[phile]

Dear Foamers,

I am new to OpenFOAM, so my question may be silly. What's the difference between nut and nuTilda? Isn't nut for kinematic turbulence viscosity? But what for nuTilda?

Thanks,
Phile

[JinBiao]

Originally Posted by phile

Dear Foamers,

I am new to OpenFOAM, so my question may be silly. What's the difference between nut and nuTilda? Isn't nut for kinematic turbulence viscosity? But what for nuTilda?

Thanks,
Phile

Hi,phile

I am also confused about this. And I have checked the OpenFOAM Programmer's C++ Reference, which you can find in the website of opencfd ltd. It seems nuTilda is a parameter for LES simulation. However, I donot know much about LES. If you can please check it and tell me about it if possible.

Jinbiao

[niklas]

perhaps it might help if you check in src/turbulenceModels/incompressible/RAS/SpalartAllmaras

and scroll to the bottom of the .C file
nut_.internalField() = fv1*nuTilda_.internalField();

[JinBiao]

Thanks. Niklas. It does help a lot.

It is great to find out the answer. nuTilda should be Spalart-Allmaras variable.
So it belongs to the Spalart-Allmaras model which is one of the RAS model.
That is to say, when I am using the k-e model, I donot need it.

Am I right?

Jinbiao

[niklas]

correct

[Philer]

I was really confused since the nuTilda file exists even with kEpsilon models in the tutorial. Now I know it is superfluous.

Thank you, guys!

Phile

[jiez]

nut (turb viscosity) is needed after V1.6

[DLC]

hello everyone!
can someone tell me how to initialize nut and nuTilda? I always used kOmegaSST and for those I found the formulas to initialize k and omega, but I couldn't find out how to initialize variables for spalartAlllmaras..
Thank you!

[tcarrigan]

If I understand correctly, If you'd like to initialize and record nut and nuTilda when using kOmegaSST, simply place them in the 0 directory and set them to 0...or you could calculate them based on k and omega.

Because nut and nuTilda aren't required when using kOmegaSST (only k and omega are required), by placing the nut and nuTilda dictionary files in the 0 directory, you're forcing OpenFOAM to record those values at each iteration. This is good for postprocessing as you don't have to calculate them yourself after the simulation is over.

Note: I believe that nut and nuTilda are only required for the SpalartAllmaras turbulence model.

Hope this helps.

[DLC]

Hi Trevis,
thanks for the quick reply.. I realized now that the post was written in an awful English...(late night...)

I well know that nut and nuTilda are not requested when using kOmegaSTT...
Now I am testing spalartAllmaras model and I don't know which values assign in the 0/nut and 0/nuTilda...
(i'm using freestream bc everywhere except for on the foil, where I'm using wallfunctions for nut and fixedvalue for nuTilde )...

Thaks!

[Akuji]

Hello!
I have a question... I use Spalart-Allmaras model, so I need nut and nuTilda. As I understand, nut is turbulent viscosity and nuTilda is kinematic turbulenr viscosity. Am I right?
And how can I calculate them?

[Ship Designer]

Originally Posted by tcarrigan

If I understand correctly, If you'd like to initialize and record nut and nuTilda when using kOmegaSST, simply place them in the 0 directory and set them to 0...or you could calculate them based on k and omega.

Because nut and nuTilda aren't required when using kOmegaSST (only k and omega are required), by placing the nut and nuTilda dictionary files in the 0 directory, you're forcing OpenFOAM to record those values at each iteration. This is good for postprocessing as you don't have to calculate them yourself after the simulation is over.

Note: I believe that nut and nuTilda are only required for the SpalartAllmaras turbulence model.

Hope this helps.

Can somebody please confirm that for RASModel kOmegaSST; the nut field is not required? I've just tried that out by renaming the nut file in the 0 directories of a parallel case and interFoam aborts with the error message attached below. Thanks!

[vidyuth]

Originally Posted by DLC

hello everyone!
can someone tell me how to initialize nut and nuTilda? I always used kOmegaSST and for those I found the formulas to initialize k and omega, but I couldn't find out how to initialize variables for spalartAlllmaras..
Thank you!

Hi, can you please tell me the formulas for determining epsilon and omega values?

[alyucel]

Originally Posted by Philer

I was really confused since the nuTilda file exists even with kEpsilon models in the tutorial. Now I know it is superfluous.

Thank you, guys!

Phile

This question was not answered yet
nut> dynamic turbulence viscosity
nutilda> kinamatic turbulence viscosity

aren't they?

and what is the different ?

[msuaeronautics]

Originally Posted by alyucel

This question was not answered yet
nut> dynamic turbulence viscosity
nutilda> kinamatic turbulence viscosity

aren't they?

and what is the different ?

The field "nuTilda" is a pseudo turbulent viscosity, which is solved for through a transport equation in all Spalart-Allmaras based turbulence models (be it SA-RANS or any of the SA-DES models).

The field "nut" is the actual "turbulent viscosity" and is proportional to "nuTilda" by the fv1 function, where fv1 is locally a function of the viscosity ratio nuTilda/nu.

One should always initialize the field "nuTilda" appropriately.

[rob3rt 0ng]

Originally Posted by niklas

perhaps it might help if you check in src/turbulenceModels/incompressible/RAS/SpalartAllmaras

and scroll to the bottom of the .C file
nut_.internalField() = fv1*nuTilda_.internalField();

Hi Nicklas or others,

What is the appropriate value for fv1? some say 0.1 the others say 0.5??

Thanks and regards,
Robert

[rob3rt 0ng]

Originally Posted by rob3rt 0ng

Hi Nicklas or others,

What is the appropriate value for fv1? some say 0.1 the others say 0.5??

Thanks and regards,
Robert

I've already got the answer. Sorry for bothering you.

Thanks and regards,
Robert

[rafamusura]

rob3rt 0ng

Can you answer the question you've asked please I don't know what value should I use!

thanks!

[rob3rt 0ng]

Hi Rafael,

From Turbulence modelling for CFD book by D. Wilcox, fv1 is defined as x^3/(x^3 + cv1^3), where x is nut/nu and cv1 is a closure coefficient of 7.1. In my case, fv1 adds up to 1.00.

Regards,
Robert

[rafamusura]

Yes but the X is defines as nutilda/nu so i'm confused. Nut is dependent only for the "arbitrary" value of nutilda. In other thread I've read that nutilda=3*nut=nu*0.1 is that correct?

Thanks