Modeling elastic waves in a solid mechanics study

Hi,

I'd like to ask a question concerning the modeling of elastic waves in the structural mechanics module. The geometry is a simple 3D beam, free on all sides. (file attached)

My goal is to study the elastic waves propagating in the z-direction - based on the full elasticity equations. However, currectly I see no good way to implement the requirement that the displacement field is a harmonic wave with wavevector K in the z-direction (explicit requirement attached).

When I simulate the beam with all-free boundary conditions, I do get some of those waves. But now it is not easy to extract the wavevector, and I get many modes that are not such elastic waves at all.

I have thought about using periodic boundary conditions, but I still didn't find a good way to just look for those modes with wavevector K.

Any help would be greatly appreciated!

Thanks