the buildings and air flow is largely above the
canopy envelope (average height H C ); the ratio
U c /U H is much less than s w /U H .
Within the canopy the flow depends on the
particular street and building configuration at
that scale. There are mean flows along streets at
an angle to the mean flows above the buildings,
which results in extra lateral diffusion.
The flow above the canopy is essentially equivalent to airflow over a wide hill with length L N and
height H C , but with a significant value of the
roughness length z 0 that is of the order of the
thickness of the shear layer over the buildings
and the “canyons” between them [23]. The flows
in the inner layer above the buildings and in the
wake downwind of the neighborhood region are
similar to those for the porous case.
Building and Street Scale
On the building/street scale there are also characteristic features of the flow corresponding to different categories of building street shape and
configuration. Various planning criteria and concepts have been proposed for defining these categories, for example, rugosity (or mean canopy
height H C ), relative rugosity (defined by building
height variability or canopy height variability H
0
C ),
sinuosity (of canyons), the Sky View Factor
(SVF), which signifies the fraction of sky dome
visible from a specific outdoor position and is
important for estimating the amount of incoming
solar radiation for energy calculation, and the
“compactness index,” which is defined as the
ratio of building surface area (excluding the plan
area) to the surface area of a cube that has the same
volume as the building. For microscale phenomena the urban morphometry is the more important,
but at the mesoscale both the geometry and surface thermal characteristics play an equal role
[24]. These, and related concepts, have been
used to guide the suggested categorization
shown in Table 3 for building/street configurations and the consequences for air flow features
illustrated in Fig. 3.
Fluid mechanical studies have shown how
flows around individual buildings become significantly distorted in the presence of other
buildings depending on the ratio b/d of the
breadth b to separation distance d from the
nearest building, on the ratio b/w of breadth to
the width w of the building, and on the relative
height to width ratio H/w. When there are many
buildings, as in urban areas, these flow interactions build up into characteristic flow patterns;
this is now examined.
Separated Buildings b/w, w/b < 2. The separation distances d are large enough that b/d 1/3
((vi) of Table 3 and Fig. 3a). The flow around each
building has approximately the same form as that
of an isolated building, with recirculating flow
regions, turbulent wakes, and horseshoe vortex
structures around the base of the building. In
slightly stable conditions the vortex structures
can persist far downwind but in most urban
cases where the flow is neutral and highly turbulent they are not significant. On the other hand the
turbulent wakes from upwind buildings do impact
on those downwind, enhancing the mixing. Isolated buildings, especially in slightly stable conditions can produce swirling wakes far downwind
[25]. When there are marked variations in the
height of adjacent buildings, the wake vorticity
shed from upwind buildings can produce sharp
down-flows and increased trailing vorticity in the
flow direction as discussed by Lawson [26]. These
effects contribute to mixing between the canopy
and external flow.
For tall buildings that are not closely packed
and where H/b > 1 (Fig. 3b), there is strong
mixing in the horizontal plane and to a lesser
extent in the vertical direction because of the
high turbulence generated. Buildings placed sufficiently close to each other may result in wake
interaction, which tends to cause downwash [27]
and strong swirl around the sides of tall buildings.
This enhances vertical mixing in the lower part
of the downwind wakes and through lateral
convergence reduces their downwind extent.
When b/d < 1/3, corresponding to long buildings
that are sufficiently separated from each other, the
flow over an upwind building can descend into the
space between the buildings. This flow is significantly sheltered and stagnation areas are larger
than for isolated buildings.
Close Packed Buildings not Aligned. In some
city centers and some types of industrial plant, the
170
Urban Air Quality: Meteorological Processes
canopy envelope (average height H C ); the ratio
U c /U H is much less than s w /U H .
Within the canopy the flow depends on the
particular street and building configuration at
that scale. There are mean flows along streets at
an angle to the mean flows above the buildings,
which results in extra lateral diffusion.
The flow above the canopy is essentially equivalent to airflow over a wide hill with length L N and
height H C , but with a significant value of the
roughness length z 0 that is of the order of the
thickness of the shear layer over the buildings
and the “canyons” between them [23]. The flows
in the inner layer above the buildings and in the
wake downwind of the neighborhood region are
similar to those for the porous case.
Building and Street Scale
On the building/street scale there are also characteristic features of the flow corresponding to different categories of building street shape and
configuration. Various planning criteria and concepts have been proposed for defining these categories, for example, rugosity (or mean canopy
height H C ), relative rugosity (defined by building
height variability or canopy height variability H
0
C ),
sinuosity (of canyons), the Sky View Factor
(SVF), which signifies the fraction of sky dome
visible from a specific outdoor position and is
important for estimating the amount of incoming
solar radiation for energy calculation, and the
“compactness index,” which is defined as the
ratio of building surface area (excluding the plan
area) to the surface area of a cube that has the same
volume as the building. For microscale phenomena the urban morphometry is the more important,
but at the mesoscale both the geometry and surface thermal characteristics play an equal role
[24]. These, and related concepts, have been
used to guide the suggested categorization
shown in Table 3 for building/street configurations and the consequences for air flow features
illustrated in Fig. 3.
Fluid mechanical studies have shown how
flows around individual buildings become significantly distorted in the presence of other
buildings depending on the ratio b/d of the
breadth b to separation distance d from the
nearest building, on the ratio b/w of breadth to
the width w of the building, and on the relative
height to width ratio H/w. When there are many
buildings, as in urban areas, these flow interactions build up into characteristic flow patterns;
this is now examined.
Separated Buildings b/w, w/b < 2. The separation distances d are large enough that b/d 1/3
((vi) of Table 3 and Fig. 3a). The flow around each
building has approximately the same form as that
of an isolated building, with recirculating flow
regions, turbulent wakes, and horseshoe vortex
structures around the base of the building. In
slightly stable conditions the vortex structures
can persist far downwind but in most urban
cases where the flow is neutral and highly turbulent they are not significant. On the other hand the
turbulent wakes from upwind buildings do impact
on those downwind, enhancing the mixing. Isolated buildings, especially in slightly stable conditions can produce swirling wakes far downwind
[25]. When there are marked variations in the
height of adjacent buildings, the wake vorticity
shed from upwind buildings can produce sharp
down-flows and increased trailing vorticity in the
flow direction as discussed by Lawson [26]. These
effects contribute to mixing between the canopy
and external flow.
For tall buildings that are not closely packed
and where H/b > 1 (Fig. 3b), there is strong
mixing in the horizontal plane and to a lesser
extent in the vertical direction because of the
high turbulence generated. Buildings placed sufficiently close to each other may result in wake
interaction, which tends to cause downwash [27]
and strong swirl around the sides of tall buildings.
This enhances vertical mixing in the lower part
of the downwind wakes and through lateral
convergence reduces their downwind extent.
When b/d < 1/3, corresponding to long buildings
that are sufficiently separated from each other, the
flow over an upwind building can descend into the
space between the buildings. This flow is significantly sheltered and stagnation areas are larger
than for isolated buildings.
Close Packed Buildings not Aligned. In some
city centers and some types of industrial plant, the
170
Urban Air Quality: Meteorological Processes