I am working on a simulation where I want to use a grid that is locally refined at selected points. I planned to carry out the refinement manually by generating the grid in parts and later assembling the grid by defining interfaces. Generally the number of nodes on ether side of an interface are different. I defined coupled interfaces to provide continuity.

The problem I am facing is that the the interfaces seem to be acting as walls in the simulation. The interfaces are generally normal to the incoming flow and the flow diverts at the interface. What am I missing?

"Coupled" option is used only for fluid-solid interface (it means that Fluent will also calculate heat transfer, if any). When would you use such interface: let's say you want to calculate temperature distribution in a turbine blade, in which case solid region mesh is often very different from fluid region mesh (usually coarser), because same mesh will then be used for structural analisys in a dedicated code. So you will have to define a non-conformal interface at the blade surface, using "coupled" option.

But in fluid-fluid interface, all you have to do is define the interface without any other options.

Razvan