[vishal_s]

Hello Everyone, did u got the correct interpretation for X??

[aerogt3]

Originally Posted by vishal_s

Hello Everyone, did u got the correct interpretation for X??

Same, question here, and how to calculate it from freestream turbulence properties!

[vonboett]

well X is specified as nutilda/nu, nutilda as zero on walls and aswell zero in the free stream, but for stability reasons values fo nutilda < 0.5 nu can be applied. In case you only need it as a starting value (and I think this is the case for Hybrid URANS_LES DES, DDES using Spalart-Allmaras) wikipedia suggests to use nutilda = 5nu in the free stream, which causes turbulence to develop whereever shear flow occurs. (see http://www.cfd-online.com/Wiki/Turbu...ary_conditions)

[SCG]

what value i have to mention in nut file for outlet

[tareqkh]

since you already got it, could you please share it.

Regards,

[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.

[aerospain]

Dear Albrecht,

A quick question about initializing nut from nuTilda (SA model) in OpenFOAM3.0.1. Should I include a nut file in 0/ directory with values calculated manually from nuTilda? (using fv1 and Cv1) or, will OF generate nut files automatically once the simulation runs?

Usually, to be on the safe side, I would initialize the nut file manually in 0/, but I was just wondering if that may create any conflict with OF as the way of dealing with some algorithms may change between versions.

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?

[pichetzh]

Originally Posted by rafamusura

Robert Ong
I'm not shure that's the way it work, because you are defining nut=fv1*nutilda, fv1=x^3/(x^3 + cv1^3), and like you say X is a function of nut, if so you're defining nut as a function of nut, nut=nutida*f(nut).

in the wiki page http://www.cfd-online.com/Wiki/Spalart-Allmaras_model, X is defined as nutlinda/nu. So what's the rigth interpretation?.

Tobi, I have the same question, I was trying to get a k-e simulation but it always crashed, so I took a step backward to a more simple model first. If you have the answer for that just let me know.

A) How does one solve for nuTilda in the following equation?

nuTilda = nuT/fv1
=nuT*(1+(CV1/chi)^3)
= nuT*(1+(CV1*nu/nuTilda)^3)

This is not easy to solve analytically, but graphically it is trivial:
1. plot y1 = x
2. plot y2 = nuT*(1+(CV1*nu/x)^3)
3. Where y1 intersects y2, say at point P* = (x*, y*), x* is your nuTilda.

B) When can I use the approximation nuTilda ~ nuT?

This approximation requires (CV1*nu/nuTilda)^3 << 1.
As a rule of thumb, let's say we want nuTilda to be within 1% of nuT, ie., nuTilda = 1.01 nuT. Then nu/nuT = 0.01^(1/3)*1.01/CV1 = 3.06%, or taking the reciprocal nuT/nu = 1/0.0306 = 32.6.

That is to say, if your turbulent viscosity is > 33 times dynamic viscosity, go ahead and use nuTilda = nuT.

[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!

[JNSN]

Nut is required

[Ship Designer]

Thanks Jan!