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Old   May 16, 2010, 10:12
Default Surface roughness
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Hi,

I'm studing the development of the ABL using a wind tunnel with variable superficial roughness. Using Fluent i'm trying to simmulate this tunnel but i'm having difficulties to specify this superficial roughness.

What should be the better way to put this in Fluent?Any usefull help would be welcome.

Thanks
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Old   May 17, 2010, 04:41
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You can set some amount of roughness in the wall boundary condition panel. The only thing you will have to change is the roughness diameter, in meters (default is 0 m). For a lot of applications this works fine.

Take care that the wall adjacent cell width is at least twice the roughness diameter.
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Old   May 17, 2010, 06:42
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My problem is that if set my wall adjacent cell width twice as high as the roughness height, my y+ values become very high, and for the correct use of standard wall functions y+ should be between 30 and 300. My roughness height goes from 2mm to 8mm and my wind tunnel height is 1m.
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Old   May 17, 2010, 07:29
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To my understanding, the wall roughness shifts the wall function down to a lower u+, but apart from this shift the profile is unchanged.

I think that when the roughness is so large that the grains reach to some y+ = 150 the effect is not limited to a u+ shift, but that you will have a quite different boundary layer altogether. I am not sure if you could still use the wall roughness boundary condition in that case ...
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