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Profile of wind gusts

Talk about the weather
Gina
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Profile of wind gusts

Postby Gina » Mon Jan 11, 2016 1:50 pm

Don't know if profile is the right word but I've been thinking about anemometers and response to gusts. I would like to know the peak wind force in gusts as that would seem to be important in damage from the wind. From feeling wind force I think the peaks can be quite narrow - but how narrow? How often does the wind strength need measuring to avoid missing gust peaks? My current anamometer has the usual magnetic sensing with magnet on the rotor and reed switch underneath. This closes twice per revolution. But I'm wondering if counting the number of pulses in 1.5s (to give a 1mph accuracy) would be short enough to catch the peaks in gusts.

Any info on this would be much appreciated - thanks in advance :)
Gina

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AllyCat
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Re: Profile of wind gusts

Postby AllyCat » Mon Jan 11, 2016 9:18 pm

Hi Gina,

Welcome back. Personally, I would consider the rotational inertia to be the primary limiting factor of gust detection by a cup anemometer. The cups are probably rather "loosely coupled" to the wind since they move at only about 1/3 of wind speed. I believe I read (somewhere) that the Met Office uses a 3 second "window" for gusts. Also I suspect that short gusts might be just "local" turbulence and not necessarily severely destructive.

Similarly, I doubt if magnetic cogging would have much effect, since it both speeds up and slows down the rotation. Reed switches can be incredibly reliable, but you could consider Hall switches (which contain little magnetic material), however each usually detects only one magnetic polarity, not both.

Slightly related to this is that in the 1990s, Derek Weston developed an (optical) cup anenometer with a "tag" on one cup. From the variation of angular speed around each revolution he could calculate the wind direction without a separate vane! A few years ago his commercial licensing agreement expired and now all the technical details (including his PIC Assembler Source code) are freely available, even for "commercial" use.

But if you really want to detect short gusts, then the "Ultrasonic Time of Flight" method is the way to go and intrinsically also delivers wind direction. AFAIK it's the only anemometer that can be calibrated from theoretical calculations alone, not by side-by-side comparison with another anemometer, or in a Wind Tunnel, etc..

Cheers, Alan.

Gina
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Re: Profile of wind gusts

Postby Gina » Mon Jan 11, 2016 11:44 pm

Thank you Alan - very useful :)
Gina

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laulau
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Re: Profile of wind gusts

Postby laulau » Tue Jan 12, 2016 9:51 pm

A link : http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4279541/from Alan ;) in an other forum (MeteoCercal WeatherDuino Pro2 project)
Laurent

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mcrossley
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Re: Profile of wind gusts

Postby mcrossley » Wed Jan 13, 2016 9:28 am

AllyCat wrote:Similarly, I doubt if magnetic cogging would have much effect, since it both speeds up and slows down the rotation.
The main problem with cogging is the effect on startup speed if you want to record from say 1 mph with small cups. Also, if you use strong rare-earth magnets, beware of the Earths magnetic field, it too produces a noticeable cogging effect - better to use weaker magnets.

AllyCat
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Re: Profile of wind gusts

Postby AllyCat » Wed Jan 13, 2016 11:07 am

Hi,

Ah, I'd forgotten that link, which I think Google just happened to throw up. ;)

Certainly Derek Weston used an optical method (since he needed to measure very precise pulse periods), but he also paid considerable attention to the bearing, which was not an off-the-shelf ball/roller type, but steel and Teflon.

Cheers, Alan.


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