MIL-HDBK-1011/2

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:

EQUATION:

Q = UWA C

(20)

where

UW is the mean air velocity at the opening center (m/sec),

is the area of the opening (m2), and

A

C

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

dimensions.

For either case, the interior wind speed velocities, U+i,, are given in terms

of the dimensionless velocity ratio:

EQUATION:

Ci = Vi /Uref

(21)

where

Uref is the reference velocity.

1.3.4.2

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

EQUATION:

(22)

where

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