CFD Online Discussion Forums - How to solve the boundary layer without wall functions?

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How to solve the boundary layer without wall functions?

Dear Foamers,
I'm using OpenFOAM 2.3.0. What conditions have I to set on k and omega parameters if I want to solve the boundary layer with k-omega SST model?
I have a mesh with a y+ less than 4 in the wall patches; however, I think the conditions with wall functions are not correct:

omega:

Code:

    "(wall_1|wall_2)"

    {

      type            omegaWallFunction;

      value           $internalField;

     }

Code:

   "(wall_1|wall_2)"

    {

                type            kqRWallFunction;

                value           $internalField;

        }

     }

If I want to solve the boundary layer with a fine mesh, without wall functions, which conditions have I to set?
I tried to use:

omega:

Code:

    "(wall_1|wall_2)"

    {

         type zeroGradient;

     }

Code:

   "(wall_1|wall_2)"

    {

        type zeroGradient;

 }

     }

However when I run the simulation, OF told me:

Code:

Selecting RAS turbulence model kOmegaSST

--> Upgrading k to employ run-time selectable wall functions

    Backup original k to k.old

    Writing updated k

--> Upgrading omega to employ run-time selectable wall functions

    Backup original omega to omega.old

    Writing updated omega

--> Creating mut to employ run-time selectable wall functions

    Writing new mut

--> Creating alphat to employ run-time selectable wall functions

    Writing new alphat

So OF set the wallfunctions again.
Witch is the correct setting?
Thanks,
WhiteW

Well you would not set k and omega to zeroGradient, but to a very small, non zero fixedValue. For example:

k: fixedValue, value uniform 1e-8

However, the omega wall function is known to work well with lowRe meshes as it only applies the correct omega value to the wall without much of damping or blending. So i'd suggest just switching k to a fixedValue.

Thanks for the reply. Now I left omega with the wallFunction and I set k as:

Code:

    "(wall_1|wall_2)"

    {

        type           fixedValue;

        value          uniform 1e-8;

    }

However, when the solver (rhosimplecFoam) run, it still changes the conditions using the wallFunctions with the value I have set:

Code:

Selecting RAS turbulence model kOmegaSST

--> Upgrading k to employ run-time selectable wall functions

    Backup original k to k.old

    Writing updated k

--> Upgrading omega to employ run-time selectable wall functions

    Backup original omega to omega.old

    Writing updated omega

--> Creating mut to employ run-time selectable wall functions

    Writing new mut

--> Creating alphat to employ run-time selectable wall functions

    Writing new alphat

the new k is:

Code:

    "(wall_1|wall_2)"

    {

        type            compressible::kqRWallFunction;

        value           uniform 1e-08;

    }

is it correct?

Quote:

Originally Posted by Kina (Post 590814)

Thank you very much for this hint. I'd like to ask you, if you have any reference for this value. Since i'd like to implement it into my thesis. :)