And lots of Finite Element Analysis packages still use Fortran under the hood.

I've noticed that Fortran comes up a lot when talking to people who do fluid dynamics simulations.

Curiously, it often does so in context of Python. One specific example I can remember was a library written in Fortran, with unit tests written in Python.

Fortran and Python go together very well (they complement each other in just the right ways and there's all the bindings / data structure compatibility with Numpy that you need). Glue code / UI in Python, Numerics in Numpy + homegrown Fortran, that's how I'd implement a numerical model from scratch today.

Just out of curiosity, what do you use for calling Fortran from Python (e.g. f2py, ctypes)? Do you have any suggestion about how to combine them together (e.g., for parallel calculations)?

So I haven't yet had the chance to do it myself, but yes I'd look at f2py first and how to integrate it with Numpy.

also a lot of computational fluid dynamics modeling (naval, aeronautical, automotive, etc.)

Thanks.