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The boundary conditions in the official tutorial motorbike

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Old   August 13, 2020, 12:57
Default The boundary conditions in the official tutorial motorbike
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Yukuan Song
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Hi,
I am studying the official tutorial of openFoam. I found there are two motorbike cases in the tutorial, one is the RANS case with SST k-omega model, and the other is the LES case. In the RANS case, the frontAndBack and the upperWall boundary of the computation domain was set as slip walls. While when computing the initial fields of the LES case with SA turbulent model, the frontAndBack and the upperWall boundaries were set as symmetryPlane. I wonder why the boundary condition of these surfaces are different between the RANS and LES cases. Any help would be great!
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Old   August 14, 2020, 22:07
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Ran
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Thanks for reporting this issue. I've checked the tutorial in OpenFOAM
version 7 in the following two files:

OpenFOAM-7-master/tutorials/incompressible/simpleFoam/motorBike/0/include/frontBackUpperPatches

`RASModel: kOmegaSST;

#+begin_src cpp
frontAndBack
{
type slip;
}
#+end_src


OpenFOAM-7-master/tutorials/incompressible/pisoFoam/LES/motorBike/motorBike/0/include/frontBackUpperPatches

`RASModel: SpalartAllmaras`

#+begin_src cpp
"(front|back)"
{
type symmetryPlane;
}
#+end_src

For the RAS case, because of the nature of simple solver, the result
will eventually converge a steady state, which the flow structures are
the same if you cut the computational domain along the center line.

Using `symmetryPlane`, it seems to me, the computational domain
extends to infinite space. Similar to when you have two mirrors and
you watch though them, one can experience infinite space.

LES will not give you a final snap-shot. Instead, it is
time-dependent. If you prescribe a `slip` boundary condition, this
will contraindicate the assumption of LES.

However, I have not tested it by myself. If the original computational
domain is large enough, such a difference in BC would only have a
marginally effect on some mean value, e.g. Mean drag coefficient.

Just some thoughts on this topic.
__________________
Yours in CFD,

Ran

Last edited by random_ran; August 14, 2020 at 23:16.
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Old   August 14, 2020, 23:34
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Yukuan Song
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Quote:
Originally Posted by random_ran View Post
Thanks for reporting this issue. I've checked the tutorial in OpenFOAM
version 7 in the following two files:

OpenFOAM-7-master/tutorials/incompressible/simpleFoam/motorBike/0/include/frontBackUpperPatches

`RASModel: kOmegaSST;

#+begin_src cpp
frontAndBack
{
type slip;
}
#+end_src


OpenFOAM-7-master/tutorials/incompressible/pisoFoam/LES/motorBike/motorBike/0/include/frontBackUpperPatches

`RASModel: SpalartAllmaras`

#+begin_src cpp
"(front|back)"
{
type symmetryPlane;
}
#+end_src

For the RAS case, because of the nature of simple solver, the result
will eventually converge a steady state, which the flow structures are
the same if you cut the computational domain along the center line.

Using `symmetryPlane`, it seems to me, the computational domain
extends to infinite space. Similar to when you have two mirrors and
you watch though them, one can experience infinite space.

LES will not give you a final snap-shot. Instead, it is
time-dependent. If you prescribe a `slip` boundary condition, this
will contraindicate the assumption of LES.

However, I have not tested it by myself. If the original computational
domain is large enough, such a difference in BC would only have a
marginally effect on some mean value, e.g. Mean drag coefficient.

Just some thoughts on this topic.

Thank you Ran.
I still have some questions. You mentioned if I prescribe a `slip` boundary condition in the LES case, this will contraindicate the assumption of LES. As I am a beginner to the LES, I cannot understand which assumption of LES you are referring to. Can you show me more information?
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