[OpenFOAM-2.1.0] kklOmega RAS Turbulence Model (low Re)
 

Hello everybody.

As referenced in the announcement of the new release version of OpenFOAM a new turbulence model has been implemented:


I have been looking for some additional information about it in the user guide but it doesn't appear. I just want to highlight that issue to discuss it and ask if someone has test it. As soon as I check the model I'll report it.

Since I'm not an expert.... Is it an "automatic near wall teatment" (like the near wall treatment implemented in ANSYS CFX)?...then It resolves the viscous layer near the wall (mesh refinement is needed)?

Thanks

Boas Alquimista (Or hi Alquimist :D)

I'm also going to begin using the transition model, I was also expecting to find some clarification about this in a tutorial or in any reference besides the one given in the openFoam site which is referencing an article of ASME.

The thing is, I suspect that the boundary conditions for "kt" and "w" are similar to a regular k-w model, though the boundary conditions for kl, since it represents the laminar fluctuations kinematic energy should be set to zeroGradient at the walls , uniform zero in the domain (for initial conditions), fixed value uniform zero at the inlet and zeroGradient at outlet.

My thoughts of course :D..

Best regards

RuiVO

Let's jointly setup a reference case...
 

Hello Gentlemen,

let's join forces and setup a reference case using the kklomega model.

How about a 2D case for a wing?

I'd prefer a case based on a stl file so it can be adapted for different geometries.

Interested?

Klaus

Boas RuiVo (or Hola jejeje)

It's nice to see someone else interested in it. By now I needn't (or I can't) use this model since I realised that my case is fully turbulent but I'm interested on the results of the model.

In my first topic I talked about the near wall treatment but I was wrong mixing the turbulence model itself with the boundary conditions specified for the wall treatment.

I think it's a good idea suggested by klausb. I propose to use the object shown in the anouncement

http://www.openfoam.org/version2.1.0/img/kklOmega.png

It would be nice to use a geometry present in some tutorial. I'm trying to find something appropiate.

Anyone knows how to set the boudndary condition for kl and kt?

For now I just copy k file and then rename to kl and kt.

I am running a square case, and so far , the strouhal number, the mean and RMS value of force coeff look quite good!

would you mind sharing your case
 

Hello,

would you mind sharing your case (post your case directory).

I'd like to play with it over the weekend.

Klaus

According to the article presenting the model, you should use kl=kt=0 and omega =zeroGradient at walls. At the inlet, k=0 and kt and omega are similar to a k-w model so based on desired inlet turbulence intensity and viscosity ratio.

Thanks, that's exactly what I have set right now.

But just for a future discussion.

I have done a lot of tests, and generally
I found kklOmega works well when it can work. But I found it do have convergence problems, especiall kl will grow very large within vortices.

Relatively, I found other LRN models are more stable.

I would vote for two test cases: 1) a classic zero-pressure gradient flat-plate boundary layer (Figs. 2-6 of Walters and Cokljat) and 2) S809 wind-turbine airfoil (Fig. 15-16 of Walters and Cokljat).

I have meshes for both, and will try to reproduce the results in this paper. If I am successful, I post case files. If not, I'll post questions and case file and we can work on this as a group.

And also a reminder, that I found the following utility is needed for using these kind of LRN turb model.

Yes, after my code skipped the time =0, it became convergent very well.

In addition, lakeat, you mean all LRN models can also solve transition flows (from laminar to turbulence), right?

1. Turbulence;
2. (Re-)Attached or Separated Turbulence;

And IMO, I think the three-equation model still needs extensive tests, and it needs to be bounded.

(Chinese) 新年快乐！

Hi 老魏, 春节快乐！ :)

I want to find the model to simulation the transition flow, I guess kklomega model is good for it. However, you think, which scheme should be chose to bound the computation values? I find, in OF, almost all schemes are bounded schemes. What about your thought?

Sandy

Correction: I dont know if it really needs to be limited. I am struggling testing with some other convection schemes.

Yes, lakeat, except Guass upwind, I will change my code schemes into bounded schemes ...

I've set up the Schubauer and Klebanoff case for natural transition (as opposed to the ERCOFTAC bypass transition cases). I'm getting closer on getting kkLOmega to work, but the results still do not look good. Here is the Cf vs. Rex plot:

http://dl.dropbox.com/u/2182201/kkLOmega.png

You can download my case from here:

http://dl.dropbox.com/u/2182201/kklOmega.tgz

Note, freestream parameters are set as follows:
U = 50.1 m/s
Tu = 0.3
kt = 0.03388 m2/s2
kl = 0 m2/s2
nut/nu = 1
nu = 1.5e-5 m2/s
omega = 2259 1/s

wall boundary conditions are set to:
kt = 0 m2/s2
kl = 0 m2/s2
omega = zeroGradient

I'd be interested to see how others would try to set this case up.

OK, found a mistake in the 0/nut. wall b.c. for nut was incorrectly set from a fully turbulent case to nutWallFunction, and is now set to zeroGradient. This improves the solution, but laminar Cf and transition location are still off quite a bit.

http://dl.dropbox.com/u/2182201/kkLOmegaCf.png

Hi egp, I guess, you gave wrong the kt value. How did you calculate the kt and omega values?

Why would you guess this? The Schubauer and Klebanoff (S&K) case is a well-known experiment with very low free-stream turbulence, i.e.,

U=50.1 m/s
Tu = 0.3%
nut/nu = 1

Given these parameters, I compute kt and omega to be:

kt = 1.5*(0.003*50.1)^2 = 0.0339
omega = k/nut = 0.0339/1.5e-5 = 2259

It's interesting to note that the paper by Walters and Cokljat (2008) did not use the S&K dataset for validation and only used the ERCOFTAC T3A-, T3A, T3B benchmarks. These latter cases have a varying degree of free-stream turbulence and are typically used to demonstrate bypass transition. In contrast, the S&K with low FST is more a test of natural transition.

Dear Eric,

Very interesting test case. did you try different mesh refinement to see if it has a large impact on the solution? spanwise and wall refinement?

Actually, I tried yesterday night for a few times, but got a worse curve.:D

Hi egp, what are the differences between natural transition with bypass transition? To natural transition, the flow features are just in laminar flow but not from laminar to turbulent, right?

In addition, do you have literatures about the ERCOFTAC T3A-, T3A, T3B benchmarks and the S&K' s ? Could you sent a copy to me? I have the paper by Walters and Cokljat, however, I could not find the literature of their test cases. Thanks.

Sandy
sandy.lee37@gmail.com

Hi, I'm trying to use the kklOmega turbolence model to simulate the flow around a 2d naca 0012; here my initial/boundary conditions:

domain:

k
inletOutlet (5.4e-3)
kt
inletOutlet (5.4e-3)
kl
inletOutlet (0)
nut
inletOutlet (1e-5)
nuTilda
inletOutlet (1e-5)
omega
inletOutlet (5.4e5)
p
outletInlet (0)
U
inletOutlet (0 2.9886 0.2615)

wall:
k
kqRWallFunction (0)
kt
inletOutlet (5.4e-3)
kl
fixedValue (0)
nut
nutkWallFunction (1e-5)
nuTilda
zeroGradient
omega
zeroGradient
p
zeroGradient
U
fixedValue (0 0 0)

Omega becomes negative after some iterations and the solutions doesn't seem to converge to a reasonable value.
I have also problems with the evaluation of the wallShearStress that becomes very high aft of the predicted transitional region.

I can't see where I fail.

Any suggestion is appreciated.

Thank you!!

The last time I used it, I used it with a VERY VERY small time step on the LRN
mesh and then everything works fine.

It seems all LRN models have convergence problem.

Hi Daniel,

Do you mean that I've to deal with a finer mesh?
I'm using simpleFoam so there shouldn't be time step problem, Am I wrong?

Thank you for the reply!

Hiya EGP, I have run a kkl-omega case and I am trying to post process the data, however I am having trouble in plotting the Cf versus (Re)x graph to find transition. I have sampled the wall shear stress to find the skin friction coefficient, but I am unsure of getting the (Re)x data from my simulation. Your help would be much appreciated!

Hi everyone

Sorry if this question sounds stupid, but has anyone tried or does anyone know if this model works well with wall functions? (y+ ~ 50)

also when I was trying to set up a case, I got this error when trying to run

can anyone shed me some light on what I have done wrong?

Thanks

strikeraj,

You need to have a y+ of less then one cuz this model uses near wall approach and you can't use a wall function, it simply does not give you transition.

Yes Strikeraj, I agree to Vahini. I guess, this method should be given the same fine grids as like as LES if you solve a three-dimension problem ...

Thanks Sandy and Vahini
I guess I missed the obvious part in the model lol

Hey!

How does one know, that the kinamatic turbulent viscosity nut and nuTilda have to be set to the viscosity of the fluid (1.5e-5)?
And why did you calculate omega with the formula:
nut = kt/omega

In the official paper from D. Keith Walters it says that:
mut = rho * Cmu,std * kt/omega
with Cmu,std = 0.09

so in your formula Cmu,std is missing.

greetings
Christian

I would like to use the new kklomega model for a Velomobile study (A typical low Reynolds case). Would anyone of you be willing to give me some advice? See http://openfoamforvelomobiling.blogspot.com/. The Velox team (http://www.hptdelft.nl/en/) have done windtunnel tests on their body. It would be very interesting to compare their results with those of the kklomega model...

what is the "new kklomega model " ?

Dear Sandy

I mean this:

Version 2.1.0 includes the k-kl-omega (low-Re) model for Reynolds-average simulation of incompressible flows with a boundary layer that undergoes a transition from laminar to turbulent flow. The model is described in D. K. Walters and D. Cokljat, J. Fluids Eng. 130:121401 (2008) and is based on the model k-w with an additional transport equation included to predict low-frequency velocity fluctuations that trigger transition in the boundary layer.

Hey,

I have a 2D airfoil simulation with settings like the above. Mesh is from blockMesh, checkMesh is fine, standard kOmegaSST converges but applying kkLOmega crashes. omega explodes, I tried all kinds of limiters but they have no effect. Any idea?

Help would be greatly appreciated :)

Mads

Last time, I used a very very small time step to keep it stable. :(

Hmm, I am running it in steady mode for a steady problem.

I have tried running this without wall functions, but openfoam will replace the files in the 0 folder with files that have wall functions and back up the old ones having a .old extension. How can I run a turbulence model (kklomega inthis case) without wall functions?

I think you need to edit the 0/nut file that openfoam creates and remove the wall function for nut.
Then you can move back your .old files to the original.

Thank you so much - this worked - it's running now without replacing the initial condition files.