MIL-HDBK-1011/2
APPENDIX C (continued)
in question is a highrise building. For such cases when a definite conflict
exists, two or more models are required. The first is a small-scale solid
model (without openings) of the building in question including all features of
the surrounding area that need to be modeled. The second and additional
models are large-scale models of the interior spaces to be naturally
ventilated. The latter models should be large enough so that all Reynolds
number constraints are satisfied, and should experience approach flows and
pressure differentials similar to those observed on the small scale model.
1.3.3
Instrumentation. The instrumentation must be able to measure mean
air velocities with accuracy (+2 percent is common) over the range of
velocities expected for the wind tunnel being used. Omnidirectional,
temperature-compensated thermistor-type air speed probes are sensitive,
reasonably durable, and relatively inexpensive. Their frequency response is
damped, but is usually adequate for ventilation studies. Probe diameters of
1/4-inch (6 mm) or less are readily available, permitting easy airflow
measurements within interior spaces of the building of the model. (Holes can
be drilled in walls, floors, and ceilings to permit the insertion of the
probe, and can be taped closed when not in use.)
1.3.3.1
Velocity Measurements. For certain studies, when wind directions
are known, Pitot-static tubes may be used to measure mean velocities. The
Pitot-static tube may be attached to a pressure transducer or a manometer.
Air speeds must be relatively high if a Pitot-static tube (or other pressure
variant) is used. For whichever velocity instrumentation is used, the
accuracy of the system over the range of velocities encountered in the study
should be documented.
1.3.3.2
Pressure Measurements. When mean pressure measurements are
required, they may be measured with pressure taps and any of the pressure
transducers typically accepted for the measurement of pressures for the design
of glass and cladding in buildings. Such a transducer has a frequency
response in excess of the needs for natural ventilation studies. Since mean
pressures are desired, lower-cost manometers may be substituted. Extremely
low differential pressures may be measured accurately with an alcohol-filled
manometer read with a measuring microscope.
1.3.4
Wind Tunnel Test Procedures. The ratios of interior airflow to
exterior wind should be determined for each critical wind direction. A
critical wind direction is one that occurs a significant proportion of the
time (over 5 percent of the time during the period that ventilation is
required). Two procedures are suggested to obtain interior airflow ratios.
1.3.4.1
Procedure 1--Direct Velocity Measurement. In the first procedure,
interior airflow velocities are measured directly. This method is applicable
for those cases when the model is sufficiently large so that all pertinent
Reynolds number requirements are satisfied, and the model is sufficiently
small so that all significant nearby features can be modeled within the wind
tunnel test section. A wind tunnel mean free stream reference wind velocity
is obtained for each critical wind direction (usually, weather stations record
wind speeds from eight, and sometimes sixteen, directions).
Reference mean wind velocities traditionally are taken at an
elevation of 33 ft (10 m) above grade. A reference wind velocity at any
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