windgen.org vawt discussion

You migt want to read about the riblet effect it reduces drag for sharks and in future planes. Works only with attached and laminar flow.

http://www.fraunhofer.de/archiv/pi-en-2004-2008/EN/press/pi/2006/12/ResearchNews12-2006-Topic4.html

titus van den brink

that golf ball simulation does not take the rotation of the ball into account.
i am no golfer but i am sure the rotation has an effect.

surprise, we will get another chance this season to see if there are any dimples on michael schumachers ferarri

There is another potential problem.  It could be that dimples will reduce drag but also reduce the lift.

It may not take a supercomputer as I believe the Mythbusters cable tv show tested both dimpled and non-dimpled cars as well as clean vs dirty cars but I don't emember the results.

Bernd wrote:

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Never the less it would be interessting to compare a waxed and unwaxed car. :)

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If I ever get any wind here, I will be comparing a dirty and clean / waxed wind turbine.

 

Just remember there is a difference between dimples and random roughness.



By the way, this is a simulation I won't be doing.  I don't have a super computer.


-- Edited by sjh7132 on Thursday 21st of January 2010 04:49:59 PM

Bernd wrote:

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How much is the surface roughness changing when the effect is so drastically ?

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That is a good question.  Unfortunately in openfoam you don't put in a surface roughness.  You put in a turbulence model with a whole bunch of values for some Greek variables.  I'm an Electrical Engineer, not a flow dynamics person so I have NO IDEA what I'm doing when it comes to turbulence.  (That may also apply to any of the simulations.)

 

Thanks for the video Zapmk !
It shows exactly what i thought. The "roughness" should be great to take effect.

Many years ago, must be around the 80th, a girlfriend of me bought the german "golf ball" helmet from the schubert manufactory
(which makes also the helmets for Michael Schumacher).
This golf ball helmet had have a significant less resistance but its "design" was not very popular.



Never the less it would be interessting to compare a waxed and unwaxed car. :)

Bernd


-- Edited by Bernd on Thursday 21st of January 2010 04:48:33 PM

http://dsc.discovery.com/videos/mythbusters-dimpled-car-minimyth.html

".. I know I never drove my car at full speed before and after I waxed it...."

Ok, if you will measure more than 1 km/h i will spend you a holiday in Germany,
where you can wax my car too. :)
May be the unwaxed car is faster, because of creating little helpfull vortex, who knows.
Must be a good job for the myth busters to discover how big or if there ever is a difference.

"... I know from OpenFoam that if I add a turbulence model for the surface, the drag changes drastically ..."

How much is the surface roughness changing when the effect is so drastically ?

Bernd

-- Edited by Bernd on Thursday 21st of January 2010 04:12:45 PM

Bernd wrote:

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It doesn't make signifikant different if a fat blade surface at normal windspeed is polished oder sandblasted.

A car didn't run faster when you polish it. :)
(Although some people think that ) :)

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I wonder if anyone has actually done experiments to prove these statements are true.  I know I never drove my car at full speed before and after I waxed it.

I know from OpenFoam that if I add a turbulence model for the surface, the drag changes drastically.  (mostly worse, but sometimes better.)  It's so drastic, and I know so little about turbulence models that I don't use it, and just assume a smooth surface.

 

Yes the nature is sometimes the best teacher. But the wal fins are not really comparable with polishing a surfaces or sandblade a surface.
Depending on the wal fin dimensions the "humps" are much greater than the roughness of a normal surface.
Like i know the humps are creating helpfull vortex, which helps to reduce the resistance of the fin.
Also we have to consider that the viscosity of water is very different to air.

Bernd

i recall alot of discusion over the shape or texture of whale fins

"... Could a surface coating on the fat wings help reduce drag ? ..."

I don't think so. I guess the form is much more important for the drag than the surface, at leat at fat blades.
Using very aerodynamik blades and/or high wind speed, like aroplanes oder golf balls, it would be different because there
you can control laminar flowing of the air using different surfaces.

I think the relationship between surface roughness and volume or thickness of the blade is responsible for the influence.
The dimple into a golf ball is, based on the diameter of the ball, fairly deep.

It doesn't make signifikant different if a fat blade surface at normal windspeed is polished oder sandblasted.
I guess it doesn't make any different except you use a very roughness surface.

A car didn't run faster when you polish it. :)
(Although some people think that ) :)

Bernd


-- Edited by Bernd on Thursday 21st of January 2010 11:02:45 AM

to reduce drag I had film forming plows sandblasted...whatcha think?

Zapmk wrote:

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Could a surface coating on the fat wings help reduce drag ?

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It works for golf balls, but they move a lot faster.   Back when I used to fly model gliders at the beach I covered my wings with 'Colortex'.  It was a shrinkable fabric that had a texture that caused turbulence on the surfaces of the wings.   It seemd to helped lift but I don't know if it reduced drag.

Unfortunately I don't think Colortex is made anymore..

Could a surface coating on the fat wings help reduce drag ?