[Fauster]

Dear CFD experts !

I am wondering what are the correct boundaries conditions for an external flow around body. Let's say around a building for exemple. The purpose is to calculate Cx, Cy and Cz.

For me the main difficulty is to have an appropriate box of fluid. Large enough to avoid edge effect.

If the box is enough large (10 times the caracteristic length of the body ?) I guess it's possible to use zero gradient for lateral faces (I mean right and left faces according to the flow direction). But what about a free stream condition on these faces ?

The same question is still remaining for the top face. Zero gradient or free stream condition ?

For the outlet face I would say a "pressure outlet" BC type. But this seems weird to me. In deed, for a building CFD analysis we should have a vertical pressure gradient on the outlet face (du to the gravity effect). If we use a "pressure outlet" BC type the pressure is going to be equal to the same value on the whole face.

So what is the correct boundary condition for this face ?

Thanks for your help