3.2.1 Wind Speed and Wind Direction
3.2.1.1 Probe placement
The
standard exposure height of wind instruments over level, open terrain is 10
m above the ground [9]. Open
terrain is defined as an area where the distance between the instrument and
any obstruction is at least ten times the height of that obstruction [2,
4, 9]. The slope of the terrain in the vicinity of the site should be
taken into account when determining the relative height of the obstruction [2].
An obstruction may be man-made (such as a building or stack) or natural
(such as a hill or a tree). The sensor height, its height above
obstructions, and the height/character of nearby obstructions should be
documented. Where such an exposure cannot be obtained, the anemometer should
be installed at such a height that it is reasonably unaffected by local
obstructions and represents the approximate wind values that would occur at
10 m in the absence of the obstructions. This height, which depends on the
extent, height, and distance of obstructions and on site availability,
should be determined on a case-by-case basis. Additional guidance on the
evaluation of vertical profiles (Section 6.1.3) and surface roughness
(Section 6.4.2) may be helpful in determining the appropriate height.
If
the source emission point is substantially above 10 m, then additional wind
measurements should be made at stack top or 100 m, whichever is lower [1].
In cases with stack heights of 200 m or above, the appropriate measurement
height should be determined by the Regional Office on a case-by-case basis.
Because maximum practical tower heights are on the order of 100 m, wind data
at heights greater than 100 m will most likely be determined by some other
means. Elevated wind measurements can be obtained via remote sensing (see
Section 9.0). Indirect values can be estimated by using a logarithmic
wind-speed profile relationship. For this purpose, instruments should be
located at multiple heights (at least three) so that site-specific wind
profiles can be developed.
3.2.1.2 Obstructions
Buildings.
Aerodynamic effects due to buildings and other major structures, such as
cooling towers, should be avoided to the extent possible in the siting of
wind sensors; such effects are significant, not only in the vicinity of the
structures themselves, but at considerable distances downwind. Procedures
for assessing aerodynamic effects have been developed from observing such
effects in wind tunnels [13], [14]. Wind sensors should only
be located on building rooftops as a last resort; in such cases, the sensors
should be located at a sufficient height above the rooftop to avoid the
aerodynamic wake. This height can be determined from on-site measurements
(e.g., smoke releases) or wind tunnel studies. As a rule of thumb, he total
depth of the building wake is estimated to be approximately 2.5 times the
height of the building [1]
Trees. In addition to the general rules
concerning obstructions noted above, additional considerations
may be important for vegetative features (e.g., growth rates). Seasonal
effects should also be considered for sites near deciduous trees. For dense,
continuous forests where an open exposure cannot be obtained, measurements
should be taken at 10m above the height of the general vegetative canopy.
Towers. Sensors mounted on towers are
frequently used to collect wind speed measurements
at more than one height. To avoid the influence of the structure itself,
closed towers, stacks, cooling towers, and similar solid structures should
not be used to support wind instruments. Open-lattice towers are preferred.
Towers should be located at or close to plant elevation in an open area
representative of the area of interest.
Wind
instruments should be mounted on booms at a distance of at least twice the
diameter/diagonal of the tower (from the nearest point on the tower) into
the prevailing wind direction or wind direction of interest [2].
Where the wind distribution is strongly bimodal from opposite directions,
such as in the case of up-valley and down-valley flows, then the booms
should be at right angles to the predominant wind directions. The booms must
be strong enough so that they will not sway or vibrate sufficiently to
influence standard deviation values in strong winds. Folding or collapsible
towers are not recommended since they may not provide sufficient support to
prevent such vibrations, and also may not be rigid enough to ensure proper
instrument orientation. The wind sensors should be located at heights of
minimum tower density (i.e., minimum number of diagonal cross-members) and
above/below horizontal cross-members [2]. Since practical
considerations may limit the maximum boom length, wind sensors on large
towers (e.g., TV towers and fire look-out towers) may only provide accurate
measurements over a certain arc. In such cases, two systems on opposite
sides of the tower may be needed to provide accurate measurements over the
entire 360°. If such a dual system is used, the method of switching from
one system to the other should be carefully specified. A wind instrument
mounted on top of a tower should be mounted at least one tower
diameter/diagonal above the top of the tower structure.
Surface roughness.
The surface roughness over
a given area reflects man-made and natural
obstructions, and general surface features. These roughness elements effect
the horizontal and vertical wind
patterns. Differences in the surface roughness over the area of interest can
create differences in the wind pattern that may necessitate additional
measurement sites. A method of estimating surface roughness length, zo , is
presented in Section 6.4.2. If an area has a surface roughness length
greater than 0.5 m, then there may be a need for special siting
considerations (see discussion in Sections 3.3 and 3.5).
3.2.1.3 Siting considerations
A
single well-located measurement site can be used to provide representative
wind measurements for non-coastal, flat terrain, rural situations. Wind
instruments should be placed taking into account the purpose of the
measurements. The instruments should be located over level, open terrain at
a height of 10 m above the ground, and at a distance of at least ten times
the height of any nearby obstruction. For elevated releases,
additional measurements should be made at stack top or 100 m, whichever is
lower [1]. In cases with stack heights of 200 m or above, the
appropriate measurement height should be determined by the Regional Office
on a case-by-case basis.
3. SITING AND EXPOSURE
3.1 Representativeness
3.1.1 Objectives for Siting
3.1.2 Factors to Consider
3.2 Simple Terrain Locations
3.2.1 Speed and Wind Direction
3.2.2 Temperature, Temperature
Difference, and Humidity
3.2.3 Precipitation
3.2.4 Pressure
3.2.5 Radiation
3.3 Complex Terrain Locations
3.3.1 Wind Speed
3.3.2 Wind Direction
3.3.3 Temperature Difference
3.4 Coastal Locations
3.5 Urban Locations
3.6 Recommendations