Does anyone know what to call this sort of fan optimization or have any reading material on how it works or how to design them?

I'm trying to find out what purpose this type of design serves and how they are designed but have hit a wall with knowing what to even look for.

This piece of the voron stealthburner is the only example I can think of off the top of my head but I have seen them before in some sort of fan duct or funnel thing.