# The boundary conditions in the official tutorial motorbike

 Register Blogs Members List Search Today's Posts Mark Forums Read

 August 13, 2020, 12:57 The boundary conditions in the official tutorial motorbike #1 New Member   Yukuan Song Join Date: Aug 2020 Posts: 2 Rep Power: 0 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!

 August 14, 2020, 22:07 #2 Member   Ran Join Date: Aug 2016 Posts: 69 Rep Power: 9 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.

August 14, 2020, 23:34
#3
New Member

Yukuan Song
Join Date: Aug 2020
Posts: 2
Rep Power: 0
Quote:
 Originally Posted by random_ran 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?