[salvoblack]

felix sorry, but I have not explained very well.
the turbulence model that I use is just the k-omega SST model.

greetings.

[aloeven]

Hi Felix,

I have a question about the correlation for ReThetac. You state that the CFX_v1.1 version is implemented according the AIAA paper of Langtry and Menter. In the source code I see that the correlation of Langtry and Menter is commented and you use a "corrected" correlation. Is this part of your own research, or is this new "corrected" correlation documented somewhere?

Furthermore, you have seem to miss something in the F1 blending function. The definition in the paper and thesis of Langtry is: F1=max(F1_orig,F3). The max is not in your implementation.

If I revert to the correlation of Langtry and Menter and include the max in the blending function, the results improve.
At 0deg AOA: Cd = 0.00579 instead of 0.0061 (experiments: 0.0055)
The simulations for higher AOA's are still running, but seem to improve even more. The lift of the original implementation is overpredicted a lot.

Alex.

[FelixL]

Hello, Alex,


wow, yes you're right. I totally forgot to add the max function when returning F1. I owe you a beer, I would've never discovered this after doublechecking the code multiple times. This makes things more interesting, I think I will rerun my old test cases with this correction. I am a bit surprised the compiler doesn't complain about returning comma separated values to a volScalarField, though.

Yeah, the "corrected" correlation is based upon my personal research. I discovered that both OpenFOAM and FLUENT lead to too early transition onset locations (see attachement 2 in this post) using the original correlation of LANGTRY and MENTER, so I recalibrated the coefficients. In said attachement you can see the original correlation (FLUENT curve) and the improved correlation (OpenFOAM curve). LANGTRY states that the correlations may have to be corrected for each solver so I assumed this correction was neccessary for OpenFOAM.

But the missing max(...) function in F1 of course changes everything. I will simulate the test cases again when I find the time and present the differences in the results.

Thanks again for finding this flaw.


Greetings,
Felix.

[FelixL]

Changelog:

    2011-02-28 : - added missing max() in the return statement of the
                   F1 blending function
                 - reverted ReThetac() to use original correlation of
                   LANGTRY and MENTER

[aloeven]

Originally Posted by FelixL

I would've never discovered this after doublechecking the code multiple times.

You're welcome. That's what this forum is for.

I noticed that something was wrong when I saw the results for higher angles of attack, they were completely wrong. I think also your results will now improve a lot.

Alex.

[FelixL]

Hello, everybody,


so I just did some of the flat plate simulations with the corrected version of the turbulence model I posted yesterday. Cf distributions along the plates for two different cases (T3A - moderate freestream turbulence intensity (FSTI), T3B - high FSTI) and as you can see the now correct F1 blending function only affects the skin friction at the transitional and turbulent regime.

In T3A the skin friction is reduced which might explain the improvement of Cd values for Alex' airfoil cases. Still, using LANGTRY and MENTER's correlation for ReThetac the transition onset location is way too early. So either there are still some errors somewhere or the correlation needs calibration.

In T3B the transition onset seems to be predicted quite well, the skin friction is much too high, though. This is a flaw of the model itself caused by the high turbulent viscosity (approx 100 times nu) in the freestream diffusing into the boundary layer. Still, the results in the paper of LANGTRY and MENTER look much better - and I don't know why.


Greetings,
Felix.

[FelixL]

It's not even that early anymore but still I forgot the attachements. So here they are.

[aloeven]

Hmm, perhaps calibrating the correlation will improve things. But indeed the difference, especialy for case T3A is quite big.

For the airfoil case, I would like to do some xfoil simulations to compare the transition location. Unfortunately, I don't have much time to spend on this the coming weeks.

[jms]

I am running some simulations right now on a thick airfoil, as soon as I will have them I will show the results in here, this should be in 1 day or 2 maximum.

[ivan_cozza]

Originally Posted by jms

I am running some simulations right now on a thick airfoil, as soon as I will have them I will show the results in here, this should be in 1 day or 2 maximum.

Hi Josè,
what's about your airfoil simulations? The new model worked well?

[jms]

Hello!

Sorry for not answering. The results I have got until now for an AoA of 8 degrees are underpredicted 8% compared to the reference I am comparing to. Be aware that I am running a thick flatback airfoil, thus it is not an "easy" airfoil to simulate.

I have thought 2 things to have a look at:
1) Please have a look at the 2nd last message in the following thread: http://www.cfd-online.com/Forums/ope...tml#post298728
2) Use the correlation factos from another autor (i.e. Sørensen--> http://onlinelibrary.wiley.com/doi/10.1002/we.325/pdf)

I have no more ideas right now. I would really like to get some suggestions of these 2 things I have posted or from other aspects you consider.

Thanks for your attention and interest.

Regards,

José

[FelixL]

Hello, José,


what physical quantity differs from literature by 8%? I assume it is the drag coefficient? If so, I think this is a pretty good start, especially for that angle of attack.

Regarding your two thoughts:

1) I'm not quite sure I understand correctly what you mean. Do you want to obtain the wall value OpenFOAM uses when applying the omegaWallFunction? It should be visible within paraView. What exactly do you want to do?

2) Yeah, of course different correlations can improve - or worsen - the results of a simulation with an empirical model. I can't access the paper but if you send me the equations/coefficients I'll be happy to implement that correlation.


Greetings,
Felix.

[jms]

Dear Felix,

Sorry for not giving information enough. The difference of 8% I told you is for both the lift and drag coefficients, so I think this difference should be lower for the lift coefficient. However, I will plot the full polar and paste it in this thread if you want.

1) I am not using wall functions for omega (i.e. I am not setting any omegaWallFunction value in the BCs files), neither for any other variable. This was just a misunderstanding of Gerard (from the other thread I pasted in here before).
What I was saying is that I run the same computation with ANSYS CFX and it gave me a different value for all the values of omega at the wall, while in OpenFOAM what I do is that I fix it to a value (based on the equation of w_wall of the following link --> http://turbmodels.larc.nasa.gov/sst.html). Thus, if I could set it to a "calculated" value somehow maybe I would get better results. Another thing that I am thinking is that maybe ANSYS CFX gives as values at the wall, values from the 1st cell....

2) I cannot attach the paper of Niels Sørensen because of its size, so tell me your e-mail address and I will send it to you by e-mail (if you don´t want to do so in here, just send me an e-mail to the address I have sent to you in a private message). I was thinking on implementing it my self but since I am not experienced with C++ it can take a while to implement it. But if you do it and upload it I will test it on this airfoils and tell you the results, of course.

What do you think about everything I have told you?


Thank you again for all your help.

Regards,

José

[FelixL]

Hello, José,


sure, go ahead and post the polar. It would be interesting, too, to compare the polar with results obtained with the k-Omega SST model, if you have that data available.

So you use fixedValue for omega at the wall using the equation proposed by Menter? This is correct, yes, but only if your first cell spacing over the airfoil surface is constant! I recommend the usage of omegaWallFunction - this BC automatically calculates the correct value and you don't have to bother with manual calculations. OmegaWallFunction is valid for the whole y+ regime. You can find some more infos about near wall values of omega here:
http://www.cfd-online.com/Forums/ope...tml#post295498

I don't know about CFX, maybe the software uses different omega values. Maybe you can find information about that in the manual?


Greetings,
Felix.

[FelixL]

I added the correlation of Sorensen (2009). If you want to use it, please uncomment it in the source code.

Changelog:

    2011-03-10 : - added ReThetac() and Flength() correlations of
                   SORENSEN (2009)

The results of my testcases didn't really improve (actually they got worse) when using this correlation (see attached PNG), but maybe you're luckier!


Greetings,
Felix.

[aerothermal]

FelixL,

Have you seen the reference of Tue in a older post?
http://www.cfd-online.com/Forums/ope...tml#post233057

He mentioned a paper that has some corrections to Langtry-Menter model. It may be another option for you.

By the way, just a final question, do you know how to fix the transition onset and end points in OpenFOAM? I am simulating a rough cylinder case and there is no correlation or prediction model for that. However, I can still use the intermittency concept (\gamma).

In a simple algebraic transition model, gamma is multiplied by the turbulent viscosity. Where gamma is zero, the flow is laminar. Where it goes to unity, the flows becomes turbulent. There are several models of intermittency function such as Narasimha, Reynolds/Kays/Kline or Abu-Ghannam & Shaw. This is not a problem. However, I got stuck in calculating the distance or Reynolds along a surface. Is this possible in OpenFoam?

Regards,

Guilherme da Silva

[taxalian]

hi Felix,
i am testing your gammaReTheta model in O.F. 1.7.1 with T3A test case and i am not able to get the skin friction magnitude/vector neither the yplus. I always get zero values for every time step. I wonder their is something wrong, may be you can help me. Looking forward to your reply.

regards,
taxalian.

[jms]

Dear all,

I am doing a master thesis using OpenFOAM to compute the flow around thick flatback airfoils. I have run some simulations in steady state until now.
Some time ago I promised I would upload my results obtained using the transition model Felix L. implemented in OpenFOAM.

This is what I am going to do in the next messages. You are very welcome to ask questions to me if necessary.

Regards,

José

[jms]

Results for a NACA0012 with a blunt trailing edge (4% of the chord is cut off). The reason why I do this is to have a mesh topology closer to the one I am using later on the thesis (which is about thick-flatback airfoils).

Comparison for the cases with and without transition for the boundary layer with OpenFOAM.

TWS=data from the book "Theory of Wing Sections"

geom_NACA0012blunt.png

FREE_FULL_OF.png

FREE_FULL_OF_CD.png

[jms]

Comparison for the cases run with OpenFOAM and ANSYS CFX. Exactly the same mesh and BCs.
FREE_OF_CFX_CL.png

FREE_OF_CFX_CD.png