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It sounds like you want floquet periodic boundary conditions (PBCs) for the sides from which the plane wave starts.

For example, if you want a plane wave going only in the x direction, select floquet PBCs for the sides of the cube on the x-axis in the boundary mode. For the xyz inputs, you can put a variable 'kx' in the x slot and 0 in the y and z slots.

You can then vary kx to, e.g., look at a band structure if you're computing eigenfrequencies.

Thanks a lot was a great help.

Thanks for the info.
What would be the changes required for PBCs with oblique waves?

I am attempting to create the band structure plots for a square lattice of circular cylinders in 2d. Essentially these plots consist of listing the degenerate eigan solutions for the first brillouin zone i.e., the reciprocal lattice. My understanding is that people create these plots by sampling a continuum of wave vectors (or altering the direction of the incident plane waves) and plotting these wave vectors against freq.

My question is this: I am using Comsol to solve Helmholtz's equation for the all of the eigan frequency solutions of the reciprocal unit cell of my lattice near the location of the principle stop band. It then returns a bunch of frequencies, some with only one degenerate and others with more. This is fantastic and all but how do I extract the wave vector direction from these solutions to assign an x-axis position to the band structure plot given that the eigan value approach uses no source terms?

see attached plot for example of what I want to create

I am using periodic boundary conditions see attached sim file

Hi,

I am looking for applications in chemical engineering that covers the topic of periodic boundary conditions. Does anyone know or has a chemical engineering model in Comsol that applies this concept?

your help will be greatly appreciated...

regards
Luis