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-   -   ROUGHNESS HEIGHT (http://www.cfd-online.com/Forums/cfx/18526-roughness-height.html)

Sumon May 21, 2001 05:41

ROUGHNESS HEIGHT
 
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

Sascha May 28, 2001 06:22

Re: ROUGHNESS HEIGHT
 
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

Sumon May 29, 2001 04:27

Re: ROUGHNESS HEIGHT
 
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

Sascha June 12, 2001 09:38

Re: ROUGHNESS HEIGHT
 
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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