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StrainRate definition 141 vs 15

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Old   September 25, 2008, 10:37
Default Hello, I compared nonNewton
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florian
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Hello,

I compared nonNewtonian results.
The pressure is not the same with 1.4.1 and with 1.5 version of OpenFoam.
When I take n=1 => newtonian flow the results are the same.
It due to the strainRate calculation.
First I had these lines in BirdCarreau in the fonction calcNu()

Info
<< "max(strainRate): " << max(strainRate()).value()
<< " min(strainRate): " << min(strainRate()).value()
<< endl;

During the caluclation OpenFoam prints line like this one.
max(strainRate): 2027.15 min(strainRate): 1.46869

when I calculate :
max(strainRate)1.4.1/max(strainRate)1.5=1,414..
min(strainRate)1.4.1/min(strainRate)1.5=1,414..

And sqrt(2)=1,4142

I searched where is the definition of strainRate.
I found it in viscosityModel.C
in OpenFoam 1.4.1
return mag(fvc::grad(U_));
in OpenFoam 1.5
return mag(symm(fvc::grad(U_)));
This is not the same

Can someone help me ?

Florian
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Old   October 9, 2008, 05:03
Default Hi, I am a beginner. I hav
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zhiwei liu
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Hi,

I am a beginner. I have some quesitons with Cross PowerLaw,I want to check the problem,
But I do not know how to print strainRate(mag(fvc::grad(U_))),can you tell me ?

Thanks!
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Old   October 10, 2008, 09:02
Default if you add Info << "max(
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if you add

Info
<< "max(strainRate): " << max(strainRate()).value()
<< " min(strainRate): " << min(strainRate()).value()
<< endl;

in the powerlow.C file it will print the max and min value of the strainRate.
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Old   November 18, 2008, 04:46
Default Hello; I am also trying to
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Bercan Siyahhan
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Hello;

I am also trying to use a non-Newtonian model, and I was also looking at the definition of strain rate, and my question is what U_ stands for?

Thanks.
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Old   November 19, 2008, 04:22
Default In Viscosities Model all const
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florian
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In Viscosities Model all constants are imported with a "_"

if you look in the "Member Functions" part of a viscosity model

k_(powerLawCoeffs_.lookup("k")),
n_(powerLawCoeffs_.lookup("n")),
nuMin_(powerLawCoeffs_.lookup("nuMin")),
nuMax_(powerLawCoeffs_.lookup("nuMax")),

I think U_=U
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Old   November 19, 2008, 05:40
Default Hi Florian, Indeed the defi
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Kerstin Heinen
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Hi Florian,

Indeed the definition of strain rate is not the same.

The more general definiton used for strain rate in rheology books fpr 3D flows is

strain rate = sqrt( 2* (D:D) )
And D = 1/2 * (grad(U) + grad(U).T)

This "D" is the deformation gradient tensor (the symmetric part of the velocity gradient tensor).

The velocity gradient Tensor L= grad(U).T can be decomposed in a symmetric part and unsymmetric part. And the unsymmetric part is called rotation velocity tensor. The unsymmetric part doesn't contribute to fluid volume deformation. This is the reason why D is usually preferred.

You can calculate both definitons and if I didn't make something wrong now after one page of tensor calculations, I get the following (with the definition of mag(T) according ProgrammersGuide)

[ mag(symm(grad(U)))]^2 = [ mag(grad(U))] ^2 +
2* dUy/dx*dUx/dy
+ 2* dUz/dx*dUx/dz + 2* dUz/dy*dUy/dz

Ux, Uy and Uz are the components of velocity vector in each direction...

So indeed this is different. So I would prefer the definition I have given above because this is the widest spread in rheology books. But I didn't look up what Fluent or CFX use so far...but who cares what Ansys does? ;-)

And a rheologist would say, that calculation of a scalar strain rate for a 3D complex flow and using a model fitted to a measurement in simple shear flow is dangerous...but I think it's the best choice available...

Kerstin
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Old   November 19, 2008, 06:26
Default I started working with OpenFoa
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I started working with OpenFoam. Doing so, a question arose for me, as I write here:

For compressible rarefied flow, it is suggested to use rhocentralFoam routine. I like to know whether it can solve very low Mach flow? I see the examples of this routine are mostly high Mach number flows. Does it an all speed flow solver?

Sincerely,
Ehsan
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Old   November 20, 2008, 05:00
Default Kerstin => Thanks ehsan =>
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Kerstin => Thanks

ehsan => I advise you to post your question in a new topic.
Or to use the search Utility and post your question in a topic which is about rhocentralFoam.
The subject of this topic is "StrainRate definition 1.4.1 vs 1.5"

Florian
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Old   November 20, 2008, 10:09
Default My mistake. The common defi
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Eugene de Villiers
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My mistake.

The common definition for shear rate in fluid mechanics is indeed:

sr = sqrt(0.5 * D:D), where D = grad(U) + grad(U).T()

In OpenFOAM, skew(T) = 0.5*(T + T^t) and mag(T) = sqrt(T:T), so

sr = sqrt(0.5 * 4* skew(U):skew(U))
sr = sqrt(2) * mag(skew(U))

The current implementation is incorrect. I will submit a bug report. Thanks for catching it!
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Old   November 20, 2008, 12:14
Default skew(T) = 0.5*(T - T^t) symm(
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skew(T) = 0.5*(T - T^t)
symm(T) = 0.5*(T + T^t)

with T = grad(U)

I think the good definition is :

sr = sqrt(2) * mag(symm(fvc::grad(U)))

Florian
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Old   December 6, 2010, 09:08
Default glopal change in shear rate for 1.7.x
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Jon Elvar Wallevik
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Hello there
Just an additional comment to the above
I am using OF 1.7.x and I have made a "glopal" change to the shear rate. What I did is as follows: (of course replace my username with yours in path, if you want to repeat)

cd /home/jonelvar/OpenFOAM/OpenFOAM-1.7.x/src/transportModels/incompressible/viscosityModels/viscosityModel

gedit viscosityModel.C

Change...
return mag(symm(fvc::grad(U_)));
...to...
return sqrt(2.0)*mag(symm(fvc::grad(U_)));

cd /home/jonelvar/OpenFOAM/OpenFOAM-1.7.x/src/transportModels/incompressible/

wmake libso

Hope this is of help
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Old   August 16, 2014, 18:19
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Dear Eugene,

In OpenFOAM, how to calculate the principal compressive strain rate? I would like to extract this quantity but did not find it in the OpenFOAM classes?

Thank you very much if you can give me some hints.

OFFO
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Old   August 18, 2014, 01:40
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Jon Elvar Wallevik
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Dear OFFO

I recently published a paper in which I derivatived the above equation for shear rate (see Section 3.1). It applies for incompressible material only.
As I understand from your post, you want to do the same for compressible material?
If so, you could follow the steps done in this paper and see where it gets you. You might have to implement it your self into OpenFOAM.

the paper is...
JE Wallevik, Effect of the hydrodynamic pressure on shaft torque for a 4-blades vane rheometer, International Journal of Heat and Fluid Flow, 2014

Cheers
J.
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