MIL-HDBK-1011/2
APPENDIX C (continued)
Table C-6
Typical Terrain Categories and Roughness Lengths
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*
*
* Terrain Category
*
Definition
Roughness Length (m)
*
*
*
*
I
Open water
0.005
*
*
*
*
II
Open terrain
0.07
*
*
*
*
III
Suburbs at considerable
0.30
*
*
distance from towns,
*
*
sparse development
*
*
with hedges and trees
*
*
*
*
IV
Towns, densely built-up
1.00
*
*
suburbs, wooded terrain
*
*
*
*
V
Centers of large cities
2.50
*
*
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1.3.1.2
Profile Limitations. It is not necessary to model the entire mean
velocity profile through the atmospheric boundary layer up to the gradient
height (the height above which effects of the earth's surface roughness are no
longer felt), but only the portion two times the height of the building and
its nearby surroundings.
Since the turbulent structure
of the atmospheric boundary layer
need not be modeled accurately, there is
no requirement for a minimum wind
tunnel air speed (or minimum wind tunnel
Reynolds number). There are,
however, minimum wind speed requirements
for airflow rates through models.
Refer to para. 1.3.2.3 of this Appendix.
1.3.2
Wind Tunnel Models. Discussed in this subsection are the minimum
requirements for models used in the wind tunnel tests.
1.3.2.1
Model Detail. The model and full-scale building must be
geometrically similar. All significant detail and relief must also be
modeled. This requires a certain feel for the problem to determine what is
and what is not significant detail. A 1-in. (25.4 mm) deep relief at
full-scale may not have any effect on natural ventilation, but a 4-in. (101.5
mm) deep relief may. On the other hand, in certain pockets where flow is
minimal, a 1-ft (0.3 m) deep relief may not be significant. If a detail is
estimated to have a significant effect upon the pressure loss through a
building (such as an insect screen), it should be included in the model. If a
detail might affect the flow in or out of an opening, it should be included in
the model. If the person conducting the experiment has little feel for what
is and what is not needed, err on the side of excessive detailing. A model
cannot have too much detail, but can have too little.
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