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Old   May 21, 2001, 05:41
Default ROUGHNESS HEIGHT
  #1
Sumon
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Hi everyone

Could anyone tell me what is the roughness height and its unit which is used as by default? In USRWTM there is an option to change the roughness height. I am egarly waiting for the reply and suggestions.

Regards,

Sumon
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Old   May 28, 2001, 06:22
Default Re: ROUGHNESS HEIGHT
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Sascha
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All surfaces in technical applications liek the surface of a car are rough with a deviation in roighness height. You have it even for very smooth surfaces. In most CFD programs a standard roughness is set. But if you have a much rough surface liek sandcorn size you have to modify the wall functions for the turbulence model with the right roughness height. Normally teh roughness height is measured in 10^-6 m but in CFX you add it also in m so be aware of that !

Sascha
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Old   May 29, 2001, 04:27
Default Re: ROUGHNESS HEIGHT
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Sumon
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Sascha

Thanks for your kind reply. But if you have a look the USRWTM, it is mentioned that for RFACT = 0.3 WOULD BE FULLY ROUGH and RFACT > 0.3 GIVES LESS ROUGHNESS. Is it correct? The equation which is used in the example, it is contradictory. Could you tell me something about this. From the equation, I think that RFACT = 0.3 WOULD BE FULLY SMOOTH and RFACT > 0.3 GIVES LESS SMOOTHNESS.

Thanks again.

Sumon
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Old   June 12, 2001, 09:38
Default Re: ROUGHNESS HEIGHT
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Sascha
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In this case the manual is wrong. When you check with the appropriate wall function which you have to use for rough walls a dimensionless roughness height as used in USRWTM (first example) will mean an increasing roughness for this parameter when u increase it.
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