Re: Is there a limit to how high a syphon can stretch and still work?

David,

Yes there is a limit, and there's an excellent description of it in the
MadSci archives (see the admin's note at the bottom of this
page).

You're right to think about friction and viscosity, as that will have some
effect on the siphon, but you could always imagine mitigating these effects
by lowering the downhill end of the siphon much farther below the uphill
end. The ultimate limit that you will never be able to overcome without
increasing the pressure on the uphill end is the one described in the link
above.

I haven't fully considered what might result from changing the diameter of
the tube (or cross-sectional area if it's not a circular tube), however I
suspect that it will not significantly aid a siphon. The dominant forces in
a typical siphon are related to gravitational potential, and the fluid
dynamic forces that could result from changing the cross-sectional area are
not likely to be important unless the ends of your siphon are very nearly
the same height. I admit I haven't put much thought into that aspect of it
because no matter what kind of fluid forces you can exert, if it doesn't
pump up the pressure on the uphill side, it won't help you overcome the
limit described in the link above. 

I hope this helps, and keep thinking of these interesting (if useless)
ideas. These kinds of ideas can lead to great breakthroughs, but even if
they don't (which is most of the time), they lead to a better personal
understanding of the Universe we live in.

David Coit