APPENDIX C (continued)
Reynolds Number for Flow through Ductways. The Reynolds number for
flow though a long rough duct such as a long hall or corridor, R+D,, shall be
greater than 2,000.
RD = LD UD/ v
LD is the minimum cross-sectional dimension of the duct (m), and
UD is the mean velocity through the duct (m/sec).
Reynolds Number for Flow in a Room.
The Reynolds number for flow
in a room, RR, shall be greater than 20,000.
RR = LR UR/ v
LR is the minimum interior room dimension (m), and
UR is the maximum air speed in the room, usually equal to UW.
Reynolds Number for Flow Through Screens and Louvers. The Reynolds
number of the flow through modeled and geometrically similar screens and
louvers will never meet the minimum criteria given above. Therefore, full
size insect screens are typically used on models; and louvers are typically
modeled to a larger scale than the building so that the minimum louver
separation is 0.15 in (4 mm). In both cases, the opening dimensions are still
small relative to building dimensions, so model/full-scale flow patterns will
still be similar. Pressure loss coefficients through rooms, windows, halls,
doors are assumed to be equal for model and full-scale (if minimum Reynolds
number requirements are met). Use of full-scale insect screens and oversized
louvers ensures that the respective model and full-scale pressure loss
coefficients are equal.
Reynolds Numbers--Reducing Margin of Error. Satisfying the above
minimum Reynolds number requirements does not guarantee Reynolds number
independent results but errors will be minimized. It is good practice to use
models that are as large as possible, limited by the wind tunnel dimensions,
and to use wind velocities as great as possible, limited only by the wind
Model Size Limitations. Model size is limited by the boundary
layer size and wind tunnel size. As mentioned earlier, the boundary layer
need not be modeled to its gradient height, but need only be modeled to a
height that fully engulfs the modeled building in question and all the nearby
buildings. The thickness of the boundary layer should be at least 200 percent
of the highest building modeled. If the models are bulky, they may be further
restricted in size by a minimum wind tunnel blockage requirement. The
projected frontal area of all buildings modeled should never exceed 10 percent
of the wind tunnel cross-sectional area.
The Reynolds number restrictions on model size often conflict with
the model height limit, the blockage requirement, and the requirement that a
significant area around the building be modeled, particularly if the building