Rename `ape` to `acoustics`

[sloede]

In some places we use ape as the abbreviation for the acoustic perturbation equations, e.g., in the elixir names. While I suggested this name originally, I've grown to dislike it since it is not a well-known abbreviation like mhd and is not self-descriptive. What about renaming it to just acoustics? True, it is a somewhat generic term, but unless we have some other equations that could interfere with it (like linearized Euler), it would be both unambiguous and self-explanatory.

[ranocha]

We should also add the APEs to the README since we use them in a number of cool demonstrations of Trixi.jl

[gregorgassner]

I would actually prefer the full name "acoustic perturbation equations".

The reason is, that there are also people around, that use the linearized Euler equations (LEE) for acoustics. Not everyone is using APE.

Hence, calling it just acoustics is not optimal in my opinion.

[ranocha]

That's a reasonable suggestion. Note that this issue is not about renaming the equations themselves, just the short name we use in the file names of example elixirs and test setups. Do you really want to use the full name there or would some abbreviated version be okay there? The current proposal of @sloede is to rename

• examples/tree_2d_dgsem/elixir_ape_gauss_wall.jl

to

• examples/tree_2d_dgsem/elixir_acoustics_gauss_wall.jl

Would you prefer something like examples/tree_2d_dgsem/elixir_acousticperturbation_gauss_wall.jl?

[sloede]

Note that this issue is not about renaming the equations themselves, just the short name we use in the file names of example elixirs and test setups.

Correct!

I thought about Gregor's argument as well, but concluded that it would be unlikely to add another acoustics-like system of equations in Trixi. Thus back then I proposed to name it just acoustics, similar to how we also use just advection for linear scalar advection. However, I could also live with acousticperturbation - I just do not think the ape is a good name (also originally suggested by myself :see_no_evil: )