turbulence modelling near walls
 

Hello Foamers,

I have a question using wall functions in OpenFOAM.
In detail I am using the RAS turbulence model (k-epsilon) with wall functions. In case one uses wall functions for simulating turbulent quantities near walls, one usually has to ensure that y+ is greater than 30 there.

Now I have the following question: How does OpenFOAM treat the wall functions in the case that y+ is around 1 at some walls? Does it just turn off the wall function (like some kind of all y+) there or nevertheless uses it (i.e. do I have to ensure y+ > 30 everywhere in advance when using wall functions)?

Thanks in advance for your answers.

Greets koimau

if your y+~1 use kw-SST

Hi koimau,

I think that the answer for your questions change depending on the turbulence model and boundary condition you use.
For the standard kEpsilon model, you might want to read the following document at page around 19:
http://www.diva-portal.org/smash/get...FULLTEXT01.pdf

Hi mechy,

Is it appropriate to use kw-SST with so refined mesh?
What is the suitable boundary condition?

Hi koiamau,

I do not really know all the details of the OpenFOAM wall treatment. Considering that you are using the std wall treatment, it does not matter what is your y+, the log layer will be applied between your first node and the wall. If you have y+=1, your wall shear stresses will be highly inaccurate.

If you try the kw (low Re turbulence model) you should try to have a y+ close to one ad lower than 3. Considering the sensitivity of this turbulence model to the BC's, the kw-SST blending model will be recomended.

You can try to see the user guide if there is a kind o Automatic/Enhacment/Scalable wall treatment...If it does, is better to use them.

Regards,

for kw-sst you should have y+<5 on walls also at last 15 cells should be in the boundary layer of wall
suitable Bc for what ?
velocity========> fixed value
pressure =======> zero gradient
k=============> kqRwallFun... or lowReWall
omega =========> omegaWallFun...

Hi mechy,

Thank you for your answer.

Is it an empirical rule?