MIL-HDBK-1011/2
nighttime downslope winds), and facing south to southeast for decreased
exposure to afternoon sun. In cooler temperate climates, sites in the middle
to upper part of the slope facing south are recommended for access to sun and
wind.
4.2.2.2
Obstructions. Obstructions include elements such as buildings,
fences, trees and other landscaping. They affect both the wind and sun
impinging on the building. Important wind effects of obstructions include
airflow at: flows on the windward face, corner flows, and wakes. Para. 4.3
and Section 3 of Appendix A discuss airflow around simple buildings and
windbreaks. Figures 7 and 8 show wake effects of complex buildings shapes.
To maximize ventilation, buildings should not be sited within the
wake of any obstruction and should be placed sufficiently far apart that each
acts in isolation. To achieve this, a clear spacing of at least 5H (five
times the height of the upwind building) is required. If the spacing is
closer, the downwind building is placed within the wake of the upwind building
resulting in lowered local air velocities and the possible establishment of a
vortex or roller of trapped air. Such rollers are stable at clear spacings of
less than 1.5H (one and one-half times the height of the upwind building) and
ventilation through the downwind building can be quite weak. For spacings
between 1.5 and 5H, the airflow oscillates between the two patterns shown in
Figure 8 and ventilation in the downwind building(s) will be sporadic and much
less effective than if properly spaced.
4.2.2.3
Pollution Sources. Because it is too difficult to filter
pollutants from the air entering naturally ventilated buildings, the
building(s) should be upwind of pollution sources. When this is not possible,
it is desirable to position them as far as possible from upwind pollution
sources, such as kitchen exhausts or major roads, so that the pollution has
space to disperse in the atmosphere before reaching the building.
Placing a New Building in a Developed Area. In positioning more
4.2.2.4
than one building, or a new building in an already developed area, provision
for air movement must be one of the most important considerations. New
buildings are not only affected by the existing buildings around them but they
can also affect the ventilation in the existing buildings and the air movement
in surrounding open spaces. Buildings and open spaces can be organized to
preserve each building's access to prevailing breezes. For the same density,
high buildings surrounded by large open spaces have better ventilation than
more closely spaced low-rise buildings.
The important influences on urban winds are:
a) dimensions of obstructions,
b) spacing between obstructions,
c) homogeneity or variability of building height,
d) orientation of streets with regard to prevailing winds, and
e) distribution, size, density, and details of planted and open
areas.
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