Over-engineered Francis turbine 2-tone siren

thingiverse

Over a year in the making, I finally finished this two-tone siren. Possible musical intervals include: * Major third = 5:4. Blade ratio = 5:4 or 10:8. * Perfect fourth = 4:3. Blade ratio = 8:6. * Perfect fifth = 3:2. Blade ratio = 6:4 or 9:6. I was inspired by a [Francis turbine](https://en.wikipedia.org/wiki/Francis_turbine), a device used in hydropower generation to convert the energy of linear fluid flow into rotational energy with over 95% efficiency. I figued, why not make it work with air, chopping the exit flow to make a tone? And while I'm at it, I'll include two runners (the turbine rotors) back-to-back with a different number of blades on each side to make it emit two tones. The number of blades on each side of the runner determine the tones emitted from each side. Ideally the number of blades should be between 4 and 9. This is customizable in the OpenSCAD script provided. The body is re-usable for different combinations of runners and caps. The STL files provided are for a siren that produces two tones in a perfect fourth interval. This would be a **great** model for a resin printer, because one can tighten up the tolerances. The STL files are for an FDM printer.