[seb62]

Hi all,

I was working on a turbulent flat plate with the
Spalart Allmaras model, and the results were not fine.
So I had a look at the source code and I think there is something wrong.
The version implemented in openFOAM 1.5 is the Spalart Allmaras with fv3 function. This version
also requires to modify fv2, which is not done (the correct fv2 consistent with the use of fv3 is on the commented line) :


tmp<volscalarfield> SpalartAllmaras::fv2
(
const volScalarField& chi,
const volScalarField& fv1
) const
{
return 1.0 - chi/(1.0 + chi*fv1);
//return 1.0/pow3(scalar(1) + chi/Cv2);
}

Thus I think either the commented fv2 should be used keeping fv3 unchanged, or fv3 should be set to 1, keeping fv2 unchanged.
For testing, I did the latter, removing fv3 in the expression for Stilda, which gives the basic form of Spalart Allmaras model :

volScalarField Stilda =
sqrt(2.0)*mag(skew(fvc::grad(U_)))
+ fv2(chi, fv1)*nuTilda_/sqr(kappa_*d_);


After recompiling, the velocity profiles and local friction agree very well with theory and other CFD codes.

Seb

[henry]

Thanks for studying this code in detail and for the bug report, yes you are correct, this is currently inconsistent. We originally implemented the standard SA model and it worked well for simple cases but when we moved to complex problems, in particular F1 cars, it proved unstable. We then implemented the "correction" from

"An Unstructured Grid Generation and Adaptive Solution Technique for High Reynolds Number Compressible Flows"
G.A. Ashford,
Ph.D. thesis, University of Michigan, 1996.

and this proved much more stable. However it looks like while switching between the models at some point the fv2 has been left inconsistent with fv3. Could you please check if you get the correct behavior with the alternative fv2 and fv3?

Thanks

H

[seb62]

Yes I will check with the fv3 version.
My test cases are however simpler than F1 cars (flat plate and backward facing step).

The fv3 version seems to be corrected for low Re flows, at least from this source :

http://turbmodels.larc.nasa.gov/spalart.html

Thanks for your comment on SA instability.

Seb

[eugene]

The LES versions of the Spalart-Allmaras model do have the correct implementation and exhibit what appears to be the correct behaviour.

Eugene

[henry]

Hi Sébastien,

Thanks for the interesting link, it seems they are not very happy with the fv3 term: " It was devised to prevent negative values of the source term, and is not recommended because of unusual transition behavior at low Reynolds numbers (see Spalart, P. R., AIAA 2000-2306, 2000)." but the source term going negative does happen in complex flow cases and causes nuTilda to go negative! so we need a fix of some kind. It is not clear if any of the alternative formulations are preferable, perhaps the SA-Edwards or SA-salsa are better; at least they both have a well-posed generation term.

H

[elorriaux]

Thanks Sébastien,

I was also getting unbelievable results on a train drag computation case with SpalartAllmaras. Since the results are satisfaying with a RNGkEps model, I was also thinking to look after the SA coeffs. You found the problem before me, thanks ;)

I'll try to run the case again as soon as I have free CPU time.

Bye, Etienne.

[henry]

I have pushed the change to fv2 to OpenFOAM-1.5.x, please let me know if this resolves the issues with this model.

Thanks

H

[egp]

I agree that the fv3 term needs to be blitzed. I did a little study of this about a year ago. Here are a couple of figures to highlight the effect of turning this term off.

[henry]

In these comparisons did you check that the fv2 term was set consistently for the cases with and without the fv3 term? The initial comparisons we did after introducing the fv3 term showed very similar results to the standard model but with improved stability which is why we kept it. Unfortunately at some point since then the fv2 has been set inconsistently with the fv3 term; this is now fixed in 1.5.x.

I would be more than happy to drop the fv3 term if a form of the SA model can be found which does not suffer from the stability problems related to negative production in the original model. Has anyone tested the many variants? They are pretty easy to implement and I would be happy to add the promising ones to the set of turbulence models in OpenFOAM if there is interest.

H

[egp]

Henry,

These plots were made doing exactly what Sebastien suggested in the original post of this thread, i.e., setting fv3 = 1, and keeping fv2 as implemented. This should give the original SA formulation.

Around the same time, I did a study of ALL of the RANS turbulence models for turbulent flat plate boundary layer, and found that most of them give poor results.

Here is a plot of some of them.


SA with fv3=1 and LaunderSharma give pretty good results. LienCubicLowRe (including a few mods that I tested) gives horrible results. kEpsilon with wall functions is OK, as long as user is careful to avoid outer part of buffer layer (cf. y+ 30 vs. 115).

The take away points from this study were:

1. be very careful if accurate prediction of wall-shear stress (and drag) is required. SA and LaunderSharma seem to do the best. Menter's kOmegaSST (no wall functions) that was out on the wiki also does a pretty good job (has that been adopted in 1.5?).

2. just because there are models in OpenFOAM, doesn't imply that they have been validated for your problem. BUYER BEWARE....

3. OpenFOAM sorely needs adaptive wall functions which can seemlessly transition from sublayer resolution to wall-function resolution.

Eric

[egp]

[egp]

[henry]

When you ran with fv3 != 1 could you clarify which form of fv2 you running with?

H

[egp]

Damn it, the image posting gremlins are back...

[egp]

tmp<volscalarfield> SpalartAllmaras::fv2
(
const volScalarField& chi,
const volScalarField& fv1
) const
{
return 1.0 - chi/(1.0 + chi*fv1);
//return pow(scalar(1) + chi/Cv2, -3);
}

[henry]

That form is correct for fv3 = 1 but to run with fv3 != 1 you will need to run with the second expression commented out, see the original post. If you have time to test this that would be great.

H

[egp]

Sure, I'd be happy to test it. I have all of the files, so it should be quick. I'll post results later today.

[paulo]

Hi All,

Maybe this is a stupid question, but I won't sleep today if I don't ask ;-)

I've read somewhere in this forum that the OF solvers (simpleFoam in particular) only have turbulence models with wall functions.

Is it true, i.e., my solution must have y+ about 30, or can I have smaller values with the viscous sublayer modeled?

Thanks in advance,

Paulo Rocha

[seb62]

I ran simulations for the fv3 version of SA, on 3 flat plate meshes (y+ around 0.2 ; 1 and 25). Reynolds number at the end of the plate is 4.10^6. There is hardly no difference compared to the basic version, except that close to the inlet, the local friction goes to a minimum before rising and going close the theoretical curve. With the basic version, the local friction keeps on decreasing, as the theoretical one does.

By the way, Etienne, as you say the RNG k epsilon gives good results, could you tell me which type of mesh you use at the walls (fine or wall-function type mesh)?

Thanks

Seb

[henry]

Not true, there are several low-Re turbulence models included in OpenFOAM:

LamBremhorstKE
LaunderSharmaKE
LienCubicKELowRe
LienLeschzinerLowRe
SpalartAllmaras

all of which will run without wall-functions. With SpalartAllmaras wall-functions are optional and should be used where the mesh is not fine enough to resolve the boundary-layer.

H