e., the sampling time, 1000Δt). This allowed us to get an appropriate plume representation, as the travel time (about 100Δt) was much smaller than both the sampling
and the emission times. Figure 9.4 shows the evolution of the dispersion process. As
expected, the isopleths of crosswind-integrated concentration show that the average
plume centerline ascends after a short downwind distance as the material is emitted
0.00
0.0
0.5
1.0
1.5
2.0
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
X *
z/h
FIGURE 9.3 Isopleths of crosswind integrated concentration averaged between last 1000
time steps, the red dotted line represents the mean plume height and the blue dotted line the
vertical dispersion parameter.
© 2010 by Taylor and Francis Group, LLC
Air Pollution and Turbulence: Modeling and Applications
9.3.6.2 Dispersion from Near-Surface Sources Convective Dispersion
The CBL is characterized by large-scale fl ow motions, which consist of strong
updrafts of hot air (i.e., thermals) and weaker downdrafts surrounding these thermals.
One of the most important features of the CBL is the difference in strength between
the updrafts and downdrafts, which causes an asymmetry of the turbulent fl ow fi eld.
This asymmetry makes the dispersion characteristics depend on the location of the
source. As a result, the dispersion from a source at the surface, primarily caused
by positive velocity fl uctuations, is different from the dispersion from an elevated
source. Moreover, the analysis of both release heights (near-surface and elevated)
for the CBL allows us to compare our results with those from the well-established
laboratory experiments of Willis and Deardorff (1976, 1978, 1981). In fact, their
classical investigations demonstrated that for an elevated source, the plume descends
within a short distance from the source until it reaches the ground. In contrast, the
average plume centerline from a near-surface source ascends after a short downwind
distance (Sorbjan and Uliasz, 1999).
To investigate convective plume dispersion from a surface source, we injected
the contaminant from a point source situated close to the ground (h/H S = 0.25) in the
CBL described in the previous section. To get proper plume behavior, the contaminant was emitted throughout the simulation period (i.e., the release time, 2000Δt),
while the concentration statistics were determined only in the second half period
(i.e., the sampling time, 1000Δt). This allowed us to get an appropriate plume representation, as the travel time (about 100Δt) was much smaller than both the sampling
and the emission times. Figure 9.4 shows the evolution of the dispersion process. As
expected, the isopleths of crosswind-integrated concentration show that the average
plume centerline ascends after a short downwind distance as the material is emitted
0.00
0.0
0.5
1.0
1.5
2.0
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
X *
z/h
FIGURE 9.3 Isopleths of crosswind integrated concentration averaged between last 1000
time steps, the red dotted line represents the mean plume height and the blue dotted line the
vertical dispersion parameter.
© 2010 by Taylor and Francis Group, LLC