## [a-w-h] Re: theoretical anem. placement

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• ... Here is how I would do it, for what that is worth... Place another anemometer well away from the rotor, maybe at 2/3 of the tower height. It does not
Message 1 of 7 , Mar 1, 2000
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>I need to think a little more about what to measure
>(and where) with the anemometers to do a proper
>comparison now.

Here is how I would do it, for what that is worth...

Place another anemometer well away from the rotor, maybe at 2/3 of the
tower height. It does not matter whether this second anemometer is
calibrated accurately although it would be nice.

Take some ten minute averages of the windspeed from the 2 anemometers and
look for a relationship between them. I would hope to find that the upper
one is a certain constant factor higher than the lower one. This factor
will probably depend on where the wind is coming from.

Do this test with and without the wind turbine running, and compare the
results. If the factor is the same either way, then you can conclude that
difference between the results then you will get an idea of what the rotor
is doing to the windspeed at the anemometer.

Hugh

At Wind and Sun Ltd
phone 07713157600
• Hugh, I appreciate the thoughts. My initial concerns are that the 2nd anemometer must be calibrated. There is nothing to gain by being +/- 1-2 mph on a
Message 2 of 7 , Mar 1, 2000
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Hugh,

I appreciate the thoughts. My initial concerns are
that the 2nd anemometer must be calibrated. There is
nothing to gain by being +/- 1-2 mph on a reference
anemometer, so I need to calibrate it. That's the
first step. I am worried that if I go down to 2/3 of
tower height, that I will get significant turbulence.
I don't think I can go down that low - and still get
good measurements. Again, those are my initial thoughts.
Perhaps after more thinking, I may wind up using more

If the exit velocity of wind from the turbine is on the
order of 1/3, then a 21 mph wind would yield a 7 mph
wind in the wake. I'm definitely not seeing that. Is my
anemometer being affected? It is possible, though I
haven't seen any conclusive evidence yet. If I had a
wind vane, it would be easier to figure out if I'm
being affected.

Mike

hugh piggott wrote:
>
> >I need to think a little more about what to measure
> >(and where) with the anemometers to do a proper
> >comparison now.
>
> Here is how I would do it, for what that is worth...
>
> Place another anemometer well away from the rotor, maybe at 2/3 of the
> tower height. It does not matter whether this second anemometer is
> calibrated accurately although it would be nice.
>
> Take some ten minute averages of the windspeed from the 2 anemometers and
> look for a relationship between them. I would hope to find that the upper
> one is a certain constant factor higher than the lower one. This factor
> will probably depend on where the wind is coming from.
>
> Do this test with and without the wind turbine running, and compare the
> results. If the factor is the same either way, then you can conclude that
> your windspeed reading is fairly accurate. If there is a significant
> difference between the results then you will get an idea of what the rotor
> is doing to the windspeed at the anemometer.
>
> Hugh
>
> At Wind and Sun Ltd
> phone 07713157600
• On Wednesday, March 01, 2000 8:01 AM, Michael Klemen [SMTP:windy@bektel.com] ... If the wind wake is swirling in the anemometer plane, would that add wind
Message 3 of 7 , Mar 1, 2000
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On Wednesday, March 01, 2000 8:01 AM, Michael Klemen [SMTP:windy@...]
wrote:
> If the exit velocity of wind from the turbine is on the
> order of 1/3, then a 21 mph wind would yield a 7 mph
> wind in the wake. I'm definitely not seeing that. Is my
> anemometer being affected?

If the wind wake is swirling in the anemometer plane,
would that 'add' wind velocity for the anemometer?
Is the theoretical 7mph wake speed measured as
time for the swirling wind to travel from the blade area
to some point in the wake? Analogy: a tornado moves
along at, say 50 mph, but the swirling wind is in excess
of 100mph.

-Chris
• ... There is no need to calibrate an anemometer if you are simply using it to compare 2 situations. The calibration constant will come out in the wash. ... now
Message 4 of 7 , Mar 1, 2000
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> My initial concerns are
>that the 2nd anemometer must be calibrated.

There is no need to calibrate an anemometer if you are simply using it to
compare 2 situations.
The calibration constant will come out in the wash.

>If the exit velocity of wind from the turbine is on the
>order of 1/3, then a 21 mph wind would yield a 7 mph
>wind in the wake. I'm definitely not seeing that.

now you have lost me. your anemometer is not in the wake is it? I thought
it was below the wind turbine.

Hugh

In Edinburgh on my way home from Wales
• Well, Hugh, ... Yeah, but if we find out that there s a 1/2 mph difference between two heights with an uncalibrated instrument, the first question I can hear
Message 5 of 7 , Mar 1, 2000
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Well, Hugh,

> > My initial concerns are
> >that the 2nd anemometer must be calibrated.
>
> There is no need to calibrate an anemometer if you are
> simply using it to compare 2 situations.
> The calibration constant will come out in the wash.

Yeah, but if we find out that there's a 1/2 mph difference
between two heights with an uncalibrated instrument, the
first question I can hear you (or me) asking is ... how does
it compare to ...? I can't do that without the calibration!

For example, how are we going to know if the 2nd anemometer
is installed in turbulent air if we don't know the wind
speed we're recording? I think we're going to want a good
solid reference point to answer those questions as they
come up. I'm not too fond of re-crunching data to get that
info if I can have it up-front, because every question that
is asked will probably need the data re-crunched. There's
no point doing that work after all of the data is acquired.
It takes too much time to "fix" it all after the fact.

> >If the exit velocity of wind from the turbine is on the
> >order of 1/3, then a 21 mph wind would yield a 7 mph
> >wind in the wake. I'm definitely not seeing that.
>
> now you have lost me. your anemometer is not in the
> wake is it? I thought it was below the wind turbine.

Well, if the calculations are right, the air behind the
turbine expands to a 14 foot rotor diameter, as you stated
previously. I am probably 2-3 feet below the blade tip, but
don't know for sure. Anything less than 2 feet 1 inch
would be inside that area. I would call that the wake, even
though it is physically below the rotor. When the anemometer
is downwind of the rotor, what else would you call that?
If my anemometer is being affected by the rotor and is
recording lower than actual wind speeds, I would consider
that in the wake, no matter if it is above or below the
turbine. Sorry for the confusion. I hope that cleared it up.
If not, let's try again. ;)

Mike
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