Boundary Definition
 

I have a voxelized random packed of spheres. I have created mesh with Avizo. I can import the mesh; however, there is no inlet or outlet boundaries. I do not know how can I define the boundaries. I would appreciate if you could help me. Thanks...

Regardless, you should have at least 1 boundary surface. The worst case is if you have only 1 boundary surface if that is the case you must work with the meshing program to write the mesh properly.

If you have all the boundary surfaces and they are correct, the only problem left is that they need to be assigned the correct boundary type. From GUI it is straightforward, on the boundary conditions menu. Select the boundary and change its type to the appropriate type inlet/outlet or whatever. From TUI use the \define.