So full overview of what I'm doing. Hopefully this will make everything come together.

The end goal of my thesis (which is what I'm doing this for) is to computationally determine the relationship between swirl, diffuser geometry, and static pressure recovery at high flow speeds (around 0.5M).

The first step to do that is to create a simulation model that can match the results of experimental results. This is the stage I'm at right now.

Right, so it's not perturbed, you're running a swirling flow.

In the experiment I'm trying to model right now, the swirl generator does create turbulence that has been measured. Would this not mean that the mean velocity is perturbed? If not, how would the turbulence at the inlet be taken into account for the LES domain?

 If the original case didn't have swirl check the cell zone settings to ensure you're actually swirling about the correct axis.

Just doubled checked and it's rotating about the correct axis.

Why do you need to include temperature, looking through you've not mentioned any heat transfer: 40m/s is well below 0.3M so the solution may not be overly compressible.

Because the simulations after the validation step are going to be high velocity, high temperature flows. Ideally, I'd like to maintain the exact same settings from the validation stage to the actual computation stage. That said, I could probably ignore thermal effects, I just figured I'd keep them in there.

I'd also look at the back flow conditions, and extend the domain so the back flow isn't influencing the system.

What specifically should I be looking at the back flow conditions for? They're currently set to the same levels of turbulence and temperature as the inlet.