The Vortex Generator is a connector for 2 bottles that demonstrates the formation of a vortex in a closed system as water travels from one bottle to another. When the bottles are turned over you create a pressure difference that you can swirl which will create a vortex as the pressure difference is resolved. You can also put a few drops of food dye or glitter in it so it looks better c:

Printing
PLA or PETG are both fine

Picking Your Perfect Bottles
Thread Type; There are 2 common threads, PCO1810 (taller) and PCO1881 (shorter) pictured in the diagram above, the PCO1810 thread type is generally stronger and you'll have an easier time tightening and preventing leaks.
Bottle Shape: A bottle with a tapered funnel shaped works best, bottles with round or odd shapes like Fanta bottles tend to have issues starting the vortexes.
Bottle Size: Anything larger than 1L is fine, 2L bottles will spin for a while, but I found that 1.25L is a perfect balance.

After picking your bottles you fill them about 70% full with water, this is when you can add dye or glitter to your bottles before attaching the top bottle. It's important not to squeeze the top bottle too hard while you screw it on otherwise it gets a bit wobbly.

The trick to creating a vortex is to put as much spin into the bottle after you flip it.