Fluid Structure Intercation
 

I would like to ask, if it is possible to build a transient FSI Model in ANSYS Fluent. Should I use UDF to make the coupling?

At the time I am building a 1 way FSI steady state model. Everything can be done in ANSYS Workbench... No need of UDF... But for the transient one?

I assume you are connecting to Mechanical. One option is to use the External Data component. Follow the doc for "Importing Multiple Data Sets" in the Workbench book

// User's Guide // Systems // Component Systems // External Data // Importing Multiple Data Sets

The other option is to ask ANSYS for the ACT (Application Customization Toolkit) files for 1-way transient FSI.

This depends entirely on what information you need to have in the model. In my experience you should go for a UDF based implementation if:

Either:

1) You also need to include other custom modeling beyond the standard capabilities of ANSYS. For example, take a look at this snow deposition/erosion model implemented on Fluent.

or:

2) The structural model can be relatively simple, so that you don't need a full-featured FEM solver to do the job. For example, I have implemented a transient model for an opening hatch using a simple spring-dashpot model (3 parameters) as a UDF. I could handle it with simple Euler-type timestepping, i.e. without interfacing a FEM solver.

Hope this helps.