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 Post subject: Windgradient graphs
PostPosted: Tue Jan 21, 2003 1:50 pm 
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Location: Denmark
I made some graphs of the windgradient - that is how much the wind speed changes when going higher above the surface.

These are the standard wind gradients, meaning how they are if the weather is "normal", without wind shear or layer/temp. effects of any kind.
I've taken into account that the water surface gets more rough, when the wind picks up.

It is very evident, that a hand held anemometer can be very erroneous, because the wind speed changes so rapidly in "hand" height, especially in high winds.
And there is always some kind of shore effect, where the wind will be slowed down when reaching the shore (where you most likely are measuring).
If the anemometer is placed on a flagpole in 7-8 meters height, the readings will often be quite accurate, regardless of ground level errors.
The shore effect still exist - but is often very small in this height.

This is common knowledge to most of you, I know - but there are many who will find it useful.

Here is the graph, which is very interesting for ALL of us, I think:

Image

If you want it in meter and m/s (European):
http://pfx.dk/1/meterms.gif

If you want it in feet and knots:
http://pfx.dk/1/feetknots.gif

I've measured and checked these wind gradients on many weather stations and wind mill's here in Denmark, and it is often quite accurate - although there can be wind shear that makes it different - but not very often.


Last edited by Peter_Frank on Thu Jan 17, 2013 8:21 pm, edited 3 times in total.

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 Post subject:
PostPosted: Tue Jan 21, 2003 2:51 pm 
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Location: Florida
Hello Peter,

Thank you for the plots. They are quite telling particularly at higher windspeeds. I try to supplement my hand windmeter and wind reports with physical observations of flag and tree displacements, white caps, etc. The additional observations improve the chances of correct kite size selection in lower end winds.

It would be interesting to see the plotted influence of thermal and wind shear effects. I was curious, what algorhthym did you use?

Rick Iossi


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PostPosted: Tue Jan 21, 2003 2:58 pm 
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Location: Argentina
Wow!!
Thanks Peter.
I never realized that there was such a big difference in wind speed going higher!
A 25 knts gust, will make a more than 37 knts gust at the end of the lines!!!
Another good reason to keep your kite low when overpowered...


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PostPosted: Tue Jan 21, 2003 8:48 pm 
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Very good! I often don't trust my wind meter, and often add 5 knots to its reading windspeed if it says it's 15, and I know I can use my X2-12, but if it's hitting 20 then I can't go out!!
It would be interesting to see a graph taking into account air temperature/power produced.. I was in Rarotonga once with the same windmeter, and it was reading 20 knots steady, and I could still put up my 11.8 Airblast and just hang on to it..


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PostPosted: Tue Jan 21, 2003 9:18 pm 
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Very useful. The extent of the variation is surprising.

So the answer is to climb the nearest flag pole. Or maybe just add between say 30-40% to your reading?


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 Post subject:
PostPosted: Tue Jan 21, 2003 10:07 pm 
As always the wind dummy meathod is the only reliable source for accurate real time wind evauation,

Nice job peter,
But I think ma nature laughs at graphs
She's got one hell of a sence of humor
god bless her

fo


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 Post subject:
PostPosted: Tue Jan 21, 2003 10:12 pm 
The model you have applied must be wrong. How can you explain that sand blows at 30 knots. According to your model the wind speed at 0 height is close to zero but if it is true no sand should blow trough the beach at 30knots. The gradient effect you describe exists but not in such an extreme way. I might be wrong this is just what i think


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PostPosted: Tue Jan 21, 2003 10:21 pm 
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According to long standing principles of Fluid Mechanics, Peter has it spot on. In theory the wind velocity AT the boundary is zero. Variable winds likely tear up this nice calm boundary layer and entrain sand particles. Checkout the diagram on page three of:

http://geosun.sjsu.edu/paula/134/pdf/lecture20.pdf

It even talks about wind transport of sand.


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 Post subject:
PostPosted: Tue Jan 21, 2003 10:57 pm 
What i am saying is that this a bit theoritical. Of course the physics laws prove that wind speed at the surface is zero but this does never happens at the beach for sure. And somethink else do you think that the wind ranges provided by the kite companies are measured or corrected to the height of 20 or 30 meters? Of course not, they are just rough values. The most important when measuring the wind is your personal experience. This means you have to watch at the wind for several minutes around you and you will have a better estimation of the wind condition. Is it too gusty or not, are there any low clouds comming that can drammatically change the wind conditions, is it more windy on the sea than on the beach. This chart though can be helpful to understand the effect of using short lines, or some changes at kite power when it is kept low. Dont use it to correct the wind speed and choose your kitesize(in fact dont use windmeters at all is my oponion). An experienced kitesurfer can estimate the wind conditions better than any windmeter. Windmeters should be useful for begginers to help them correlate the wind-speed with the observations that they make on the beach. Advanced and intermidiate riders if they are unsure for the wind they should know that the smaller size should be tested first rather than the big one
(Sorry for my bad English)


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PostPosted: Tue Jan 21, 2003 11:14 pm 
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Very interesting.

It helps explain a few things:

- We can ride in a huge wind range because even when it's only 10 knots at head height, it's really about 13 at the kite (about 21m high with 30m lines angling up at 45 degrees), yet when it's 25 at head height we can fly so low it's really only 25 at the kite.

- We can jump much higher in stronger winds because there's a greater increase in airspeed when we fly the kite up overhead.

- Parked power is greatest with the lines angling up at about 45 degrees (or less?) because above that the wind doesn't increase much, yet the kite is presenting less surface to the wind.

- We can get lofted higher than seems reasonable because even when the wind "only" gusts from 10 to 25 at head height, at zenith it's gusting from 13 to 37, which is OVER EIGHT TIMES the power (since it's proportional to the square of the airspeed).


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