Boundary condition at the interface of two fluids
 

In flow-3D, I am running simulations using two fluids. Can anyone please let me know what would be the boundary condition that should be applied at the interface. i.e., I am creating two mesh blocks for two fluids, the top surface of the bottom block and the bottom surface of top mesh block is the interface. What boundary conditions do I need to apply here.

The solver automatically takes care of the inter-block boundary. You do not need to set it.