Langtry-Menter transition modell issue?

April 22, 2006, 03:34

Using CFX 10.0.1, SST turbulence model with Langtry/Menter correlation (Gamma Theta Model) at default settings on a NACA 63-012 profile at alfa = 4 deg, Re=6500000.

I get a very strange result where the transition onset is jumping around every second or third grid point. I have refined my mesh to be well inside every specification discussed in the Modelling Manual p103-111. Increasing the y axis mesh density does not make the problem go away, it only compresses it

The images below is from a mesh with:

y+ around 1-2

90 streamwise nodes

Normal expansion 1.2

30 layers in the boundary mesh

(for reference XFoil says transition is at x/C = 0.65 for this case)

Any ideas what causes this weirdness?

/C

kdrbrk · June 8, 2010, 12:18

as far as I know, NACA 63 series gives laminar flow to %30 of the chord (the 3 after the 6)
For %65 laminar flow, try NACA 66 or 67 series.

ghorrocks · June 8, 2010, 19:37

The behaviour described above is typical of turbulence transition models. Just like real turbulence transition points they move around and advance and retreat on the surface. In my experience you frequently have to run it 3D and transient even though the flow looks 2D and steady.

Will Anderson · July 3, 2013, 11:41

Has anyone got the Transition-SST model to capture purely laminar flow, before the onset of transition? Particularly on perfectly round objects like cylinders or spheres?

I cite here a figure from Langtry's own thesis, in which the laminar region doesn't appear to have been accurately computed, encircled in blue:

Langtry does write:

"It is possible that smaller time steps are required to properly
capture the drag coefficient.. However, what is more likely is that a hybrid RANS-LES approach such as Detached Eddy Simulation (DES) will be required to predict this case correctly."

Has anyone done that?

ghorrocks · July 3, 2013, 20:09

It has been a while since I did this sort of stuff but I do not recall a problem in the laminar region. I cannot cite and reference for this, just a vague recollection of ancient history.

Jonathan · September 13, 2015, 09:44

Hi Will,

i know this is an old post but ... you published figures from Langtry's thesis. I was able to get a partial pdf copy (excluding chap 6 -> onwards) from the Stuttgart Uni site, but cannot find a full copy with the 3D validation cases / references / appendices etc.

I was hoping you might have this and wouldnt mind passing it on if you did?

many thanks and regards
jonathan

April 22, 2006, 03:34	Langtry-Menter transition modell issue?	#1
Chebeba Guest Posts: n/a	Using CFX 10.0.1, SST turbulence model with Langtry/Menter correlation (Gamma Theta Model) at default settings on a NACA 63-012 profile at alfa = 4 deg, Re=6500000. I get a very strange result where the transition onset is jumping around every second or third grid point. I have refined my mesh to be well inside every specification discussed in the Modelling Manual p103-111. Increasing the y axis mesh density does not make the problem go away, it only compresses it The images below is from a mesh with: y+ around 1-2 90 streamwise nodes Normal expansion 1.2 30 layers in the boundary mesh (for reference XFoil says transition is at x/C = 0.65 for this case) Any ideas what causes this weirdness? /C

July 3, 2013, 11:41	laminar flow using turbulent transition model?	#4
Will Anderson Member W.N. Anderson Join Date: Jul 2010 Location: Zürich, Switzerland Posts: 46 Rep Power: 16	Has anyone got the Transition-SST model to capture purely laminar flow, before the onset of transition? Particularly on perfectly round objects like cylinders or spheres? I cite here a figure from Langtry's own thesis, in which the laminar region doesn't appear to have been accurately computed, encircled in blue: Langtry does write: "It is possible that smaller time steps are required to properly capture the drag coefficient.. However, what is more likely is that a hybrid RANS-LES approach such as Detached Eddy Simulation (DES) will be required to predict this case correctly." Has anyone done that?

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June 8, 2010, 12:18		#2
kdrbrk Member Burak Join Date: May 2009 Posts: 90 Rep Power: 17	as far as I know, NACA 63 series gives laminar flow to %30 of the chord (the 3 after the 6) For %65 laminar flow, try NACA 66 or 67 series.

June 8, 2010, 19:37		#3
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,871 Rep Power: 144	The behaviour described above is typical of turbulence transition models. Just like real turbulence transition points they move around and advance and retreat on the surface. In my experience you frequently have to run it 3D and transient even though the flow looks 2D and steady.

July 3, 2013, 20:09		#5
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,871 Rep Power: 144	It has been a while since I did this sort of stuff but I do not recall a problem in the laminar region. I cannot cite and reference for this, just a vague recollection of ancient history.

September 13, 2015, 09:44		#6
Jonathan Senior Member Join Date: Mar 2010 Posts: 181 Rep Power: 17	Hi Will, i know this is an old post but ... you published figures from Langtry's thesis. I was able to get a partial pdf copy (excluding chap 6 -> onwards) from the Stuttgart Uni site, but cannot find a full copy with the 3D validation cases / references / appendices etc. I was hoping you might have this and wouldnt mind passing it on if you did? many thanks and regards jonathan