[cfdmarkus]

Hi,
I have extended the kOmegaSST turbulence model in OF1.5 by a low Reynolds number correction.
The model extension was straightforward, however, I am having problems specifying appropriate
wall boundary condition for "omega". In the low Reynolds number version the wall boundary
condition for omega is a function of the laminar viscosity and the distance to the first grid point.
As a consequence, I cannot use the "standard" zeroGradient and fixedValue boundary conditions for omega
at the wall.

My first idea was to create a new boundary condition which computes and specifies omega at the wall.
I already had a look at the default fixedValue and some other derived boundary conditions.
Since I am still a beginner in OpenFoam, my knowledge is not sufficient to adapt any of the
existing boundary condition for my purposes.

Could anybody please give me some guidlines with which of the existing boundary condition I should start with and what are the important lines that need to be changed.

Thanks
Markus

[ngj]

Hi Markus

Search the forum for a thread called "Calculation of Pressure Loss". In there is a discussion of boundary conditions for omega over rough boundaries.

Even though Wilcox states that the boundary condition should work on smooth boundaries I have not had any good experience with it.

In the same thread there is also a implementation of the Wilcox rough boundary. This you could use to get inspired.

Best regards,

Niels

[cfdmarkus]

Hi Niels

Thanks a lot - I will have a look at it.

Markus

[cfdmarkus]

Hi again,

I have now solved the problem of specifying the boundary conditions! (using: omega_boundaryField()[patchi]== X and omega_.correctBoundaryConditions()).

However, I discovered some unusual bahaviour which I cannot explain: In order to check that I am using the correct values to compute the boundary conditions, I have printed the values for the wall distance and the laminar viscosity on the wallpatch and in the cells attached to the wallpatch. During this check, I did not compute new boundary conditions for omega at the wall.

As a result, I get incorrect values for the wall distance and the lam. viscosity in the first grid cell in exactly two cells, all other wall-cells have the correct value for the wall distance and lam. viscosity. Also, in the first grid cell, nu() has changed, even though, I am using incompressible-Newtonian-simpleFoam. The values of nu() in the same two cells as before are again completely unreasonable.

When I include the computation of wall boundary conditions for omega, the situation becomes even worse. Now, only the values at wallpatch are correct and the values in the cells are all incorrect.

Does anybody know what is happening here.
Markus






WITHOUT omega_boundaryField()[patchi]== X and omega_.correctBoundaryConditions() included:
wallpatch dy: 1e-15 --- wallcell dy: 1.15459e-312 --- wallpatch nu: 1.461e-05 ---- wallcell nu: 1.15459e-312
wallpatch dy: 1e-15 --- wallcell dy: 1.15459e-312 --- wallpatch nu: 1.461e-05 ---- wallcell nu: 1.15459e-312
wallpatch dy: 1e-15 --- wallcell dy: 2.5e-05 --- wallpatch nu: 1.461e-05 ---- wallcell nu: 2.5e-05
wallpatch dy: 1e-15 --- wallcell dy: 2.5e-05 --- wallpatch nu: 1.461e-05 ---- wallcell nu: 2.5e-05
.....

WITH omega_boundaryField()[patchi]== X and and omega_.correctBoundaryConditions()included:
wallpatch dy: 1e-15 --- wallcell dy: 1.15459e-312 --- wallpatch nu: 1.461e-05 ---- wallcell nu: 1.15459e-312
wallpatch dy: 1e-15 --- wallcell dy: 4.79861e-317 --- wallpatch nu: 1.461e-05 ---- wallcell nu: 4.79861e-317
wallpatch dy: 1e-15 --- wallcell dy: 6.25e-10 --- wallpatch nu: 1.461e-05 ---- wallcell nu: 6.25e-10
wallpatch dy: 1e-15 --- wallcell dy: 6.25e-10 --- wallpatch nu: 1.461e-05 ---- wallcell nu: 6.25e-10
....

[mattijs]

The dy values are not set it seems. Compile the code with -DFULLDEBUG -g -O0 and run through valgrind.

[cfdmarkus]

Ok thanks again - I will run through valgrind and see what happens.

[waynezw0618]

Hi

would you mind to explain different of the boundary layer calculating method for OF and CFX of Mentor`s.and what about the Markus` low Reynolds number correction ?is there any reference paper of the SST in OF and Markus` low Reynolds number correction ? you see i am calculate the torque for the blade of pump impeller,but with the flow separation the standard wall function of log-law is not suitable for this calculation as i have tried,and made the analysis by RE method. i found a huge error of this modeling method not numerical works. would you mind to give me some good idea?


wayne

[cfdmarkus]

Hi

I don't know anything about the SST model in CFX. The SST model in OF uses a wall function approach.
In my opinion, wall function may work ok for very simple flows. If you want to simulate more complex flows (e.g. flow separation under adverse pressure gradient) I would always fully resolve the boundary layer up to the wall. The SST model in its original formulation is derived for high Reynolds numbers. This however does not mean you have to use wall functions. You still can fully resolve the boundary layer and with that avoid the assumptions inherent in the wall function approach.

Since the original SST model is derived for high Re, you cannot expect to get accurate profiles for the turbulent quantities very close to the wall where the flow has a low local Re. If you want "more" accurate profiles for the turbulence quantities close to the wall you need low Re corrections. This only corrects turbulence quantities, the mean velocity profile for high Re flows will be largly unaffected since the turbulent viscosity is small where the low Re corrections becomes active.

Maybe that helps a little bit.
Markus

[ivan_cozza]

Hi Markus,
I'm very interested in your approach for Low-Re version of k-omega.
I'm calculating the turbulence statistics in the bl of a thick flat plate ( thickness = 3% of the chord), so with y+ 30 I cant resolve allmost nothing of the bl.
Can you tell me what to do to modify the actual SST for low-Re correction?
Thank you,
Ivan

[cfdmarkus]

Hi Ivan,

you have to create a copy of the kOmegaSST.C and reomve everthing that is related to the wall-functions. In addition you have to take care of the wall boundary condition for omega.
This gives you a model which allows integration to the wall.

For additional low-Re corrections you can consult the Fluent manual:
http://202.185.100.7/homepage/fluent...ug/node432.htm

The low-Re corrections are not really necessary unless you want to improve the turbulence quantities close to the wall by a little bit.

Regards
Markus

[ivan_cozza]

Hi Markus,
I'm very interested in your approach for Low-Re version of k-omega.
I'm calculating the turbulence statistics in the bl of a thick flat plate ( thickness = 3% of the chord), so with y+ 30 I cant resolve allmost nothing of the bl.
Can you tell me what to do to modify the actual SST for low-Re correction?
Thank you,
Ivan

[ivan_cozza]

Thank you Markus,
so, I have to cancel any inclusion of "kOmegaWallFunctionsI.H", and correct the wall value of _omega as you do, with a new boundary condition for omega.
Can you show an example of this boundary condition?
Thank you, Ivan
P.S. actually I work with an y+ of about 1.5, in your opinion is good for the model without the wall functions?

[cfdmarkus]

you can find the solution in the thread called "Calculation of Pressure Loss". In there is a discussion of boundary conditions for omega over rough boundaries.

I tend to use y+<1. But I suggest that you do a sensitivity study for y+.

Regards
Markus

[waynezw0618]

Hi Markus

would mind tell me what is meaning of :

omega_.correctBoundaryConditions()

anyway,if i don`t want to use the wall function,how can i give the wall boundary for k and omega ? zeroGradient ?


thanks

yours wayne

[florian_krause]

Deal all,
the thread is already quite old, so there are maybe others (than Markus) who successfully implemented the low-Reynolds version of the k-omega SST model...

I am about to implement the low-Reynolds version following (file is too large, but should have free access if you google it):

Florian Menter, Jorge Carregal Ferreira, Thomas Eesch, and Brad Konno (2003), The SST Turbulence Model with Improved Wall Treatment for Heat Transfer Predictions in Gas Turbines

Everything is clear, except one thing: In the standard (high-Re) formulation betaStar is constant with a value of 0.09. In the low-Re formulation betaStar1 is corrected (eqn. 20) and blended with betaStar2 (which is constant) to calculate betaStar (eqn. 10). Now, the blending function F1 (eqn. 3) itself uses betaStar within arg1 (eqn. 5).
So, basically, either I overread/misunderstand something trivial here or betaStar is blended by F1 using the belnded betaStar?!

How did you guys overcome this? Did you just use the blended betaStar from the previous iteration for the blending function?

As always, any help is appreciated.

Thanks,
Florian

[florian_krause]

I have apparently solved it by myself...

Best,
Florian

[nikola_m]

Hi! Are you still working on this? I need someone to discuss these things trough. I'm reading the same paper you cited at the momment.

Nikola