Erik Kostson
Ansys Employee

I do not think Fluent is appropriate even though in theory it is possible using our compressible solver (will need to be fully coupled to a transient structural to account for the solid rod structure).

If the couplant is really of interest we can model it with acoustic elements, assuming we care how the wave is transmitted through there, and the structure with structural elements. We can use the new coupled field transient system to do this in 2021 R2 (this is an implicit coupled analysis). We can though only use it for 3D analysis (If one wants 2D then the standard structural transient system is needed, just as you have in your example which we helped you with, but with some command snippets to change to acoustic elements).

For Explicit, one can use acoustic elements in (MAT_ACOUSTIC, and element form 8) in LS-Dyna coupled to normal structural elements, but this approach can not be used in Workbench (does not contain acoustic elements) so you need to create or modify the input .k file and run in LS-Dyna (LS-RUN say).

Normally the couplant layer is so thin that it will require very thin elements, and from what I have seen not many people model this of course for ultrasonic FEA.

All the best