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.
-- VJW, Jun 11 2011

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]

So... the bearings which separate this surface from the chassis will generate less friction than the air does?
-- pertinax, Jun 11 2011


I am not sure if it is possible to predict this with theorotical calculations. Some one needs to try this out to make sure.
-- VJW, Jun 11 2011


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?
-- RayfordSteele, Jun 11 2011


This is like a caterpiller wheel, except that it is for the air instead of road surface.

[Rayf] I think light weight material should suffice for the rotating surface.
-- VJW, Jun 11 2011


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.
-- pertinax, Jun 11 2011


Skin friction is not the only problem. You are still an object making a whole in the air.

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.

You would of course have the same amount of skin friction on the inside causing effects too complicated for me to guess at.
-- marklar, Jun 11 2011


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.

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.
-- doctorremulac3, Jun 11 2011


//if it were a passive mechanism that just turned quickly due to the air friction//

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.

Which sounds a lot like laminar flow. So, wouldn't a passive system be, at best, exactly as good as streamlining and no better?

[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.
-- mouseposture, Jun 11 2011


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.

We now have something like this idea, except with *only* the friction-reducing mechanism, and no means of propulsion.

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.)

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.
-- mouseposture, Jun 11 2011


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.

I posted an associated link after reading about Reynolds numbers that's kind of interesting.
-- doctorremulac3, Jun 11 2011


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 %.
-- RayfordSteele, Jun 11 2011


In theory, it'd be helpful with those fluid dynamic issues?
-- Sir_Misspeller, Jul 11 2011


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.

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.
-- Twizz, Jul 11 2011


So you cover your car with oogluk and you'll get better mpg?
-- Alterother, Jul 11 2011


//outer surface// Are you quite sure it's the outer surface, rather than the inner volume?
-- mouseposture, Jul 12 2011


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.

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]

No chance of greater efficiency, but might be fun.
-- csea, Jul 12 2011


[csea], I'm not sure if I want a super-cavitating Honda Accord driving past my house....
-- Alterother, Jul 13 2011


MP; the two are interchangeable. Inner volume fluid turns to outer surface jelly at the front, and back to inner fluid at the back.

Amoebae use this for propulsion.

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.
-- Twizz, Jul 13 2011


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.
-- mouseposture, Jul 13 2011


[bigsleep] correctly points out three surfaces that would cause friction in this mechanism. Fixable with the application of vacuum and maglev bearings.
-- caspian, Jul 29 2011


Small ball-bearings driven magnetically along the surface of the car ?

Air heated by the exhaust or radiator would be rarer, thus less fricty, than ambient air.
-- FlyingToaster, Jul 29 2011



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