A problem about setting boundary conditions
 

Hi,

Here I have a question associated with setting boundary conditions.

Currently, I am using Gila(a finite element codes0 to solve a 3-D problem. Gila only provides two options to set the boundary conditions: natural and prescribed. For natural conditions, one doesn't need input anything in the input data file. For prescribed conditions, one need to input the specific values for the boundary conditions. When my boundaries are normal or paralell to the selected coordiantes, there are no problems to set the boundary conditions. However, there is a plane which does not satisfy these conditions in my problem. This plane is a slip plane which means there is no mass crossing this plane and the derivative of the velocity in the tangent direction with respect to the unit vector normal to this plane is also zero. My problem is: how can I set boundary conditions for this plane? There is no way to modify the Gila codes. In one case, I just assumed the natural conditions for this plane. But the visualized results turned out to be incorrect. I was really confused by this problem.

I'll appreciate if any people can give me some advices.