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Calculation of Pressure Loss

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Old   May 22, 2008, 06:37
Default Hey Niels, That's to bad, I
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Bart Boonacker
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Hey Niels,

That's to bad, I am also working on my master thesis, but I only have OF
Thanks anyway for the quick answer.

Bart
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Old   May 26, 2008, 10:28
Default Hi Bart, If your geometry i
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Jean-Luc Pelerin
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Hi Bart,

If your geometry is a "simple" extrusion of a 2D profile, it should not be too difficult to get good results. But if the profile changes periodically along the axis it may become much harder.

I do not use directMappedPatch anymore but cyclic boundary conditions associated with a source term in the momentum equation. I think it is the best way to compute a pressure drop.

Best regards,

Jean-Luc
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Old   June 18, 2008, 05:33
Default Hi Jean-Luc and Bart I know
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Niels Gjoel Jacobsen
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Hi Jean-Luc and Bart

I know that you both have been working with kOmegaSST, and I have just begun reading the source for the turbulence module, and something is quite strange.

Basicly it is line 265 in kOmegaSST.C. In this line it is stated that if the production of turbulence is larger than c1 times the dissipation of turbulence, then the production is limited to c1 times the dissipation. Is this some kind of numerical issue, which needs to limit the production?

Hope you can help,

Best regards,

Niels
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Old   June 23, 2008, 09:05
Default Hi Niels, Accordind to http
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Jean-Luc Pelerin
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Hi Niels,

Accordind to http://www.cfd-online.com/Wiki/SST_k-omega_model the term is from the original model and not OpenFOAM specific. But I have no idea of why this term is used.

Sorry I can't help you, you should probably contact the author of the OpenFOAM implementation of SST...

Jean-Luc
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Old   June 23, 2008, 09:46
Default Thanks Jean-Luc I have made
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Niels Gjoel Jacobsen
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Thanks Jean-Luc

I have made an implementation without the wall functions, i.e. in the way we discussed above, as I was not fond of the results with the standard implementation. I have begun testing it, and it looks good. As I have removed the wall function I have also removed the limitation on the production term, as I think it is the reason for the limit. Have ordered the article by Menter, thus will soon know explicitly.
Further I am also testing a body-force correcter, which should help you approach a certain flux faster.

/ Niels
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Old   June 26, 2008, 02:54
Default Hi Niels, That is a very go
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Hi Niels,

That is a very good news!!!

Do you plan to implement a roughness model?
Do you plan to share it?
Do you need help?

Thank you,

Jean-Luc
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Old   June 26, 2008, 04:05
Default Hi Jean-Luc > Do you plan t
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Hi Jean-Luc

> Do you plan to implement a roughness model?
I have already modified kOmegaSST -> kOmegaSSTRough, i.e. removed all functionality with regard to the wall functions and implemented two different boundary conditions for omega: Wilcox (2006) and Roulund (2005).
> Do you plan to share it?
Actually had not considered that point yet, but yes.
> Do you need help?
For the testing phase, I would indeed appreciate some help. I will return to you, when I think that stage is reached.

/ Niels
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Old   June 26, 2008, 05:17
Default Niels, Sorry for my first q
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Niels,

Sorry for my first question that didn't make any sense... Of course it is a roughness model!

I would be glad to test it. I can run it on pipes and on our geometry.
As it is confidential I won't be able to give the cases set up but I will be able to give you an error.

Jean-Luc
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Old   June 26, 2008, 07:38
Default Hi Jean-Luc This is my work
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Hi Jean-Luc

This is my working version, but haven't started the tested yet. I have implemented two versions of a omega-BC. The first is the one given in Wilcox (2006) and the second is a version by Roulund (2005).

I have added two parameters two the turbulence-list. Namely d50, which is the median grain diameter of the roughness, and omegaBC, which can take the values as seen below in the coeffs list.
The present boundary condition should be working for both rough, transitional and smooth boundaries (Wilcox (2006)), where d50 just need to be chosen to be some appropriate value for smooth boundaries.

As we have previously discussed the boundary condition on k needs to be considered as a function of the roughness in wall coordinates, but as of now there is no generic implementation, i.e. still controlled by /0/k.

I'll be looking forward to hear about your experiences. If any questions, etc, you can also reach me by email.

/ Niels

Place the files in ~/src/turbulence/incompressible and compile with the other turbulence-schemes.
kOmegaSSTRough.tar.gz

To be copy-pasted into turbulenceProperties:

kOmegaSSTRoughCoeffs
{
alphaK1 alphaK1 [0 0 0 0 0 0 0] 0.85034;
alphaK2 alphaK1 [0 0 0 0 0 0 0] 1.0;
alphaOmega1 alphaOmega1 [0 0 0 0 0 0 0] 0.5;
alphaOmega2 alphaOmega2 [0 0 0 0 0 0 0] 0.85616;
gamma1 gamma1 [0 0 0 0 0 0 0] 0.5532;
gamma2 gamma2 [0 0 0 0 0 0 0] 0.4403;
beta1 beta1 [0 0 0 0 0 0 0] 0.0750;
beta2 beta2 [0 0 0 0 0 0 0] 0.0828;
betaStar betaStar [0 0 0 0 0 0 0] 0.09;
a1 a1 [0 0 0 0 0 0 0] 0.31;
c1 c1 [0 0 0 0 0 0 0] 1000;

Cmu Cmu [0 0 0 0 0 0 0] 0.09;
d50 d50 [0 1 0 0 0 0 0] 0.001;
omegaBC wilcox2006; //roulund2005
}
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Old   June 26, 2008, 08:03
Default Niels, Thanks a lot. I will
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Niels,

Thanks a lot. I will start working on it now :-)

Jean-Luc
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Old   June 26, 2008, 08:18
Default BTW: forgot to mention that th
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BTW: forgot to mention that this procedure is highly y+ dependent, i.e. U_f deltaY_wall / nu should be around 1 (at least).

/ Niels
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Old   September 19, 2008, 15:34
Default There is a method proposed by
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There is a method proposed by Hellsten to reduce the y+ dependency of the results using either Wilcox's or Menter's appraoches of the k-omega model.

I am attaching the document here.


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Old   September 19, 2008, 15:37
Default Hi, There is a method propo
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Hi,

There is a method proposed by Hellsten to reduce the y+ dependency of the friction coefficient using either Wilcox's or Menter's approaches of the k-omega model.

You can view the full report online: http://www.aero.hut.fi/pubs/reports/B50_abstract.html

Also, I am trying to implement this B.C. in the current kOmegaSST model of OF.


regards,

-Louis
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Old   September 23, 2008, 03:30
Default Hi Nils, nice work! Though
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Hi Nils,

nice work! Though I have a question about the omega implementation. As I understand it, you modified compared to the OpenFoam version you modify all wall-patches in a averaged (using patchi) way and do not access the each 'faceCelli' on these wall separetly. Is there any reason for this?

Fabian
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Old   September 23, 2008, 04:59
Default Hi Fabian If you consider e
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Hi Fabian

If you consider e.g. omegaWilcox.H, then I loop over all boundaries. Those which are wall-type are further investigated.

In either I work with fields, thus I calculate the individual shear stress in either boundary face cell and as you will se, both wallStress and Sr are scalarFields, thus the values set to the boundary is face-specific.

If this does not answer you question, please elaborate on your uncertainty.

Further I would definitely recommend Wilcox over Roulund, as some small investigation shows that Roulund is unreliable.

Further the report given by Louis gives another way around, but I do not like the modified roughness model, as my intuition tells me that it will not give Reynolds independent results in the limit Re->inf.

Best,

Niels
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Old   September 23, 2008, 05:00
Default Even though a proof would be s
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Even though a proof would be significantly better than intuition
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Old   September 23, 2008, 05:41
Default Hi Niels, (sorry for the missp
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Hi Niels, (sorry for the misspelled name!)

thanks; though I do not understand it...maybe I do not get the implementation of the OpenFOAM SST model. There one loops over all patches (mesh_.boundaries) and if is of type wall, one further loops over each cell of theses wall!?

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Old   September 23, 2008, 07:18
Default Hi Fabian If the patch is a
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Hi Fabian

If the patch is a wall, it is correct that further computations are done.

E.g. "vectorField uGrad" means that the velocity gradient normal to the wall is computed in _every_ face on the patch in question. Thus if there are a 100 faces on that patch, uGrad will consist of 100 vectors. All computations, except for Sr is done in that way, i.e. working on fields instead of individual faces.

The forAll(kPlus,kI) then loops over all faces, because I at that time didn't use Foam::pos as extensively as today. The use of Foam::pos would actually make the loop unnecessary.

Best,

Niels
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Old   September 23, 2008, 10:07
Default Hi Niels, Nice to know you
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Hi Niels,

Nice to know you were interested in the report! What I will actually try to implement from this report is the modification to Menter's method (basically changing the wall estimate of the omega value by a factor of 1.5/10), and that will, at the same time, help me understand how the 1.5 version of the kOmegaSST model is implemented (because it has wall functions that force the use of zeroGradient conditions).. I'll keep you posted when I have something coded.

Cheers,

-Louis
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Old   September 23, 2008, 11:09
Default Hi Louis The forcing of zer
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Hi Louis

The forcing of zeroGradient is basically removed by deleting the files in the kOmegaSST, which makes that exact inquiry.

It is done in my own implementation above, if that could be of any help.

/ Niels
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