# Empty boundary condition

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 May 31, 2017, 05:41 Empty boundary condition #1 New Member   behrooz Join Date: May 2017 Posts: 3 Rep Power: 7 Hi all we are preparing a finite volume code to simulate flow over a airfoil with FORTRAN. we have a mesh which is extruded one layer (Nz=1). we need to impose EMPTY boundary condition in two faces in Z direction. but we don't know how to apply this. in other word, we don't know what are the velocity and pressure values on the boundaries nodes (what is the meaning of the EMPTY boundary condition???)

 May 31, 2017, 06:53 #2 New Member   D. N. Venturi Join Date: Jan 2013 Location: Brazil Posts: 25 Rep Power: 11 Hi, I don't know which software you are using, but if you mean EMPTY boundary condition of OpenFOAM, I think it is the same as symmetry or periodic in other solvers. Other users may confirm this. leflix likes this.

 June 7, 2017, 03:44 #3 Senior Member   Santiago Lopez Castano Join Date: Nov 2012 Posts: 344 Rep Power: 13 Empty is not symmetry nor periodic, it means that the Gauss theorem shouldn't use the faces defined as EMPTY. This basically sets all coefficients related to the direction normal to the empty faces to nil. Sent from my Lenovo A5000 using CFD Online Forum mobile app

 August 5, 2017, 14:16 #4 Senior Member   Join Date: Aug 2011 Posts: 270 Rep Power: 14 In fact you are both Venturi and Santiago right and you say exactly the same thing. As Venturi mentioned it EMPTY boundary condition in Open foam corresponds to symmetry boundary condition over this surface. Santiago when you set the coefficient of your discretized equation to zero you impose a normal Gradient to this boundary =0 or if you prefer d(Phi)/dn=0 and this is exactly the boundary condition you should impose for symmetry BC. If you want to perform a 2D simulation (x, y) when you have a 3D solver as in Openfoam you must use only one cell in the z direction and you have to apply symmetry boundary condition on the two side surfaces normal to the z direction.

 August 5, 2017, 14:35 #5 Senior Member   Join Date: Aug 2011 Posts: 270 Rep Power: 14 By the way Santiago this is not true that if the Gauss theorem do not use the faces defined as EMPTY that it sets all coefficients related to the direction normal to the empty faces to null. In a pure diffusive equation yes, in a convective-diffusive one no because you would have the convective flux contribution in the calculation of this coefficient. And this is exactly because for symmetry BC convective fluxes are null that the coefficient is then null because all its contribution are null (diffusive flux with Gauss theorem and convective flux).

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