|MadSci Network: Physics|
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
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