Why should I use the new 95 inch mounting boom?

[johng]

Hello,

What are good applications for the new 95" boom? 

I
was thinking about using it on a standard 60-m tower instead of the 60"
boom.  Looking at a tower shadow sketch, I see that getting a sensor
only four tower diameters away from the tower would give a reading of
at worst +/- 2% of free stream.  For a 10" tube, that's only 40", and
the standard boom is 60". 

So even if I were trying to get
very clean readings at 10m AGL (on the 10" part of the tower), the 60"
boom would be fine for every circumstance except for when the sensor is
directly downwind (when you're screwed no matter what). 

Best regards,

Alex DePillis

 

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The short answer is that the new 95 inch boom can be used in applications
which require lower measurement uncertainty than the 60 inch boom. 

As for the long answer... 

I believe you might be looking at Figure G.5 in IEC 61400-12-1 which is
based on 2 dimensional Navier-Stokes computations.

If so, the figures you indicate are about right (4 tower diameters equates
to about a 2.3% centerline disturbance). 
Another consideration is that the IEC model measures the offset distance
as tower centerline to sensor centerline, which really places the 60 inch boom
at about 6.5 diameters (for a 10 inch tower) equating to a centerline
disturbance of about 1%.

However, here is another way to look at it...what level of disturbance is
acceptable for your wind resource assessment campaign?

Many in the industry would say "let?s make it 0.00%!"...

Basically, the further the sensor is away from the tower, the lower the
disturbance.  In reality, the law of
diminishing returns comes into play and no matter how long a boom is
(realistically) the disturbance will not (in theory) be 0.00%.  At about
10 diameters, the disturbance curve flattens out and approaches
horizontal.  This is really the sweet spot of boom design as the
centerline disturbance is less than 0.5% (some might say it is about 0.3% but I
prefer to be a bit conservative).  I have
also heard that some in the industry believe disturbances are actually greater
than what the Navier-Stokes model predicts. 
If anyone has seen research papers in this area, please add links to
this thread!

Of course, other design factors are involved such as robustness, cost, ease
of installation and transport (the new booms are cost effective, easy to
install and are also UPS shippable).

In summary, NRG will continue to offer both the 60 inch boom and the 95 inch
boom.  The new 95 inch boom can be used in applications
which require lower measurement uncertainty than the 60 inch boom.

 

 

[Matthew]

Here is study on the matter. Effects of Tower Shadowing on Anemometer Data, by William David Lubitz

http://www.iawe.org/Proceedings/11ACWE/11ACWE-Lubitz.pdf, archived at http://www.webcitation.org/5xw8H8vUN

 

From the conclusions section:

"The results of the two case studies examined support the recommendation by prior investigators that mounting anemometers on booms greater than 7 tower diameters should limit tower flow distortion effects to a lower magnitude than the sensor uncertainty of the anemometer itself."