There is also lateral air movement which flow along the building sides in a
symmetrical horseshoe-shaped flow. Eventually, these lateral flows along various
buildings merge, with a prevalence of turbulence from the tallest building. If the
buildings have empty spaces at their base (e.g., building raised on pillars or if there
is a walkway) there will be a concentration of streamlines with a crossflow in their
spaces. In this context, instead of providing shelter, taller buildings promote the
deflection of faster moving upper air down to the ground level. These low-level
winds can have velocities about three times higher (Fig. 5.4) than on a flat open
space (Oke 1992).
In clusters of buildings that vary considerably in shape and configuration, air
circulation regimes are complex and asymmetrical, depending on buildings heights,
distances between them, and the characteristics of mean airflow. Low buildings
located upwind of tall buildings affect the configuration of symmetrical
horseshoe-shaped eddies, which tend to wrap around downstream buildings, while
the cavity and wake regions of the taller buildings are only slightly affected (Arya
1988). In this case, taller buildings impose a downwind movement to the mean
flow, generating various types of air displacement (Fig. 5.4). On the contrary, small
buildings are negatively affected by recirculation inherent to the cavity and wake
regions if they are located downwind from taller ones. (Hosker 1984).
For urban hemispherical geometries (Fig. 5.9), horseshoe-shaped eddies dominate the flow, and wakes persist over long distances. Studies in wind tunnels have
shown that the vortex lines approaching the hemispheric obstacle are rotated
towards a longitudinal direction and stretched downwind as they overpass the
obstacle (Hansen and Cermak 1975). As the wakes immerse in a boundary layer
with tangential stresses, their mean velocity increases vertically. Downwind eddies
to the hemisphere move in an oscillating pattern and cannot be measured at a single
point (Fig. 5.9). Downstream the hemisphere obstacle, a huge vertical momentum
transport occurs between the top of the boundary layer and the ground, causing an
increase in longitudinal mean velocity (Rohatgi and Nelson 1994).
Knowledge of the physical and environmental behavior of wind in urban areas
with tall buildings, can be used in protecting against adverse effects of air circulation, with resultant savings in maintenance and improve the well-being of
pedestrians and city dwellers as well as dispersion of pollutants (Oke 1992). The
buildings are programmed to withstand loads due to high-velocity airflow both in
terms of mean flow and turbulence. In designing buildings, it is necessary to
consider characteristics such as resistance loads, compactness, porosity, roof
inclination, pressure distribution, as well as the location of flow separation areas and
suction effects.
Figure. 5.10 illustrates the velocity profile upwind and downwind of a prismatic
building with two symmetrical sloped roofs. The area just above the roof has greater
turbulence, similarly, to flow over the hills, described below.
Winds over urban canopies are also associated with the various forms of precipitation, related moisture regimes, and consequent surface wear. The location and
configurations of arrays of urban buildings also affect the radiation fields in terms of
sunny and shade spaces, thermal load accumulation.
144
5 Flow Over Modified Surfaces
symmetrical horseshoe-shaped flow. Eventually, these lateral flows along various
buildings merge, with a prevalence of turbulence from the tallest building. If the
buildings have empty spaces at their base (e.g., building raised on pillars or if there
is a walkway) there will be a concentration of streamlines with a crossflow in their
spaces. In this context, instead of providing shelter, taller buildings promote the
deflection of faster moving upper air down to the ground level. These low-level
winds can have velocities about three times higher (Fig. 5.4) than on a flat open
space (Oke 1992).
In clusters of buildings that vary considerably in shape and configuration, air
circulation regimes are complex and asymmetrical, depending on buildings heights,
distances between them, and the characteristics of mean airflow. Low buildings
located upwind of tall buildings affect the configuration of symmetrical
horseshoe-shaped eddies, which tend to wrap around downstream buildings, while
the cavity and wake regions of the taller buildings are only slightly affected (Arya
1988). In this case, taller buildings impose a downwind movement to the mean
flow, generating various types of air displacement (Fig. 5.4). On the contrary, small
buildings are negatively affected by recirculation inherent to the cavity and wake
regions if they are located downwind from taller ones. (Hosker 1984).
For urban hemispherical geometries (Fig. 5.9), horseshoe-shaped eddies dominate the flow, and wakes persist over long distances. Studies in wind tunnels have
shown that the vortex lines approaching the hemispheric obstacle are rotated
towards a longitudinal direction and stretched downwind as they overpass the
obstacle (Hansen and Cermak 1975). As the wakes immerse in a boundary layer
with tangential stresses, their mean velocity increases vertically. Downwind eddies
to the hemisphere move in an oscillating pattern and cannot be measured at a single
point (Fig. 5.9). Downstream the hemisphere obstacle, a huge vertical momentum
transport occurs between the top of the boundary layer and the ground, causing an
increase in longitudinal mean velocity (Rohatgi and Nelson 1994).
Knowledge of the physical and environmental behavior of wind in urban areas
with tall buildings, can be used in protecting against adverse effects of air circulation, with resultant savings in maintenance and improve the well-being of
pedestrians and city dwellers as well as dispersion of pollutants (Oke 1992). The
buildings are programmed to withstand loads due to high-velocity airflow both in
terms of mean flow and turbulence. In designing buildings, it is necessary to
consider characteristics such as resistance loads, compactness, porosity, roof
inclination, pressure distribution, as well as the location of flow separation areas and
suction effects.
Figure. 5.10 illustrates the velocity profile upwind and downwind of a prismatic
building with two symmetrical sloped roofs. The area just above the roof has greater
turbulence, similarly, to flow over the hills, described below.
Winds over urban canopies are also associated with the various forms of precipitation, related moisture regimes, and consequent surface wear. The location and
configurations of arrays of urban buildings also affect the radiation fields in terms of
sunny and shade spaces, thermal load accumulation.
144
5 Flow Over Modified Surfaces