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

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

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

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

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

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

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 ??

It's a good idea.Bob.

Thanks a lot. I will do in this way.

Hannah