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Initialization of nut

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Old   September 25, 2020, 06:44
Post Initialization of nut
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Lucie Recurt
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Hello!


I want to run a k-epsilon simpleFoam simulation of a pipe but I got some questions:



1- in the tutorials, they initialize nut at 0 in the internal field and for every boundary. I don't really understand why because k and espilon are different of 0 and the approximation of nut is rho*Cmu*(k/eps)


2- do I have to use a calculator like this one: https://cfd-training.com/language/fr...de-turbulence/ to calculate epsilon and k's boundary condition or it doesn't give the value for the boundaries?





Thank you in advance
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Old   September 26, 2020, 11:13
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Those are just the initial conditions. It is indeed better and recommended to initialise them to non-zero values.

You can find approximated values, as you said, from analytical formulas. From your Re, the turbulence intensity, k, eps, nut, etc.
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Old   September 28, 2020, 10:55
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Internal field is just initial conditon but boundary conditions no...


And I tried to put approximated values from analytical formulas but when I do that, I get a non-physical solution
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Old   September 28, 2020, 17:56
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If your initial values are OK, you should check if the boundary conditions are ok. Also, check if your mesh is ok and even the size of the model (sometimes geometry files are in mm and we forget to apply a scaling when we mesh).
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Old   September 29, 2020, 05:55
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To give a more valuable and resonable answer to your question (it seems that high quality answers are not given anymore since Bruno Santos and other people who has high advanced experience in numerical analysis are not entering the forum anymore).

So the initial value for your internal field can be in most cases of arbitrary value. Commonly, the higher it is set, the higher your turbulent viscosity gets, the more smooth are your gradients, which in fact, is good for your stability. However, this does make sense especially for steady-state analysis. For transient analysis, you should set your conditions more carefully, also your internal field values.

The boundary conditions for inlets should be set to fixed values which are pre-estimated carefully or you simply can use the turbulentIntensityKineticEnergyInlet for k and for the other (epsilon or omega) a mixingLength.... condition in which you specify the mixing length.

If you are dealing with transient analysis and the flow is set to zero at the beginning, while it establishes during the analysis, you are welcomed to set the turbulent quantities to a small value such as 0.01 for k and for omega and epsilon too. However, all these values (except of the boundary conditions) change during each iteration.


Cheers.
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Old   September 30, 2020, 10:56
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Thank you for your answer Tobias!



However one thing seems weird for me. How can OpenFoam calculate a value at the boundary? Because it has to be the last cell and cfd software use discretization to calculate a value in one cell. But at the boundary, there is not (n+1) cell...
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