MIL-HDBK-1011/2

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)

ti

=

average indoor air temperature, degrees F (degrees C)

to

=

temperature of outdoor air, degrees F (degrees C)

For further discussion, see DM-3.03.

1.2.4

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.

1.3

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

tunnel facilities, instrumentation, and wind tunnel testing procedures to

ensure the acceptability of the obtained airflow rates or pressures.

1.3.1

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):

EQUATION:

U(z) = 2.5 u * ln(z/zo)

(14)

where

U(z) = mean velocity at elevation z above grade, ft(m/sec),

u

= the shear velocity, mph (m/sec)

zo = the roughness length, a measure of surface roughness,

ft (m).

Appropriate values of roughness lengths for various terrain categories are

given in Table C-6.

Variations from Theoretical Mean. If experimentally obtained mean

1.3.1.1

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.

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