# replace cfx scalable wall function

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 October 27, 2016, 09:20 replace cfx scalable wall function #1 New Member   G Ionut Join Date: Sep 2016 Posts: 29 Rep Power: 9 Hi, I want to replace the scalable wall function used in the k-epsilon turbulence model, with my own wall function that I could use it for k-epsilon turbulence model. Until now i tried with user defined wall function (ch 4.2.4. User Defined Wall Functions) but it isn't working. How can I do it? Thank you

 October 27, 2016, 17:15 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,728 Rep Power: 143 What isn't working about it? That feature should do what you are looking for.

October 31, 2016, 10:24
#3
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G Ionut
Join Date: Sep 2016
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Quote:
 Originally Posted by ghorrocks What isn't working about it? That feature should do what you are looking for.
Hi Glenn, thank you for the reply.

I am doing a test in a diffuser, and for the Wall Function Transfer Coefficient I entered a value of 5 [m/s], then I run the simulation.
When I represent the velocity in the diffuser I have values from 0.2 to 1.4 m/s, shouldn't I have 5 m/s over the whole velocity?
Attached Files
 test.txt (4.5 KB, 13 views)

 October 31, 2016, 13:56 #4 Senior Member   Join Date: Jun 2009 Posts: 1,815 Rep Power: 32 Not sure what is your understanding of the wall function concept. For the momentum equation, the wall function transfer coefficient relates the wall shear with the velocity at the wall and the control volume, i.e. Wall Shear = WallFunction Transfer Coefficient * (Vertex Velocity - Wall Velocity) The analogy to heat transfer Heat Flux = Heat Transfer Wall Function Transfer Coefficient * (Vertex Temperature - Wall Temperature) Hope the above helps,

November 1, 2016, 01:42
#5
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G Ionut
Join Date: Sep 2016
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Quote:
 Originally Posted by Opaque Not sure what is your understanding of the wall function concept. For the momentum equation, the wall function transfer coefficient relates the wall shear with the velocity at the wall and the control volume, i.e. Wall Shear = WallFunction Transfer Coefficient * (Vertex Velocity - Wall Velocity) The analogy to heat transfer Heat Flux = Heat Transfer Wall Function Transfer Coefficient * (Vertex Temperature - Wall Temperature) Hope the above helps,
Hi Opaque, thank you for the reply.

 November 1, 2016, 16:50 #6 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,728 Rep Power: 143 Any CFD textbook will have a chapter on turbulence modelling which covers this. Or you can probably find some descriptions of this stuff with a google search too - it should not be too hard to find.

 Tags cfx, user defined function, wall function