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If we fix a car with rotating surfaces like treadmill on the front, and may be on top surfaces of the car, I think it may reduce wind drag. The rotating surface should be of the same width as width of car. Its linear speed should be same as speed of car, and the direction, opposite that of the car.
It will be rotated using engine power or electric motor.
For the wind moving over the car, the tradmill surface will be absolutely stationary, hence no or minmal drag.
It will create problem with visibility, but something can be worked out using cameras at sides etc.
Boundry layer stuff
http://en.wikipedia...okes_boundary_layer
[doctorremulac3, Jun 11 2011]
Bubbles for speed
http://gizmodo.com/...s-bubbles-for-speed
Just make the bubbles less dense than air, should help. [csea, Jul 12 2011]
Water Willy
http://www.coolmagn.../images/slipper.jpg
Very hard to keep hold of! [Twizz, Jul 13 2011]
Ahem... Sort of halfbaked?
Virtually_20Spiraling_20Automobile
Elves promote no shame! [daseva, Jul 29 2011]
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So... the bearings which separate this surface from the chassis will generate less friction than the air does? |
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I am not sure if it is possible to predict this with theorotical calculations. Some one needs to try this out to make sure. |
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This treadmill will have to travel across a three-
dimensional curve, smoothly, and not significantly
contribute to surface area by its shape. Even if you
could get it to work, the weight alone would
probably offset any fuel savings. Perhaps some golf-
ball dents? |
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This is like a caterpiller wheel, except that it is for the air instead of road surface. |
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[Rayf] I think light weight material should suffice for the rotating surface. |
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Good point, [21 Quest]. Maybe this treadmill surface would have to pass externally over the top of the car, then pass into a slot somewhere under the rear bumper for a return journey *inside* the chassis, to re-emerge at the front from under the radiator grille. |
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Skin friction is not the only problem. You are still
an object making a whole in the air. |
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It would be interesting to try this with a free-
rolling panel on the sides of a lorry. That way you
would know the ratio between the skin friction
and the bearing friction. |
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You would of course have the same amount of skin
friction on the inside causing effects too
complicated for me to guess at. |
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Pertinax hit it. You've got a lot of friction from
where the treadmill contacts the car with
bearings,
rollers etc. If you put power into it that's power
you
could just as easily put into turning the wheels.
My guess is it would be a net loss. |
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That being said, if it were a passive mechanism
that just turned quickly due to the air friction,
that turning would represent decreased drag. How
much? Dhunno. Might be an interesting wind
tunnel model experiment or a Mythbusters
episode. |
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//if it were a passive mechanism that just turned
quickly due to the air friction// |
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And solve the problem of curving in two planes by
making it a gas or liquid, which flows over the
surface of the car. The substance would have to
adhere tightly to the surface, while sliding over it
with low friction. |
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Which sounds a lot like laminar flow. So, wouldn't
a passive system be, at best, exactly as good as
streamlining and no better? |
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[marklar] in your lorry experiment, put, on the
inside of the truck, another roller, separated from
the first by a narrow gap, with servocontrolled
motors keeping it moving at exactly the same
speed as the inner surface of the primary roller,
whatever that happened to be at any moment.
This would eliminate the "effects too complicated
to guess at" and enable you to measure just the
quantities you want. |
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Let's push this idea to an extreme, and see what
happens. Imagine an aquatic creature, which
swims very slowly. It has no movable fins, and it's
body is rigid (can't swim by undulating or by
moving flukes or a tailfin). Instead, it's body
surface is covered with microscopic cilia, which
beat rhythmically, pushing the layer of water next
to the skin backwards. They're "feathered" on the
backstroke, so there's net movement of water
from front to back. (I think rotifers use something
like this means of propulsion.) This removes the
problem of the equal and opposite friction on the
inside of the rollers. |
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We now have something like this idea, except
with *only* the friction-reducing mechanism, and
no means of propulsion. |
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But the friction-reducing mechanism is revealed
as being, in fact, a means of propulsion. You put
energy into it, and it moves the fish/vehicle
forward. (Or the surrounding medium backward.
Same thing.) |
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So the question is, can this ever be more efficient
than putting the same energy into
fins/flukes/wheels? It seems intuitively
obvious that it cannot be *at high velocity* It also
seems intuitively likely that it cannot be *in air* or
for a *large* vehicle in water. It seems intuitively
possible that it might be more efficient for a very
small vehicle in water or other heavy medium. |
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Good anno Spidy. I think I'd heard the term Reynolds number before but you just hipped me to what it means. Pretty important concept to figure in when talking fluid mechanics. |
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I posted an associated link after reading about Reynolds numbers that's kind of interesting. |
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Every 100 lbs added to a vehicle reduces your mileage
by about 2 %. I could see adding this system adding
easily 300 lbs. I very much doubt the skin friction is
worth 6 %. Add to that the mechanical losses of
pushing a treadmill around at high speed as well as
the motor energy to move it, some means to power
that, and your losses are well north of 10 %. |
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In theory, it'd be helpful with those fluid dynamic issues? |
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To make this work, you'd need to forget about conventional hardware and think more in terms of the kind of mechanism used by amoebae. |
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Amoebae have an outer surface which can change viscosity from a relatively stiff jelly to a relatively runny fluid. At the 'back' end (relative to the amoeba's motion) the jelly changes to fluid, moves forward through the cell and changes back to jelly at the front. |
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So you cover your car with oogluk and you'll get better
mpg? |
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//outer surface// Are you quite sure it's the outer
surface, rather than the inner volume? |
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I thinks [RayfordS] has a good idea with golf-ball
dimples. Possibly could be tested on cars
subjected
to golf-ball size hail after replacing the glass. |
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Another possibility is to perforate the vehicle skin
with tiny holes, and express a lower-viscosity gas
(helium?) to detach the boundary layer. I recall
reading about this being used on submarines to
good effect. [link] |
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No chance of greater efficiency, but might be fun. |
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[csea], I'm not sure if I want a super-cavitating Honda
Accord driving past my house.... |
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MP; the two are interchangeable. Inner volume fluid turns to outer surface jelly at the front, and back to inner fluid at the back. |
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Amoebae use this for propulsion. |
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Another example is the toy referred to coloquially as a 'water willy'. (I've been trying since yesterday to find out the correct name for it - see link). This is a tube, turned in on itself and one end sealed to the other with the resultant volume filled with water. |
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Ah, thank you. I assumed for no good reason that all
the cytoplasmic flow was happening inside a
membrane that did not, itself participate. But as a
result, I now know that
"ectoplasm" has a real meaning in biology, which is
worth a little embarrassment to learn. |
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[bigsleep] correctly points out three surfaces that would
cause friction in this mechanism. Fixable with the
application of vacuum and maglev bearings. |
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Small ball-bearings driven magnetically along the surface of the car ? |
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Air heated by the exhaust or radiator would be rarer, thus less fricty, than ambient air. |
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