APPENDIX C (continued)
Since airflow rates through interior spaces of buildings
are to be
studied, interior furnishings having significant blockage should be
Furnishings having significant blockage include easy chairs, sofas,
bookshelves, desks, beds, cabinets, dressers, bathroom fixtures and
fixtures. Items such as lamps, tables, and dining room chairs most
would not have to be modeled.
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.
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.
Reynolds Number for Flow Around Bluff Bodies. The Reynolds number
for flow around bluff bodies such as building exteriors, RB, shall be greater
RB = LB UB / v
LB is the typical building dimension (m),
UB is the typical approach velocity (m/sec),
Reynolds Number for Flow Through Window Openings. The Reynolds
number for flow through window openings, RW, shall be greater than 300.
RW = LW UW / v
LW is the minimum window dimension (m), and
UW is the mean velocity through the window (m/sec).