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PostPosted: Tue Jan 21, 2003 11:27 pm 
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The other thing that is important to think about when interpreting these types of graphs is that they are for the average wind speed as a function of height. Averaged over some period of time longer then the turbulence time scale. The type of flow they describe is turbulent flow over a rough boundary.

For kiting it is important to remember that the instanteous variation of velocity with height may not look anything like these graphs. Turbulence is often descibed as sweeps of fast air moving down from above, during which the velocity at your 2m wind meter may be close to the velocity higher up , and then ejections of slow parcels of air from close to the boundary up into the higher flow. Under the right condtions it can average to something like those graphs, but your kite pretty much feels the instantaneous wind with a pretty fast reaction time. So if you want to be safe with your wind meter look at the gusts and base it on that. But still relying soley on a wind meter is probably a bad idea and you should use your experience tell you what kite to rig based on many factors..


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PostPosted: Wed Jan 22, 2003 8:14 am 
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As allways Peter has it spot on. I totally agree with what he says and those graphs mean quite a bit too... many a time that I've taken wind readings that have only said 15 knots whereas a wind station has said over 20 knots.... good job Peter....

This is a great Thread!!

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PostPosted: Wed Jan 22, 2003 8:17 am 
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On 2003-01-21 21:18, blizzard wrote:
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?


Another way would be too get a 30m pole and attach the wind meter to the end of it....

Even better would be to attach a wind meter to the leading edge of your kite (preferably a small one if checking for high winds)...

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PostPosted: Wed Jan 22, 2003 8:22 am 
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On 2003-01-21 13:50, Peter_Frank wrote:
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://home20.inet.tele.dk/pf/images/meterms.gif

If you want it in feet and knots:
http://home20.inet.tele.dk/pf/images/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.



Anyone know what type of curve that is?? Logrithmic, Exponential, Parabolic, Hyperbolic etc......

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PostPosted: Wed Jan 22, 2003 9:07 am 
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looks hyperbolic because of the horizontal flat bits near the origin

would need to have a look at the exponents in the equation i guess

but hey i'm a computer tech instead of a civil engineer becoz i failed calculus twice!!!!!


<font size=-1>[ This Message was edited by: doofus on 2003-01-22 09:14 ]</font>


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PostPosted: Wed Jan 22, 2003 11:02 am 
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Yikes that goin back a while!!! 10 years to be exact for me. I was stupid enough to do scholarship Calculus and got 18% in the exam :grin:

Since i'm a homosapian Nuclear fuel production technician i guess i should have gotten higher aye.

But yep it amazing how Physics finds it's way into everything....

Benouliis Principle
Laminar and Turbilant flow
Lift to Drag ratios
Foil Draft
Angle of attack
Wind shear
Vector analysis
Apparant wind
Buoyancy
The art of planing

The thumbnail effect
Edge to wedgie ratio

Man aint it good :grin: :grin: :grin: :grin: :grin: :grin:

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PostPosted: Wed Jan 22, 2003 12:07 pm 
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On 2003-01-21 20:48, aklbob wrote:
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..


Okay Aklbob - you got me going this time...
Here you can see how a 14.0 m2 kite (at sea level 20C/68°F) would feel like in different temperatures and location heights :

Image

The factors that influences power, NOT related to wind speed, can be clarified in an (extreme) example:

1. At 1500 meter, low pressure 955mb(at sea level), 30°C, Humid air - wind speed 10 m/s.

2. At sea level, high pressure 1050mb, 0°C, Dry air - wind speed 10 m/s.

If you have a kite in the first situation, then the same size will lift exactly 50% more in situation 2, so a 10m2 will lift as a 15m2 !

If the air was totally dry in 1500 meter, and humid at sea level, the difference would be about 47% - so humidity plays no important role, only 3% in extreme cases.

The height difference gives about 21% power difference, the "weather" pressure about 10%, the temperature about 10%, and the humidity about 3% - in this example which is a fictive and extreme case.

But it gives us an idea of where the levels are, and what we are talking about.

I like to understand how things work, at least some dedicated issues !

Apart from that, I think Fo is very right, when he says that mother nature laughs at us - because things are often very very different.
Especially gusts - they often penetrate from higher wind layers, and they blow upwards again at some point, which will give a slight change in wind direction, and lift at some point too.
It is an issue that have been studied a lot, in order to perform better tactical on a race course.
You don't really have thermals on the sea, but gusts can in fact give more lift than just what happens because of the increased wind.
This is why parking your kite at 12 on land should be avoided !
Here there are tons of evident things that can get you lifted high in a split second - and if mother nature has one of her tricky days too - then you are history !

It is winter here now, with only rare good kitesurf sessions, as you can guess :smile:


Last edited by Peter_Frank on Thu Jan 17, 2013 8:23 pm, edited 3 times in total.

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PostPosted: Thu Jan 23, 2003 1:58 am 
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Quote:
On 2003-01-22 12:07, Peter_Frank wrote:
Quote:
The height difference gives about 21% power difference, the "weather" pressure about 10%, the temperature about 10%, and the humidity about 3% - in this example which is a fictive and extreme case.


If I understand you correctly, by "height" here you're meaning water surface altitude (high lake compared to ocean), which at 21% power difference is practically negligible, by comparison to the difference between surface-level & kite-level (with 30m lines angling up at 45 degrees) power, which can vary by more than 800% (if I calculate correctly) due to the gradient shown in your original chart.

It would be interesting to see a chart of kite power vs. line angle (with 30m lines) in various wind speeds.


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PostPosted: Thu Jan 23, 2003 9:32 am 
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So true Mel - but we are used to the difference in kitepower at different heights (0-30 meter) as we kite in this everyday.

The other effect (wind power changing at THE SAME windspeeds) is what most people don't know the effect and magnitude of.

It is of no real importance, because as mentioned it will most likely max. be 1/2 a kite size change in power only, for most riders in their present conditions, and this is really no danger.
But still an interesting fact, and I know that many have been discussing the size of this effect !


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PostPosted: Thu Jan 23, 2003 10:25 am 
This is known from years and years back. in the 80´s they made the powerhead sails to catch the higher speed wind @4-5m height. They were really babd sails regarding point of gravity, though.
The same thing causes a phenomena which makes the kite wanting to dive into water when flown close enough the water surface. This holds true specially for bigger kites with a large span. The upper part of the kite flies in a higher wind than the lower part of it. The lower part brakes in comparison to the upper part and the kite wants to turn downwards. Try it with a well trimmed kite.


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