APPENDIX C (continued)
location or at any elevation is appropriate as long as it is well defined.
Mean interior airflow velocities are measured throughout the interior spaces
to be naturally ventilated. If the entire three dimensional flow field is to
be determined in the interior spaces, then a sufficient number of point
measurements must be recorded to define that flow field.
Often, however, air exchange rates are desired. To measure an air
exchange rate, only the airflow rates into, or out of, a confined space need
be measured. When the inlet, or outlet, consists of a single opening, the
airflow rate may be determined from a single measurement, and is equal to:
Q = UWA C
UW is the mean air velocity at the opening center (m/sec),
is the area of the opening (m2), and
is a coefficient, 0.8 to 0.9, determined experimentally or
theoretically for a set of similar openings to account for the
nonuniform mean velocity distribution over the opening
For either case, the interior wind speed velocities, U+i,, are given in terms
of the dimensionless velocity ratio:
Ci = Vi /Uref
Uref is the reference velocity.
Procedure 2--Indirect Velocity Measurement. The second procedure
applies when a small scale model is to be used in conjunction with a large
scale model of the interior spaces. For each critical wind direction a
reference wind tunnel mean free stream velocity is measured on the small-scale
solid model. In addition, a mean pressure differential from the inlet to
outlet location is measured for each interior space under consideration.
The large-scale models of the interior spaces are then used to
determine interior airflow rates for a given pressure differential across the
large-scale model. The large-scale model is placed within the wind tunnel,
the openings are blocked, and the mean pressure differential is measured
across the model from the assumed inlet and outlet. The openings are then
opened, and the airflow rates, and/or three dimensional flow filed are
measured as before. Interior air speeds for the small-scale model, U+ns,, are
then computed in the following manner.
Across each interior space on the small scale model the total
pressure differential is given by:
delta Pns = (1/2 P Uns2)Cpn