Re: Spin Doktors, and the Bathtub Vortex
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Re: Spin Doktors, and the Bathtub Vortex
 
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Dear Peter, There HAS to be SOME lesson for us handwavers. If one charts the pressure-relative rate of flow as one allows and disturbs the formation of sink-vortices, does one see increases and decreases? Ok, so let me try and go empirical on you. I put a measured amount of water in the basin, and let it out, and observed which kind of vortex it forms: counter clockwise. (In the northern hemisphere, for something moving downward, is that cyclonic or anti cyclonic? Now I put a drop of milk in the water to establish that there is no obvious vortex in the water left from the filling. Finally, I observe the draining under three conditions. Passive: I just let the water out. Sometimes it forms a clockwise vortex, sometimes an anti-clockwise one. Ten seconds Vortex discouraged: I put a plastic colander over the sink drain. Ten seconds Vorticity encouraged: anticlockwise; I give the water a counter-clockwise turn to get it moving. I try to do it smoothly so that I don’t make waves but get the water moving as fast as it can without rocking or sloshing. More than 20 seconds. Vorticity encouraged Clockwise: Same as above only clockwise. More than 20 seconds. Closer to 25, actually. Conclusion: Artificial discouragement of symmetry breaking doesn’t seem to hurt. Artificial breaking of symmetry does not help but hinders flow. I know you don’t like thought experiments, but they are usually the only ones I can do, so humor me a bit. Imagine that I have a parabolic basin with water in it. The basin can be rotated around its drain at high speed. I set the basin to rotating until the water climbs up the sides of the basin. Now I stop the rotation and pull the plug. At that instant there is a marvelous vortex but centrifugal force keeps the water from getting to the drain. My sense is that if I went on watching this I would find that for a given amount of water, etc., the formation of the vortex occurs at a particular stage in the draining of the basin. Once the vortex forms, the water draining seems to slow. The artificial vortex doesn’t seem to speed the draining itself, but to speed the formation of the natural vortex. (Not sure what I mean by these terms, but if you try it, I think you will see what I mean.) So, the vortex at the end of the draining process, that impedes drainage, goes on longer and is more powerful. Ok. Now I have played the empirical game, could we play the philosophical game for a few rounds? IF it were the case that a dissipatory structure formed that IMPEDED dissipation, what would a Kaufmannite CALL such a structure? And, in Kaufman’s world, is there any room for “anti-dissipatory” structures? Nick From: [hidden email] [mailto:[hidden email]] On Behalf Of [hidden email] The “Bathtub Vortex” has been much studied, by good hydrodynamicists and others, like me. It is a fine example of the Navier-Stokes Equations in their full glory, and can be solved (more or less) by techniques of Computational Fluid Dymanics (CFD). Turbulence, that often occurs, is dealt with, approximately, by Reynolds Averaging (RA) or Large Eddy Simulation (LES). Occasionally the theoretical solutions are supported by test. I have had humbling experiences trying to predict this flow for real aircraft vortex wakes and “validating” my results with flight tests on a B-707. Truth can be brutal! I would not venture into hand waving or word waffling on this topic. Friam folks may be entertained by Todor von Karman’s take. He relates that, in the waning days of the Austro-Hungarian Empire, the Viennese trolley cars had a passive ventilator on their roofs – a kind of funky S shaped impeller on a vertical axis that spun around in the breeze, and putatively sucked out the smoke from the professor’s cigars. The aero students asked their prof. to explain how it worked. He went into patient, painstaking detail with figures, equations and other Eulerian stratagems, finally deriving the sense in which it would rotate (but not the speed). The students then triumphantly noted, “But, Herr Professor, it goes the other way!” “Ah”, said the learned prof, “Zen I can explain zat, too!” I assume that aerodynamics is the same in Vienna as everywhere else, although we know, happily, the air itself isn’t. It is full of Arias, Bel Cante and Leit Motifs swirling, drifting in the breeze, that make Alt Wien truly the “City of any Dream”. If folks are interested in the “discovery” of the Karman Vortex Street, a classic phenomenon of unsymmetrical flow, I will be glad to post my article on this, as related by Todor himself, if someone can show me how to. On these subjects one resorts to a statement I’ve often made in expert witness work. “Theory crumbles before the Facts”.
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