BenB
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| posted on 27/2/11 at 05:53 PM |
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Wierd engineering thought
Just curious about something (don't ask!)
Does a completely smooth surface shaft spinning round create any airflow? Does it move the air surrounding it at all? What happens at the junction
between the metal surface and the air molecules? Are the air molecules still and the metal moving at a kind of shear layer? With a perfectly smooth
surface you won't get any drag created by "grip", but perhaps Van De Waals forces between the air and metal would create some
movement?
If you introduced a non-rotating (fixed) shaft next to the rotating shaft and slowly brought them closer together would you get any
"compression" occurring between the two or would the air molecules be completely ignoring the metal molecules of the shaft shooting round
and round?
Obviously in real life there's no thing as perfectly smooth so it'll be a function of drag, rpm and shaft diameter but I'm just
curious. Nothing like a bit of theoretical surface physics to wake up the old noggin....
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contaminated
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| posted on 27/2/11 at 06:00 PM |
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I'm a scientist as well mate, but it's Sunday. Sounds like you need a beer to me 
Tiger Super Six Independent
www.southernkitcars.com
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Steve Hignett
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| posted on 27/2/11 at 06:02 PM |
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I am certainly no engineer, and it's a shame to admit it, but never will be either!
However, the way I try to find a solution or theorise something that is "above" me is to simplify it as much as poss...
The only way I could simplify your example is to imagine that the spinning shaft isn't something as small as a driveshaft etc, but if it were of
a huge diameter (metres etc) then I cannot imagine it NOT displacing some air...
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BenB
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| posted on 27/2/11 at 06:10 PM |
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yes, I probably do.
but I agree- a massive diameter shaft rotating at some crazily high RPM must surely displace air.
hohum.
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smart51
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| posted on 27/2/11 at 06:12 PM |
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fluids (including gas) stick to moving things. The layer of gas at the surface of the moving thing will be moving at almost the same speed. The next
layer will be a bit slower and so on. Moving things stir up the air, even smooth ones. Smooth, aerodynamic shapes stir up air less than rough bluff
things but they still do it.
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BenB
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| posted on 27/2/11 at 06:13 PM |
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Aha, so there will be some compression between the spinning shaft and the stationary one?
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zilspeed
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| posted on 27/2/11 at 06:24 PM |
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Forgetting the dynaimcs for a second, where both materials meet, i.e. air meets metal, there will be some adhesive forces, just like there always are.
Same forces as create a meniscus.
Now spin the shaft, and it all starts to get turbulent.
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britishtrident
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| posted on 27/2/11 at 06:26 PM |
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Look up "foil bearings" on Wikipedia
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balidey
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| posted on 27/2/11 at 06:29 PM |
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Turning shaft will generate friction with the surrounding air molecules.
The stationary shaft would not turn unless its mass so much the same, or lower than the air molecules otherwise the friction between the surrounding
molecules would just displace the surrounding air rather than turn the shaft. Sorry, not explained that too well. but basically, in air it won't
move it. Maybe in another fluid.
Dutch bears have terrible skin due to their clogged paws
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tegwin
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| posted on 27/2/11 at 06:44 PM |
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Look up tesla turbines.... they use the surface contact firction of fluid/air to turn the turbine
------------------------------------------------------------------------------------------------------------------------
Would the last person who leaves the country please switch off the lights and close the door!
www.verticalhorizonsmedia.tv
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designer
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| posted on 27/2/11 at 06:52 PM |
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Even a smooth shaft will 'drag' air.
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BenB
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| posted on 27/2/11 at 06:56 PM |
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Cool! Love the foil bearings and tesla turbines!!
I had a sneaky feeling it would move the air!
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Mr Whippy
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| posted on 27/2/11 at 07:11 PM |
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maybe this will answer your question -
linky
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BenB
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| posted on 27/2/11 at 07:23 PM |
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quote:
Originally posted by Mr Whippy
maybe this will answer your question -
linky
It certainly muddies the water nicely Although in that test the tube is moving relative to the surrounding air hence the lift. But certainly the
induced drag it talks about are applicable to the initial thing I was trying to get my head round
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Ninehigh
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| posted on 27/2/11 at 07:48 PM |
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I suppose it would be theoretically possible if the shaft was perfectly smooth down to a sub-molecular level (I mean right down smaller than
electrons)
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JoelP
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| posted on 27/2/11 at 08:29 PM |
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If it was only down to shape, maybe it wouldnt drag anything along, but there will always be electromagnetic or even gravitational effects.
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indykid
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| posted on 27/2/11 at 10:31 PM |
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quote:
Originally posted by Ninehigh
I suppose it would be theoretically possible if the shaft was perfectly smooth down to a sub-molecular level (I mean right down smaller than
electrons)
the boundary layer will always move at the same speed as the surface.
even at a molecular level, it's due to electrostatic forces rather than mechanical interface. if you could have a perfectly smooth surface,
(which at an atomic level is impossible, sub atomic level even less possible) and could then negate the elctrostatic forces, you'd have a
shaft spinning with zero influence on the surrounding fluid
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