Does anyone have an idea about what software they use to simulate this stuff?

I'm wondering if they can even make use of the newfound GPU power or are just going ahead with ancient CPU based software because too much work has already been put in.

SpaceX is doing the best work on simulation. The adaptive multiscale work is a million times more important this moving to GPU, but of course they did that too:

https://www.nextplatform.com/2015/03/27/rockets-shake-and-ra...

Looks interesting. It looks like they are creating the CFD software for their own specific application. While that's cool and all, I doubt any other companies have the resources/motivation to write complex software from scratch, let alone underfunded postdocs.

I'm wondering about all the research that goes on in all the universities where large investments have been made on CPU based clusters. The simulation in the article you linked was run on NERSC servers, which are Cray supercomputers[1], which pretty much are Intel Xeon class servers with fancy interconnects.

So looks like it is CPU based, but I'm still interested in the software they use.

[1]: https://my.nersc.gov/nowcomputing-cs.php

It's usually highly-parallelized Fortran ran on the world's largest supercomputers, utilizing thousands of CPU cores. There are several codes like the ones in the study above (google "gyrokinetic equation solver"), and somehow more pop up year by year. So it's not a matter of sunk costs.

And yes, GPUs are increasingly being utilized, depending on the algorithm. But, GPUs aren't magic; they don't speed up every kind of problem.