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blowing up with k-epsilon model

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Old   August 5, 2010, 04:56
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  #21
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Sebastian Saegeler
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Hello!

Thank you a lot for all your advices. I think it will take me a while untill I have tested it all.
But I dont really understand this expression :
Quote:
Gauss linearUpwind cellLimited Gauss linear 1;
Can you explain me the the meaning of it, what it does?

Over night I run a calculation where I have changed the fixed values at the inlets for k and epsilon. I looked into the fluent solution, where I have variable BCs according to turbulent viscosity ratio and turbulent intensity. Now what I did is, I took the values from there and changed the one in OF. Then I continued the calculation from it500 and it didnt diverge at it600. Its still running and now I am at it 2000. Still have the bounding problem, but at least its running.
So I also expect its not good to use the fixedValue bc at the inlets for k and epsilon. I already tried to use the "turbulentIntensityKineticEnergyInlet" and the "turbulentMixingLengthDissipationRateInlet". But as mentioned, the calculation crashes after only a few iterations.
No idea want went wrong. Can anybody have a look at my case. You will find my bc for k and epsilon in /0. The files are epsilon.functions and k.functions.

Thanks!!

Sebastian
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Old   August 5, 2010, 05:06
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Christian Lucas
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Hi,

just a short remark. In the epsilon inlet function you used the wrong length. The length has to be given in meter not mm.

Regards,
Christian
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Old   August 5, 2010, 09:27
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Sebastian Saegeler
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Hi Christian!

Of course, youre right, i forgot the e-3 behind the values.
Now it starts calculating and at least its still running

But for the turbulentIntensityKineticEnergyInlet something still is going wrong!

I have calculated the initial values like this:
Initial U=10 m/s, turbulence intensity is 5%, so: k=3/2(0,05*10)^2=0.325 ... and so on.

Here, thats my k file, maybe anybody has an idea:



dimensions [0 2 -2 0 0 0 0];

internalField uniform 0.0001;

boundaryField
{
core_inlet
{
type turbulentIntensityKineticEnergyInlet;
intensity 0.05; // 5% turbulence
value uniform 0.375;
}
fan_inlet
{
type turbulentIntensityKineticEnergyInlet;
intensity 0.05; // 5% turbulence
value uniform 0.375;
}
farfield_inlet
{
type turbulentIntensityKineticEnergyInlet;
intensity 0.001; // 0.1% turbulence
value uniform 1.5e-4;
}
walls
{
type kqRWallFunction;
value uniform 1;
}
farfield_outlet
{
type zeroGradient;
}
}


If I do initialize everything with k=1, and also set the bc at the inlet to this fixed value, its running.. but thats not a good way to do so..

I am also not so sure about, if I have set the turbulent lenght scales correctly in my turbulentMixingLengthDissipationRateInlet bc.
At the Inlets of my core and fan stream, I just said its 5% of my channel height. At the Inlet of my farfield, I also used 5% of my domain height. Do you think thats a reasonable guess??

I would be pleased if I get some further help of you!! Thanks a lot!


Sebastian
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Old   September 18, 2010, 09:04
Default units turbulentMixingLengthDissipationRateInlet
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Patty
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Hi
I am not a specialist but having the same problem (blowing up)
I had a look at Sebastian 0/e file and he mentioned a factor
turbulentMixingLengthDissipationRateInlet = 15 with a comment //15mm

In the following web reference

/html/classturbulentMixingLengthDissipationRateInletFvPa tchScalarField.html

it is said the units to be specified are meter (and not mm). Are you sure of your units? it would change drastically the turbulence isn't it?

Have you a comment? did you solve your problem? how?

Best regards
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Old   September 18, 2010, 09:17
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maddalena
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Hello,
Quote:
Originally Posted by ptbs View Post
it is said the units to be specified are meter (and not mm). Are you sure of your units? it would change drastically the turbulence isn't it? Have you a comment? did you solve your problem? how?
this has been already pointed out and solved! look here.
cheers

mad
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