effectiveness in winds with a 45-degree incidence angle (see Figure 32).
Theoptimal shape has been found to be eight times as wide as tall, however
smaller width-to-height ratios are also effective.
Square and vertical shapes exhibit peak performance in
perpendicular winds. If the wind incidence angle is confined to a narrow band
and openings can be placed perpendicular to the wind, then square openings
will also work effectively. However, if the wind incidence angle varies, then
horizontal openings will work more effectively under a greater variety of
conditions and should be used. Tall openings exhibit a lower effectiveness
than both horizontal and square shapes for all wind incidences.
Size. The effect of window size depends on whether or not openings
are cross ventilating. If openings are on one surface only, size has little
affect on airflow. In cross ventilated rooms, airflow is determined mainly by
the area of the smallest openings; average indoor velocity and number of air
changes is highest when inlet area is equal to or slightly less than outlet
area as in Equation 1.
outlet area/inlet area = 1.25
Ventilation is more efficient for a greater number of incidence
angles when inlets larger than the outlets. If concentrated flow in a
restricted area of the room is desired, the inlets may be sized smaller than
the outlets and placed immediately adjacent to the living space to be
ventilated (see Figure 33). In general, use the largest area of openings
possible with inlet area equal to or slightly less than outlet area. To
determine the size of windows necessary to obtain a given air change rate,
refer to Appendix C, Section 1.