CFD Online Discussion Forums

CFD Online Discussion Forums (http://www.cfd-online.com/Forums/)
-   CFX (http://www.cfd-online.com/Forums/cfx/)
-   -   about roughness hight Yr (http://www.cfd-online.com/Forums/cfx/19092-about-roughness-hight-yr.html)

Hannah liang July 26, 2002 19:43

about roughness hight Yr
 
Hi, CFX friends, who knows how to specify the roughness (Yr)of the wall in order to simulate the resistance of duct when I failed to use inflation near the wall in CFX 5.5? Thanks a lot.

Hannah

Michael July 27, 2002 01:33

Re: about roughness hight Yr
 
Hi Hannah, if I understand this correctly, then you are trying to avoid the use of wall inflation and set a roughness height instead? I'm assuming that your near-wall mesh is very course then? The roughness height setting is not meant to be used as a substitute to a fine near wall mesh. Using a no-slip wall will simulate the resistance of the duct. If the duct is not smooth, then also use a roughness height to simulate this. You should still use a fine near wall mesh for accurate results (together with scalable wall functions). The roughness height setting is based on an equivalent sand grain roughness - see the reference in the documentation in the roughness height section. Hope this helps, Michael

Hannah liang July 27, 2002 11:18

Re: about roughness hight Yr
 
Hi, Michael, thanks for your replying. Actually, the duct I simulated has a definite roughness. In order to get the result as accurately as possible, the inflation should be used definitely, at the same time, roughness should be considered. My problem is, firstly, when I use the inflation, the def. file cannot be written, but when I delete the inflation, it can be writen. So frustrated! Secondly, how to specify the Yr value in actual case?

Hannah

Michael July 28, 2002 21:03

Re: about roughness hight Yr
 
Hi Hannah, you specify the wall roughness height in the Wall Boundary Condition Values panel. However, it isn't always available (e.g. with some turbulence models), it will be available for k-epsilon. Note that the actual height of your duct roughness is NOT the height you specify - you specify an equivalent sand grain roughness. Not sure about writing the def file with inflation. If you are using 5.5.1 try the new inflation option where you specify the height of the first layer and the number of layers (see doc). It give a smoother transistion to the tet mesh. Hope this helps, Regards, Michael

Bob July 29, 2002 05:08

Re: about roughness hight Yr
 
Hi Hannah, Is you model just of a plane duct, with no other more complex geometry attached to the duct or going through the duct ? Do you get a mesh out even when you specify including inflation in the definition file setup form within build or do you just get an error ? regards Bob

Hannah liang July 29, 2002 09:22

Re: about roughness hight Yr
 
Hi, Bob, sometime I can get a mesh out with inflation, but most time I cannot. Some error code will come out. I even cannot understand the meaning of the code. Where I can check or read the code and get the meaning of the error code in CFX 5.5?

Thanks

Hannah

Bob July 31, 2002 06:55

Re: about roughness hight Yr
 
Hi Hannah, some of the error messages are included in the online help pages, some are not. Some of the error messages can be quite cryptic. Why don't you post your error message, then some one may be able to help with it ??

Hannah liang July 31, 2002 09:26

Re: about roughness hight Yr
 
It's a good idea.Bob.

Thanks a lot. I will do in this way.

Hannah


All times are GMT -4. The time now is 13:46.