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-   -   Transition Model - Gamma Re Theta (http://www.cfd-online.com/Forums/star-ccm/86628-transition-model-gamma-re-theta.html)

nomad March 28, 2011 15:34

Transition Model - Gamma Re Theta
 
Hi all,

I would like to know what your experiences with the Gamma Re Theta transition model in STAR CCM+ have been like for airfoils at really high angles of attack near the stall region.

The transition model in STAR CCM+ underpredicts lift forces for an airfoil section for angles of attack that are near the stall region due to early separation. I am interested in controlling flow separation so that this separation is delayed and better correlation with experimental results can be made at high angles using this model.

Does anyone know how the field functions for the F_length and Re_crit correlations for the Gamma Re Theta model can be written in order to define the transition point along the surface of an airfoil?

Also, if you have been able to predict stall accurately for multi-element airfoils(2D or 3D to within <5%) with any of the models in STAR CCM+, I would be very interested in knowing how this was done.

Thank you.

nomad March 28, 2011 17:40

I would like to add that my mesh is fine enough, with a wall y+ <1 everywhere, with a dense wake refinement region, and lift and drag coefficients within 5% of experimental data for all angles except for within 2-3 degrees of stall.

nomad March 28, 2011 18:01

Hidden away in the documentation, here it is for Re_crit and f_length for the modified option:

min(300,exp(7.168 - 0.01173*$TransitionReTheta) + 0.5)
min($TransitionReTheta, $TransitionReTheta*0.615 + 61.5)


and


(100*$TurbulenceIntensity > 1.3) ? 331.5*pow(100*$TurbulenceIntensity - 0.5658, -0.671) : (1173.51 - 589.428*100*$TurbulenceIntensity+0.2196/(10000*$TurbulenceIntensity*$TurbulenceIntensity))



for the user-defined option.


References can be found in the following paper:



Calibrating the gamma-ReTheta Transition Model for Commercial CFD, by Paul Malan.



I would still like to know if anyone's been able to model wing stall accurately with any of the models in STAR CCM+.


Thank you.

f-w March 28, 2011 18:32

I can't answer your question from experience, since I haven't run any transition models; however, I was planning on doing so, and have done some research on it.

First of all, if you're interested in the paper you referenced, it can be found here:
http://cfd.mace.manchester.ac.uk/twi...a-re-theta.pdf

Also, you will find of interest the following link to high lift prediction workshop papers:
http://hiliftpw.larc.nasa.gov/Workshop1/asm2011.html
(the Ansys CFX paper used the same transition model and had bad agreement at high alpha too, as you did)

nomad March 28, 2011 22:34

Thank you for the workshop presentations, they're a valuable resource.

Even though most of these guys are using custom codes (with mesh densities of around ~20-100 million cells), some of the modifications they employ at high flap settings might shed some useful insight on the right approach.

screech1987 April 17, 2011 08:34

I seem to rememeber that the default Re_theta model has a number of co-efficients that can be altered at the users discretion. I think one of these holds some sway over transition location, while not having to resort going ground up with a UDF. I think it may be 's1', but don't hold me to that.


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