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PostPosted: Thu Jan 23, 2003 10:30 am 
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Location: Pembrokeshire
Peter,

First allow me a moment to bow down and admire someone who understands which laws to apply to create this stuff!

Secondly I was out yesterday on a 9.1m Mach1 having a really hard time controlling it when trying to walk up the beach (still do the walk of shame when I can't stay upwind)the air temp was about 8C and my wind meter said 25mph this agrees with the data buoy nearby. Gusts were coming in at around 30mph. Towards the backend of last year I was going out in solid 30mph gusts to 35mph with the same kite and having a blast! Air temp around 16C.

Am I just having a bad patch or does this make sense? I appreciate there are many variables played by nature and have always just accepted this (like we have a choice) but if there was a scientific explanation it would feel better to understand why.

Thanks.


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PostPosted: Tue Feb 25, 2003 9:13 pm 
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you get around boi.

i have no real explanation for the difference we feel with 10-oc shift in temp, but ten degrees temp difference makes about the same difference as 2-3knots on one of my foils. the facts don't add up on this one, perhaps we just try to be more accurate when its cold and thats what makes the difference our level of concetration.


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PostPosted: Wed Feb 26, 2003 2:24 am 
Peter - thanks to the internet, Toby's forum and kitesurfing...one who did not graduate from high school without three years of summer terms...including the final year's summer term. Has gotten a whole new look at physics from you.

I remember the post about the wind at various angles of your lines, and thought then: Gee! I would have never thought of that?! It's too bad I lost it. Maybe Toby should have a library of 'educational topics'.

Thanks for helping me think. I won't identify myself directly, but not having been educated has not kept me from prosperity. Some of us just have to work harder at it than others.

Again, many thanks for the informative posts.


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PostPosted: Tue May 18, 2004 9:03 am 
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Location: Miami @ 6" Flat Butter!
Peter,

Thanks for the good brilliant work . :thumb:

DrLightWind


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PostPosted: Fri Jun 04, 2004 11:43 pm 
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Peter_Frank wrote:
I've taken into account that the water surface gets more rough, when the wind picks up.


Hai Peter,
thank You for the interesting graphs. Would You mind telling us, what model and Parameters You used? I would be most interested in the roughness parameters (z0 for a logarithmic profile?) belonging to various Wind speeds on the Water. Guess You have assumed steady Wind with a fetch of a few 100 kilometers for at least 24 hours?


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PostPosted: Sat Jun 05, 2004 4:30 am 
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Well it helps explain why you need so much less wind to kite on the snow or ice in the winter: the denser, colder air really provides much more power at the same wind speed.


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PostPosted: Sat Jun 12, 2004 6:42 pm 
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Klaus (c:E wrote:
Peter_Frank wrote:
I've taken into account that the water surface gets more rough, when the wind picks up.


Hai Peter,
thank You for the interesting graphs. Would You mind telling us, what model and Parameters You used? I would be most interested in the roughness parameters (z0 for a logarithmic profile?) belonging to various Wind speeds on the Water. Guess You have assumed steady Wind with a fetch of a few 100 kilometers for at least 24 hours?


I sure will :thumb:

V = Vo * ln(H/R) / ln(Ho/R)

V = Wind speed
H = Height
R = Roughness of surface at current wind conditions
Vo = Wind Speed at 1.75meter
Ho = 1.75meter

Based on the water surface and usual wave development/height, the R constants are approx. as follows, in my eyes and evaluation :
5m/s: R = 0.0003
7.5m/s: R = 0.0007
10m/s: R = 0.0018
12.5m/s: R = 0.005
10m/s: R = 0.015

Above is based on my experiences and measurements, and our Danish wind turbine "physics" and measurements, which we have a lot of knowledge about, as we have huge amounts of wind turbines at the coasts and in the country sites, here in Denmark :thumb:


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