APPENDIX C (continued)
single openings, and average height difference
between bottom of the inlets and top of the outlets
for rooms with multiple openings, ft (m)
average indoor air temperature, degrees F (degrees C)
temperature of outdoor air, degrees F (degrees C)
For further discussion, see DM-3.03.
Combining Terms. As a rough rule of thumb, when flow due to the
stack effect and flow due to winds are equal, the actual combined flow is 30
percent greater than the flow caused by either force alone.
Wind Tunnel Testing. Wind tunnels are used to determine the
airflow rates through interior spaces of buildings for each relevant wind
direction. The airflow rates are expressed as ratios of interior velocity
over a "reference velocity" obtained from historical climatological records.
When combined with the climate's probability distributions of wind speed, wind
direction, temperature, and humidity, the acceptability of natural ventilation
can be determined. This is discussed in the Climate Analysis Method, Appendix
B. In certain cases, the wind tunnel will be used to produce mean pressure
distributions, as functions of a reference wind speed and direction. For such
cases, the mean airflow rates through interior spaces of the building are
computed analytically rather than being obtained experimentally.
Presented in this section are the minimum requirements for wind
ensure the acceptability of the obtained airflow rates or pressures.
Wind Tunnel Test Facilities. Because the objectives of wind tunnel
testing for natural ventilation studies are mean airflow rates or mean
pressure distributions, the turbulence characteristics of the atmospheric
boundary layer need not be fully modeled. The principal requirement is that
the mean velocity profile, expected at the building site, be modeled
accurately in the wind tunnel. An appropriate set of target mean velocity
profiles are given by the logarithmic law (Equation 15):
U(z) = 2.5 u * ln(z/zo)
U(z) = mean velocity at elevation z above grade, ft(m/sec),
= the shear velocity, mph (m/sec)
zo = the roughness length, a measure of surface roughness,
Appropriate values of roughness lengths for various terrain categories are
given in Table C-6.
Variations from Theoretical Mean. If experimentally obtained mean
velocities from the theoretical target mean velocity profile is less than
0.10, then the mean velocity profile is assumed to be modeled acceptably. A
presentation of the experimentally obtained mean velocity profile (or
profiles) must be included in the documentation of the wind tunnel testing.