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This is a very simple tube to create a water tornado. There are no threads so you'll need to use hot glue or epoxy to connect the bottles together. Here are two options: print big enough to glue the caps into the connector or glue the bottles directly to the connector.
Experiment with different sized nozzle diameters, small diameters are more difficult to get the vortex but take longer to drain.
Strongly recommend using the Customizer in OpenSCAD, the stl is for a random Chinese water bottle and not the same dimensions as a standard soda bottle.