# Equations as boundary condition

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 December 11, 2018, 15:17 Equations as boundary condition #1 Senior Member   Join Date: Jan 2013 Posts: 134 Rep Power: 12 Hi, I'm wondering if it is possible to specify the boundary condition as a set of equations rather than specific variable. I've trying to solve the potential flow with an unusual boundary condition dP/dn=f, in which P is the potential, n is the surface normal and f is an explicit variable. I know that in potentialFoam, the boundary condition enters by specifying velocity. By definition of potential flow we have dP/dx=Ux, dP/dy=Uy. Thus, the boundary condition breaks down to Ux*nx+Uy*ny=f, in which nx and ny are the two components of the surface normal vector. How should I set the this kind of boundary condition?

 December 11, 2018, 23:09 #2 Senior Member   Join Date: Jan 2013 Posts: 134 Rep Power: 12 I think this kind of boundary is very similar to the slip boundary, in which Ux*nx+Uy*ny=0 is ensured for the no-penetration condition. I wonder how openfoam implement such kind of boundary condition, in which the components of the velocity vector form an equation rather than appearing as explicit values.

 December 12, 2018, 23:13 #3 Senior Member   Join Date: Jan 2013 Posts: 134 Rep Power: 12 After a few's days search I found surfaceNormalFixedValue may suit my needs.

December 13, 2018, 14:24
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I'm testing flow around a cylinder with both slip boundary condition and surfaceNormalFixedValue with refValue=0 on the cylinder wall. I suppose these two conditions are equivalent. However, I'm getting totally different results.
For the slip, I get a stable flow (left figure), while for the surfaceNormalFixedValue I get periodic shedding (right figure).

I'm wondering why is this happening.
Attached Images
 huayi.jpg (38.1 KB, 16 views) normal.jpg (45.6 KB, 17 views)

 Tags boundary condition, potential flow