Difference between Junction BOX routine & CEL expression in a 1 way FSI
 

ANSYS 11.0 has this tutorial 20: Fluid structure interaction and mesh deformation which says one way FSI can be performed by junction box as well as by CEL expression.

ANSYS 12.0 does have the same tutorial but does not have the mention of a junction box.

Can anyone tell me what the primary difference is ???

The version 11.0 tutorial used a Junction Box to read in a set of meshes that were created ahead of time, with each timestep getting a new mesh. There's no interpolation here - the mesh topology is identical but the nodes are at different positions in each mesh which results in mesh motion. The CEL approach uses "standard" mesh deformation.