Gamma- Re Theta Transition Model CFX Roughness Modeling

prakhar_32 · June 18, 2014, 04:53

I have been using ANSYS CFX for my work . I had a question pertaining to the transition model available in cfx and would be glad if you could help me on that.

I am modelling the effects of surface roughness on the flow field in the compressor and for that I define patches of roughness elements in a domain say compressor impeller. My doubt is that if I specify a value of roughness height (geometric) for a patch for which equivalent roughness is specified , then is that value applied to all the walls in the domain. If yes does it effect the flow field despite the fact other walls have no equivalent roughness specified??

Looking forward to your response .

Kind Regards
Prakhar

ghorrocks · June 18, 2014, 06:38

You can define wall roughness to be whatever you like:
* You could split your wall boundary into blades and everything else - then put the wall roughness on the blades only.
* You could make the wall roughness a function of something, maybe radial position. In the radial position of the blades you could write an expression which defines a wall roughness.

prakhar_32 · June 18, 2014, 07:16

Dear Glenn!!!

Thanks for your quick reply. I am really grateful for that. I am also including a picture of my impeller blade with the split patches. What I intend to do is that I want to apply roughness to just patches one at a time, keeping rest of the blade smooth. For that I need to specify geometric roughness height in the roughness correlation field of the impeller domain. My only concern is that, does this roughness height applies to the all the walls or just the wall I have specified as rough with a specific value of equivalent roughness. I got confused because the Manual foe Fluent says that it is applied to all the walls, however there is nothing mentioned in the CFX documentations.

Kind Regards
Prakhar

ghorrocks · June 18, 2014, 18:52

Don't worry about Fluent. What ever the Fluent manual says has nothing to do with CFX.

If you set the wall roughness for that wall patch to a certain value then it applies to that patch only. If you do not believe me then do a test and check you get the difference between patches.

June 18, 2014, 04:53	Gamma- Re Theta Transition Model CFX Roughness Modeling	#1
prakhar_32 New Member Prakhar Kapoor Join Date: Jun 2014 Location: NETHERLANDS Posts: 2 Rep Power: 0	I have been using ANSYS CFX for my work . I had a question pertaining to the transition model available in cfx and would be glad if you could help me on that. I am modelling the effects of surface roughness on the flow field in the compressor and for that I define patches of roughness elements in a domain say compressor impeller. My doubt is that if I specify a value of roughness height (geometric) for a patch for which equivalent roughness is specified , then is that value applied to all the walls in the domain. If yes does it effect the flow field despite the fact other walls have no equivalent roughness specified?? Looking forward to your response . Kind Regards Prakhar

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June 18, 2014, 06:38		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	You can define wall roughness to be whatever you like: * You could split your wall boundary into blades and everything else - then put the wall roughness on the blades only. * You could make the wall roughness a function of something, maybe radial position. In the radial position of the blades you could write an expression which defines a wall roughness.

June 18, 2014, 18:52		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	Don't worry about Fluent. What ever the Fluent manual says has nothing to do with CFX. If you set the wall roughness for that wall patch to a certain value then it applies to that patch only. If you do not believe me then do a test and check you get the difference between patches.