APPENDIX A (continued)
Wake Geometry. The geometry of the wake is important because it
defines the limits of significant air movement. Outside the wake, the air
movement is similar to the free-stream, but the area within the wake may be
considered as a cavity of relatively still air, where the pressure differences
needed for building ventilation are unlikely to occur.
Airflow Around Multiple Buildings. Airflow around groups of
buildings or other obstructions is very complex. The following are a few of
the general airflow patterns that are commonly found to produce strong winds.
These patterns may be used to benefit the ventilation of buildings in their
path, but the designer should be aware that they might also adversely affect
the comfort of pedestrians outside the buildings, or in semi-enclosed lobbies,
corridors, or balconies.
Downwash at the Foot of a Tall Building. Some of the strongest
winds around buildings are found at the windward side and edges of tall
buildings protruding above the surrounding general level of development. This
effect occurs because winds aloft are stronger than at ground level, causing
higher pressures at the top of the building's windward face than at its base.
This pressure difference creates a strong downward flow on the windward face
(see Figure A-10).
Corner Effect. Strong winds occur at building corners as the
airflows from the high pressure zone on a building's windward side to the low
pressure zone on the leeward side (see Figure A-11). Accelerated wind is
generally restricted to an area with radius no longer than the building's
width. The taller and wider the building, the more intense the effect. If
two towers of 30 stories or more are placed less than two building widths
apart, an acceleration will fill the entire space between them.