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Old   April 23, 2013, 20:30
Default Question regarding Gamma Re Theta Model
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Helly everyone,

I'm trying to simulate a LP Axial Turbine (2 stages, air ideal gas, steady and transient) using the SST turbulence model.

From experimental data I know that the hub and tip regions are fully turbulent. On the suction and pressure side of the blades there is transition though. I want to model the transition like this in my CFD setup.

So my question would be: is there any possibilty to specify the hub and tip regions as fully turbulent, while using the Gamma ReTheta model for the rest of the domain ? Can I specify certain surfaces as fully turbulent and other surfaces as transitional at the same time ?
My mesh should be fine enough (in fact, it was used in another code, however, with a different transition model)

I'd be very thankful for any help

PS: sorry, english is not my native language. If I didn't explain my problem well enough, feel free to ask and I'll give it another shot

Thank you !
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Old   April 24, 2013, 07:17
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Of course you can specify turbulent regions by setting the inttermittency, but if you want to mix this and the gamme-theta model it is a little more tricky. But not too tricky I suspect - try a source term of intermittency. Then you should be able to fix the intermittency to turbulent in those regions and leave the rest up to the model.

On the other hand, if you know the tip and hub are turbulent then the gamma-theta model may well predict this anyway with no need for you to do anything. I would try this first, before artificially forcing it using source terms.

By the way: Your English is fine and your question very clear.
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Old   April 24, 2013, 10:52
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Quote:
Originally Posted by ghorrocks View Post
but if you want to mix this and the gamme-theta model it is a little more tricky. But not too tricky I suspect - try a source term of intermittency. Then you should be able to fix the intermittency to turbulent in those regions and leave the rest up to the model.
Thank you ghorrocks ! That's exactly what I'd like to do.

I have to admit, I'm a newbie to CFX but I've worked through all tutorials and almost every page of the manual.
Unfortunately, you can't use the variable "Turbulence Intermittency" in CFX-Pre (see Variables and Predefined Expressions Available in CEL Expressions, Reference Guide). So there is no chance to set up a CEL Expression or CEL Routine for the Intermittency.
Or is it ?!

Could you guide me roughly through your idea ?

I would appreciate any help I can get on that issue.
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Old   April 25, 2013, 07:12
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On second thoughts I am not sure CFX will let you put source terms on the intermittency equation anyway. I do not have CFX available right now so cannot check.

If CFX-Pre does not have the variable available then just used the CCL editor to put it in manually. Just make sure you get the variable name right and it will be there in the solver.
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Old   April 25, 2013, 13:40
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On second thoughts I am not sure CFX will let you put source terms on the intermittency equation anyway. I do not have CFX available right now so cannot check.
No, you can't put a source term on the intermittency or theta equation.

I think it is not possible to specify intermittency in certain regions while using Gamma-Theta for the rest of the domain
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Old   April 25, 2013, 18:45
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Just checked and it looks like you are right.

But looking at the RULES file suggests you can specify intermittency as a boundary condition at an inlet. It is not available in the CFX-Pre GUI, but if you put it in as a CCL text object it should work. This might do the trick for you. You can define the flow as turbulent at the inlet in the tip and hub regions and leave the rest of the flow as laminar.
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Old   April 25, 2013, 19:00
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Hey ghorrocks,

Funny, I was looking at the RULES file today and I tried that (Transitional Intermittency at my Inlet). I guess I'll just go with that. On a test run with just my first stator it looked promising.

Thank you very much for your time and help. Much appreciated !
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Old   May 14, 2014, 12:12
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Hi, where can i find RULES file?thank you
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Old   May 14, 2014, 18:31
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<CFX_ROOT>/etc/RULES
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Old   May 15, 2014, 13:59
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thanks! it helps a lot

Last edited by guxin7005; May 15, 2014 at 15:03.
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