E N G I N E E R I N G C O N S U L T I N G I N F O R M A T I O N S E R V I C E S THE TEN TIMES RULE WHERE DID IT COME FROM? IS IT REALLY VALID? T. Edward Fenstermacher and Mark J. Abrams

A LITTLE HISTORY OF THE 10 TIMES RULE Regulatory Guide 1.23 Rev. 1 states that sensors should be mounted at least 10 times the height of nearby obstructions. References ANSI/ANS ANSI/ANS similar language, but with “where practicable”. Vague reference to Hilfiker “Air Pollution Meteorology Manual August 1973” ANSI/AMS 3.11 references ASTM D , Standard Practice for Characterizing Surface Wind … ASTM D references “On-site Meteorological Program Guidance… EPA 450/ June EPA 450 references “Quality Assurance Handbook …EPA and On-site Meteorological Instrumentation Requirements…Workshop Report EPA 600/ , 1981” No direct reference could be found to work that arrived at the factor of Ten.

Effects of Obstructions on Meteorological Measurements Reduces wind speed downwind of object — Velocity Defect Wind direction may change as a result of wind speed reduction Little change in temperature, unless obstruction is a heat source

Technical References Randerson, Darryl, Atmospheric Science and Power Production, USDOE, 1984, DOE/TIC — Chapter 7, Flow and Diffusion Near Obstacles by R. P. Hosker, Jr., including critical data from: Hosker (1979) Sforza and Mons (1970) Counihan, Hunt and Jackson (1974) Hunt (1974)

Cavity Length One way to estimate the effect of a wake is to estimate the cavity length — At the end of the cavity is the stagnation point where, the velocity defect is –1 — Use a power law to estimate the decrease in the velocity defect past that point

Wind Flow and Cavity Around a Cylinder

Velocity Defect From Hosker 7-4.3, the decay of the far wake follow the power m=–0.95 for two dimensional objects, m=-1 for wide three dimensional objects, and m=–2.6 for square objects (Sforza and Mons)

Two-Dimensional Obstacles Wind Wide in Crosswind Direction

Three-Dimensional Obstacles Wind Narrow in Crosswind Direction

Velocity Defect in Power-Law Wind Fields Vertical profiles scales proportional to z, inversely proportional to x 1/(n+2) For two-dimensional obstacles, size of velocity defect scales as x -1 For three-dimensional obstacles, size of velocity defect scales as x -(3+n)/(2+n) In the experiments, n=.125 for two- dimensional obstacles, n=.15 for three- dimensional obstacles

Effect on Measured Wind Speed at 10 m The analytical models for the velocity defect were used to determine the magnitude of the expected measurement error at 10 m as a function of — Obstacle size — Downwind distance Obstacle heights from 1 m to 10 m shown

Effect of a Two-Dimensional Obstacle on Measured Wind Speed at 10 m

Effect of a Three-Dimensional Obstacle on Measured Wind Speed at 10 m

The Ten Times Rule The size of the obstacle was varied with the downwind distance so that x=10H, the limit of the Ten Times Rule The effects of two-dimensional and three dimensional obstructions are shown

The Ten Times Rule with a Two-Dimensional Obstruction

The Ten Times Rule with a Three- Dimensional Obstruction

Sample Problem Velocity Defect 600 m downwind of a cooling tower 160 m high and 100 m in diameter

Sample Problem Using Cavity Length Method Using cavity length method — A=2.328 — B= — x c =256.1 m — X of met. tower = 600m — |  U/U|=10.93%

Sample Problem Using Velocity Defect Profile Method Using Velocity Defect Profile — z sc = — |  U/U|=3.71% Velocity Defect Profile method gives more realistic, less conservative results

Conclusions Two-dimensional obstructions that are very wide compared to their height are expected to have much larger impact than three- dimensional objects of similar height The Velocity Defect Profile method is less conservative than the Wake Cavity method for estimating the impact on meteorological instrument, but either method should be used with caution.

Conclusions For three-dimensional obstructions, the Ten Times Rule should restrict the errors in measured wind speed to less than 10%. All in all the Ten Times Rule seems to be a good one.