[Fauster]

Dear Foamers,

I am quite lost with wall treatment in openFOAM. As far as I know Omega based models can be solved in the viscous sublayer. For exemple :

standard k-omega and k-omega SST.

With these turbulence models it's possible to use low and high reynolds boundary conditions for wall :

type omegaWallFunction; (valid for both low and high reynolds. The condition based the blanding of viscous and log layer)
type kqRWallFunction; (For high Re, zero gradient condition)
type kLowReWallFunction; (for high and low Re, the condition based on the position of y+)

But when using k-epsilon model, for instance Standard, RNG or Realizable, is it possible to go down in to the viscous sublayer ? By using this BC :

type epsilonLowReWallFunction (for High and Low Re, the condition based on the position of y+)

I am more than doubtful that it's possible to use this BC with the standard k-epsilon turbulence model.

If you have any ideas on the subject I would be very grateful.

Regards

[piu58]

If you mesh you geometry in a way that you resolve the Prandtl layer you don't need any turbulence model: You don't model the turbulence in that layer in an indirect way, but cover it by the NS equations.

If you use a model anyway the coefficients get very small and switch of the modelling this way.

[jherb]

As piu58 wrote depending on your y+ values you might need wall functions or not (there are several forum threads about this topic). But nevertheless, you will need a turbulence model (if you don't resolve all of your domain with such a fine mesh that you can do DNS).

[Fauster]

I don't understand your answers. Are you sure is it legit to calculate the viscous sub layer (go to y + ~1) with a k-epsilon type model when using the epsilonLowReWallFunction BC ?

I mean there is no damping function in the k-epsilon models available in OpenFOAM. As far as I know there is no low reynolds formulation for k-espilon model implemented in OpenFOAM.

Theoretically the turbulent dissipation goes to infinity when approaching the wall. That's why k-epsilon models need wall functions, or a two layers wall treatment with Wolfstein model for example, or damping functions.

BUT in OpenFOAM there is a boundary condition epsilonLowReWallFunction. The code is here :

https://cpp.openfoam.org/v4/epsilonL...8C_source.html

This BC set two different values of the turbulent dissipation depending on the position of the first cell (below or upper y+~11)

So :when using this BC what is the recommendations for y+ ? And moreover, Is it ok to use this BC with (for exemple) the standard k-epsilon model ?

For exemple : In commercial code CFX, when using k-epsilon model it's only possible to use scalable wall function. This wall treatment avoid the deterioration of standard wall functions under grid refinement below y+ = 11. "CFX force the usage of the log law in conjunction with the standard wall functions approach by limiting the minimal y+ computed at the wall. So in CFX it's not possible to go below y+~11 wen using a k-epsilon type model"



Regards

[Fauster]

I guess I found the answer. I met some difficulties because I thought that the boundary condition epsilonLowReWallFunction was based on the standard wall treatment which is pretty bad when going below y+~11.

In fact the boundary condition epsilonLowReWallFunction is more like the Non-Equilibrium Wall Functions treatment available in FLUENT which is design to avoid the main inconvenient of standard wall functions.

Do you agree with me ?

Regards

[CFD_10]

Originally Posted by Fauster

I mean there is no damping function in the k-epsilon models available in OpenFOAM. As far as I know there is no low reynolds formulation for k-espilon model implemented in OpenFOAM.

I think the LaunderSharmaKE is a low reynolds formulation for k-epsilon turbulence model. For more details, take a look at this page:https://www.openfoam.com/documentati...E.html#details

[Iose]

Originally Posted by Fauster

Dear Foamers,

I am quite lost with wall treatment in openFOAM. As far as I know Omega based models can be solved in the viscous sublayer. For exemple :

standard k-omega and k-omega SST.

With these turbulence models it's possible to use low and high reynolds boundary conditions for wall :

type omegaWallFunction; (valid for both low and high reynolds. The condition based the blanding of viscous and log layer)
type kqRWallFunction; (For high Re, zero gradient condition)
type kLowReWallFunction; (for high and low Re, the condition based on the position of y+)

But when using k-epsilon model, for instance Standard, RNG or Realizable, is it possible to go down in to the viscous sublayer ? By using this BC :

type epsilonLowReWallFunction (for High and Low Re, the condition based on the position of y+)

I am more than doubtful that it's possible to use this BC with the standard k-epsilon turbulence model.

If you have any ideas on the subject I would be very grateful.

Regards

Hello Fauster,
Did you tested the setup for LowRe kepsilon group of models for some basic cases?
Because I will try it, simple straight circular tube (ID 33 mm) and results are not good, even if I have yplus below 1...
Thanks for your comment.