Slip and Symmetry boundary conditions

giuse · November 22, 2016, 11:12

Hello all,

I'm trying to understand how slip and symmetry are implemented in OpenFoam.
In general a slip boundary condition should guarantee zero shear on the surface over which is imposed, but this is not the case in openFoam.
Anyway, imposing the symmetry condition on that surface, zero shear can be obtained.
Looking at the boundary condition implementation, instead, they seems to do exactly the same thing, i.e. substituting the value of the fields with the field itself plus an Householder reflection of the field. At least theoretically, if one calculate the wall shear for the new field, one can obtain that it is exactly null, provided that the normal gradient of the tangential component of the field is zero (and this is that you can find as definition of slip in the user guide of openfoam).
The only difference between slip and symmetry seems to be a check on the type of patch: in case of symmetry, there is an "if" which gives an error if you specify a different type of boundary condition while in the polymesh/boundary file symmetry is set.

There is someone who could tell me where is the difference in the implementation of the two boundary conditions (in the IMPLEMENTATION, not the theoretical difference) and where this difference is placed?

I have also tried to compile a new condition based on symmetry and, commenting the "if" I have cited before (where inside there are only some print on screen), my new symmetry bc gives the same (wrong) results of slip.

Thank you

vatavuk · November 25, 2016, 05:11

Hi Giuseppe,

I'm not sure, but I think that simmetry condition can be used only for plane surfaces.

Best Regards,
Paulo

giuse · November 25, 2016, 05:30

Hi Paulo,

this is not true, there are two different types of symmetries:
symmetryPlane --> only for planes,
symmetry--> also for curved surfaces.

Moreover, I understood the difference. Slip and symmetry are the same for the fields over which are imposed, but the symmetry condition modifies also the fields internal to the solvers (e.g.phiHbyA etc), so the obtained results are different.

Finally, there is no already implemented condition to impose free-shear over a curved surface.

hua1015 · December 20, 2016, 04:21

Hi Giuseppe,
I agree with you that there is no already implemented condition to impose free-shear over a curved surface. Would you mind give me some suggestions about how to implement such BC.
Thanks.

November 22, 2016, 11:12	Slip and Symmetry boundary conditions	#1
giuse New Member Giuseppe Join Date: Jul 2014 Posts: 3 Rep Power: 12	Hello all, I'm trying to understand how slip and symmetry are implemented in OpenFoam. In general a slip boundary condition should guarantee zero shear on the surface over which is imposed, but this is not the case in openFoam. Anyway, imposing the symmetry condition on that surface, zero shear can be obtained. Looking at the boundary condition implementation, instead, they seems to do exactly the same thing, i.e. substituting the value of the fields with the field itself plus an Householder reflection of the field. At least theoretically, if one calculate the wall shear for the new field, one can obtain that it is exactly null, provided that the normal gradient of the tangential component of the field is zero (and this is that you can find as definition of slip in the user guide of openfoam). The only difference between slip and symmetry seems to be a check on the type of patch: in case of symmetry, there is an "if" which gives an error if you specify a different type of boundary condition while in the polymesh/boundary file symmetry is set. There is someone who could tell me where is the difference in the implementation of the two boundary conditions (in the IMPLEMENTATION, not the theoretical difference) and where this difference is placed? I have also tried to compile a new condition based on symmetry and, commenting the "if" I have cited before (where inside there are only some print on screen), my new symmetry bc gives the same (wrong) results of slip. Thank you

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November 25, 2016, 05:11		#2
vatavuk Senior Member Paulo Vatavuk Join Date: Mar 2009 Location: Campinas, Brasil Posts: 200 Rep Power: 18	Hi Giuseppe, I'm not sure, but I think that simmetry condition can be used only for plane surfaces. Best Regards, Paulo

November 25, 2016, 05:30		#3
giuse New Member Giuseppe Join Date: Jul 2014 Posts: 3 Rep Power: 12	Hi Paulo, this is not true, there are two different types of symmetries: symmetryPlane --> only for planes, symmetry--> also for curved surfaces. Moreover, I understood the difference. Slip and symmetry are the same for the fields over which are imposed, but the symmetry condition modifies also the fields internal to the solvers (e.g.phiHbyA etc), so the obtained results are different. Finally, there is no already implemented condition to impose free-shear over a curved surface.

December 20, 2016, 04:21		#4
hua1015 Member Hua Join Date: May 2012 Posts: 31 Rep Power: 14	Hi Giuseppe, I agree with you that there is no already implemented condition to impose free-shear over a curved surface. Would you mind give me some suggestions about how to implement such BC. Thanks.