Learning Forum

[Sofen Kumar Jena]

For Wall function approach the k and epsilon equations are not solved upto the wall, rather their value is pescribed at the first cell centre adjucent to the wall boundary. Ansys Fluent theory guide tells this is how the values of k and epsilon are solved in the first cell centre. 

I want to customize these values as per my requirements. Please inform how to pescribed the customized values of the epsilon at the first cell centre. 

No where the same is discussed with clarity . 

[Essence]

Hello,

Did you try using the expressions or the UDFs in the BC panel for k and epsilon?

[Sofen Kumar Jena]

I do not find any option in the BC panel for K and Epsilon, Kindly inform me how to enable the same or use the same through UDf with minimal example.

Cheers

Sofen 

[Essence]

Ok, I now understood what you wanted. There are some options you can try.

First is to create the Cell registers and assign fixed values using the expressions.

Another method is to re-create the mesh segergating the fluid domain in two parts. One, near the wall boundary (adjacent to the cell) and two, rest of the domain. This way, you will have a separate fluid mesh near the wall and you can fix the k and epsilon values. 

[Sofen Kumar Jena]

I an able to mark the first cells adjecent to the boundary through UDF only, by looping it over the desired faces using the following codes in a UDF

c2 = F_C0(f2,tf2);
tc2 = THREAD_T0(tf2);

Then the question is how to proceed next. We need to provide the first cell velocity, and epsilon value as well as a Neuman condition for K (two scale approach). How to do that ? Provide some minimal example. 

 

 

[Essence]

Look, you can just use the cell registers, separate them out in Domain > Separate > Cells. Then just click on the cell registers and the cell zone in which it is and click Separate. Then you can just apply the fixed values of variables you need, in the Cell Zone conditions.

No need to use UDFs. I just gave you couple of options in which, cell registers is easy (according to my opinion).

[Sofen Kumar Jena]

Thanks, Now I have sucessfully able to separate the first layer cell using cell register. I think now I can able to provide the modified value of epsilon in this layer of cell using fixed values. In Two scale approach, I too need to provide the Neumann boundary for k at the wall boundary. How to provide the same ? 

[Essence]

This might be useful.

https://ansyshelp.ansys.com/account/secured?returnurl=/Views/Secured/corp/v241/en/flu_udf/flu_udf_ModelSpecificDEFINE.html%23flu_udf_sec_define_wall_functions