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How to determine the boundary conditions in LES simulation

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Old   October 22, 2018, 11:42
Default How to determine the boundary conditions in LES simulation
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Yuan Jiahui
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Hi everyone,


I am trying to simulate a case using LES model (dynamicKEqn model). The case is similar to pitzDaily and I only know the initial inlet velocity field (0.455 0 0). I checked online for the boundary conditions that I should use and I found that I should use fixedvalue for inlet and inletoutlet for the outlet, but I have doubt about the initial values of k and other boundary conditions.


And I also want to get better understand of dynamicKEqn model. Any information about it helps.


Regards
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Old   October 22, 2018, 13:30
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Michael Alletto
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A constant velocity at the inlet is not a good idea for LES. Since an inlet is an artificial boundary condition the boundary condition should resample as close as possible what happens in reality there.

For turbulent flows you'll never have a constant profile.

Usually one has to either synthetic turbulence at the inflow or use a precursor simulation with periodic boundary conditions as inflow.
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Old   October 22, 2018, 14:05
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To add to this, here's a paper that compares choices for the inlet velocity bc (assuming incompressibility) : https://www.sciencedirect.com/scienc...95717711006820.

The reference for the dynamicKEqn model is also provided in the corresponding .H file. It is :

Kim, W and Menon, S. (1995).
A new dynamic one-equation subgrid-scale model for
large eddy simulation.
In 33rd Aerospace Sciences Meeting and Exhibit, Reno, NV, 1995.

Caelan
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Old   October 23, 2018, 09:20
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Yuan Jiahui
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Quote:
Originally Posted by clapointe View Post
To add to this, here's a paper that compares choices for the inlet velocity bc (assuming incompressibility) : https://www.sciencedirect.com/scienc...95717711006820.

The reference for the dynamicKEqn model is also provided in the corresponding .H file. It is :

Kim, W and Menon, S. (1995).
A new dynamic one-equation subgrid-scale model for
large eddy simulation.
In 33rd Aerospace Sciences Meeting and Exhibit, Reno, NV, 1995.

Caelan
I am sorry to bother you again. I can not determine the value of k through the reference. Is there any expirical formula to get initial boundary condition for k in LES?
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Old   October 23, 2018, 11:57
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Michael Alletto
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by aware that k in for LES means the subgrid scale turbulent kinetic energy. I.e. the part of the spectrum which is not resolved. So it depends strongly on the grid resolution and of the problem.

What are you trying to simulate?
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Old   October 23, 2018, 12:10
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Quote:
by aware that k in for LES means the subgrid scale turbulent kinetic energy. I.e. the part of the spectrum which is not resolved. So it depends strongly on the grid resolution and of the problem.
And it is dynamic so an initial value will be overridden by one that is "figured out". This is not to say that it will automatically find an use a correct value --a least squares problem is often solved to find the "best" coefficient/coefficients for the flow at a given time/conditions.

Caelan
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