# Modelling Turbulent Wall Functions

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August 25, 2010, 09:44
Modelling Turbulent Wall Functions
#1
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Victor Azevedo
Join Date: Aug 2010
Posts: 5
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Hi everybody,

I'm doing a multiphase flow simulation and I have to model the wall roughness by using "turbulent wall functions". The CFX Theory Guide have an expression for this, but the big problem is in one of the variables.

The turbulent wall function uses the variable u* (I think that's the friction velocity or shear velocity). This variable is available for me in the Solver, but when I run the case, in the Post this variable does not appear (CFX says that it not exist). I'm needing this variable to calculate the film thickness and other variables of my case.

So, I would like to ask you how can I put this variable in my expressions. Is there any syntax? Remember that's a multiphase flow case.

I attached my CCL file, so you can look. The u* is in the "Uaster" expression, and that's the origin of all my problem here. The "wall turbulent function" is in the "WTFC" expression.

Thanks for your help.
Attached Files
 TCC.txt (23.5 KB, 50 views)

 August 25, 2010, 18:03 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,749 Rep Power: 143 Your function looks OK for a single phase flow. I suspect you need to get ustar for the phase you are interested in, not sure how to do that off the top of my head. Does the ustar function work for a single phase flow?

 August 26, 2010, 16:49 #3 New Member   Victor Azevedo Join Date: Aug 2010 Posts: 5 Rep Power: 15 Thanks for your answer ghorrocks. I tried to run the case as single phase. Same problem on the ustar function. I'll try to run in other computer, and see if the same problem occur. thanks again.

 January 13, 2015, 09:38 implemanted new wall function #4 New Member   adelo Join Date: Jan 2015 Posts: 10 Rep Power: 11 Hi everybody, I'm doing a multiphase flow simulation and I want to implemanted a new wall function in CFX( So, I would like to ask who can I add this new function(U*, C,K ) Thanks for your help.

 January 13, 2015, 10:06 #5 Senior Member   Join Date: Jun 2009 Posts: 1,825 Rep Power: 33 You can read the following section in the documentation: CFX-Solver Modeling Guide -> Turbulence and Near-Wall Modeling -> Modeling Flow Near the Wall -> User Defined Wall Functions

 January 13, 2015, 14:02 #6 New Member   adelo Join Date: Jan 2015 Posts: 10 Rep Power: 11 thank you very much I'm going to read this document

January 13, 2015, 14:22
#7
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adelo
Join Date: Jan 2015
Posts: 10
Rep Power: 11
I want to write new formulas for this parameter (see document). in the indicated section I understood it shows me how to change a constant
Attached Images
 parametrs.jpg (16.3 KB, 34 views)

 January 13, 2015, 14:49 #8 Senior Member   Join Date: Jun 2009 Posts: 1,825 Rep Power: 33 The software does not let the user modify the ingredients of the built-in wall function. User modifications may not be compatible to what the developers implemented; therefore, the burden is on the user to implement it all. Use the example provided in the documentation, and see where you can introduce the modifications you need. markos9149 likes this.

 January 13, 2015, 14:58 #9 New Member   adelo Join Date: Jan 2015 Posts: 10 Rep Power: 11 you speak of this exemple in (ANSYS CFX-Solver Modeling Guide 4.2.4.3. Example of User-Defined Wall Function for Heat Flux)

 January 14, 2015, 11:32 #10 Senior Member   Join Date: Jun 2009 Posts: 1,825 Rep Power: 33 Yes.. You can use that example as a template to create your custom wall function markos9149 likes this.

 January 14, 2015, 13:53 #11 New Member   adelo Join Date: Jan 2015 Posts: 10 Rep Power: 11 thank you very much

 January 14, 2015, 15:48 #12 New Member   adelo Join Date: Jan 2015 Posts: 10 Rep Power: 11 before i do the ilmplemtation of my new wall function i need just to know how can I activate the wall function in simple case two phase flow thanks for you help

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