MIL-HDBK-1011/2

APPENDIX C (continued)

Since airflow rates through interior spaces of buildings

are to be

studied, interior furnishings having significant blockage should be

modeled.

Furnishings having significant blockage include easy chairs, sofas,

bookshelves, desks, beds, cabinets, dressers, bathroom fixtures and

kitchen

fixtures. Items such as lamps, tables, and dining room chairs most

likely

would not have to be modeled.

1.3.2.2

Immediate Surroundings. It is important to model all nearby

buildings and structures, expected foliage, and variations in terrain that

exceed a few feet in height. With low-rise buildings it is typical to model

all such features within a radius of five times the height of the subject

building, and the rough massing of significant building and obstructions

beyond that for a minimum of 500 feet (150 m) for any upwind direction tested.

For buildings above 4 stories, the radius within which detailed modeling is

needed can be reduced. The aerodynamic effects of features beyond this

minimum are modeled by the mean velocity profile selected.

Trees are also modeled with overscaled pores and foliage elements,

usually made of screening or furnace air filter material.

1.3.2.3

Model Size and Wind Tunnel Speed. The minimum model size and

reference wind tunnel speed are governed by a set of minimum Reynolds number

requirements. The Reynolds number is a measure of the ratio of inertial to

viscous forces. Model dimensions and velocities are usually less than

full-scale values, however model viscosity typically equals full-scale

viscosity (if air is the testing fluid). Therefore, relatively speaking,

airflow through models is much more viscous than it is through the full-scale

building. In nearly all full-scale building flows, the flow patterns and

pressure losses are dominated by inertial rather than viscous effects. Air

flow rates in the model of such a building must therefore be sufficiently

great that the flow is dominated by inertial effects. This is guaranteed by

maintaining an appropriate minimum Reynolds numbers for each of the flow

situations in the model.

1.3.2.4

Reynolds Number for Flow Around Bluff Bodies. The Reynolds number

for flow around bluff bodies such as building exteriors, RB, shall be greater

than 20,000.

EQUATION:

RB = LB UB / v

(16)

where

LB is the typical building dimension (m),

UB is the typical approach velocity (m/sec),

v

1.3.2.5

Reynolds Number for Flow Through Window Openings. The Reynolds

number for flow through window openings, RW, shall be greater than 300.

EQUATION:

RW = LW UW / v

(17)

where

LW is the minimum window dimension (m), and

UW is the mean velocity through the window (m/sec).

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