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Roughness

Up to Using openWind

Roughness

Posted by Joan Miquel Angles at January 11. 2010

Hi Nick, I've got the following question:


I used to calculate windmaps using only terrain info and wind frequency tables. However, some time later I started using also roughness length as well.


I thought I should get windmap with lower windspeeds, but I get windmaps with higher windspeeds. ¿Is it normal? ¿Does anyone know what should I get in most cases?


What I do to implement roughness is drawing a polygon and then I add in its attributes a field called "Roughness" with a certain value (0.4 in the case of forrests) and then I create a raster from this layer. I add this raster to the Windmap and I calculate.


Thanks!


Re: Roughness

Posted by nick at February 10. 2010

Sorry for the delay Joan,


I've been crazy busy trying to get a commercial version of openWind validated and ready for release.


The answer to your question is "not necessarily". If the default roughness was set to 0.04 (there is ALWAYS a roughness value - just maybe not yours :) ) when you weren't inputting a roughness map and now you have a map with areas of higher roughness and your hub height is above your met mast measurement height then the greater shear coefficient will lead to faster winds at hub height.


Make sense?


Nick


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