k-kl-omega not producing any transition

Kina · April 26, 2016, 15:06

Hi all!
I am currently investigating a thick flatback airfoil using the k-kl-omega model as fully turbulent and even gamma-ReThetat failed to reproduce the experimental lift curve. The k-kl-omega converges. However, opposed to the experiment that shows flow separation at AOA=12, the simulation doesn't show any separation, even at AOA=20.
My kT and omega are set according to the initial turbulence parameters eqns given here: http://www.cfd-online.com/Wiki/Turbu...rbulent_energy

As my Tu is about 0.05%, I am dealing with natural transition with k at something about 1e-4 m2/s2 and omega around 14/s.
This is correct according to Walters' initial article presenting the k-kl-omega model.

I have set kt = kl = 1e-12 at walls, kl = 0 in freestream, and w = zeroGradient at walls. My y+ is around 0.75 and my boundary layer aspect ratio is approx. 800.

Is there any way to tweak the k-kl-omega model other than increasing inlet freestream turbulence? I have read many articles concerning the model but nothing helped me in getting this model to predict some kind of turbulence.

Cheers
Alex

malv83 · June 17, 2016, 15:05

Hi, if you tweak the model, then the model will not be reliable for other test cases... it is a common practice to change the inlet turbulent intensities to get a faster transition if we intend to simulate turbulent flows..

I want to introduce you to a new turbulence model, a new version of the k-kl-omega model, which is more reliable and with a wider range of applications. it is the k-v2-omega model, it is a transitional model developed as an alternative to the k-kl-omega model.. the model was developed by Walters group (same developers of k-kl-omega). the model is explained in:

Maurin Lopez. D. K. Walters. “Prediction of transitional and fully turbulent free shear flows using an alternative to the laminar kinetic energy approach”. Journal of Turbulence, Vol 17, Iss. 3, 2016.

The recommendation is to use this new model from now on instead of the k-kl-omega model.

Also, if you see the paper, check the Jet flow test case in the paper, The flow is fully turbulent, but the inlet TI is low 0.5%, so what they do in this case, is to run a fully turbulent model first and then use those values as new values for the transitional model, that way, the transition occurs quickly.

If your case include separation, then definitely you should use this new model.

April 26, 2016, 15:06	k-kl-omega not producing any transition	#1
Kina Senior Member Alex Join Date: Jan 2014 Posts: 126 Rep Power: 12	Hi all! I am currently investigating a thick flatback airfoil using the k-kl-omega model as fully turbulent and even gamma-ReThetat failed to reproduce the experimental lift curve. The k-kl-omega converges. However, opposed to the experiment that shows flow separation at AOA=12, the simulation doesn't show any separation, even at AOA=20. My kT and omega are set according to the initial turbulence parameters eqns given here: http://www.cfd-online.com/Wiki/Turbu...rbulent_energy As my Tu is about 0.05%, I am dealing with natural transition with k at something about 1e-4 m2/s2 and omega around 14/s. This is correct according to Walters' initial article presenting the k-kl-omega model. I have set kt = kl = 1e-12 at walls, kl = 0 in freestream, and w = zeroGradient at walls. My y+ is around 0.75 and my boundary layer aspect ratio is approx. 800. Is there any way to tweak the k-kl-omega model other than increasing inlet freestream turbulence? I have read many articles concerning the model but nothing helped me in getting this model to predict some kind of turbulence. Cheers Alex

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June 17, 2016, 15:05		#2
malv83 Member Alberto Join Date: Sep 2013 Posts: 37 Rep Power: 12	Hi, if you tweak the model, then the model will not be reliable for other test cases... it is a common practice to change the inlet turbulent intensities to get a faster transition if we intend to simulate turbulent flows.. I want to introduce you to a new turbulence model, a new version of the k-kl-omega model, which is more reliable and with a wider range of applications. it is the k-v2-omega model, it is a transitional model developed as an alternative to the k-kl-omega model.. the model was developed by Walters group (same developers of k-kl-omega). the model is explained in: Maurin Lopez. D. K. Walters. “Prediction of transitional and fully turbulent free shear flows using an alternative to the laminar kinetic energy approach”. Journal of Turbulence, Vol 17, Iss. 3, 2016. The recommendation is to use this new model from now on instead of the k-kl-omega model. Also, if you see the paper, check the Jet flow test case in the paper, The flow is fully turbulent, but the inlet TI is low 0.5%, so what they do in this case, is to run a fully turbulent model first and then use those values as new values for the transitional model, that way, the transition occurs quickly. If your case include separation, then definitely you should use this new model.