Naked Science Forum
Non Life Sciences => Technology => Topic started by: Michele Benn on 24/12/2008 15:58:57
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Michele Benn asked the Naked Scientists:
Is it possible to capture the wind that is produced from driving a automobile to power the vehicle. It has to be good for something.
What do you think?
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At a guess, I would say that the wind wouldn't be nearly powerful enough.
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That 'wind' isn't really wind, at all. The air may well be stationary. What you feel is 'drag' because you keep pushing air out of the way, losing energy in the process. You won't get that energy back.
You could produce a greater force by having a wider vehicle but that would involve even more loss of energy (supplied by the fuel).
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There is a trade off, but how much of a trade off is it? Only a build to test real time, will give a true effect to measure.
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What if you had tiny turbines that spin around on the side of the road from the cars dragging the air past, they might be able to power a light or two.
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There is a trade off,
It's hardly a trade off: slowing the vehicle a bit or a lot, in order to get a small amount of energy.
If you wanted energy from the engine you would be better to take it directly off the shaft.
As for using energy from passing traffic. Low drag bodies are the best value for motor cars so they would produce relatively little power in the form of 'wind'. Anything which took substantial power from the disturbed air from passing cars would almost certainly couple itself to the cars and slow them down a bit. You can't get owt for nowt, lad.
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It wouldn't be practical no, but how would it slow the car down? I've nearly been blown over while walking along the highway by a truck passing me at 100km/ph, did I somehow slow the truck down?
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Have you ever tried 'slipstreaming' as you get in position just behind a big truck? You are getting energy from it 'for free'. If you pull ahead, it will drag you back - it's getting energy from you.
If the vehicles are near enough for you to get energy from them, then they will almost certainly, lose some. This, of course, assumes that the vehicles have sensible profiles - you could, just, imagine where an appropriate shape of 'tunnel', you could get some energy out and eliminate some of the original losses. BUT no one would design a vehicle like that!
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The truck is doing the work to push the air out of the way and creating a slipstream whether you're in it or not though, you're not slowing a truck down by tailgating it
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Short answer: No.
Longer answer: No, it is not possible. The movement of your car was created by burning gasoline or Diesel, or whatever. The source of energy you used to get moving created the "wind" you feel. You never create energy, you just transform it from one state into another. There are losses in each conversion of energy. Gasoline engines are not efficient (I remember %40 max.). The wind turbine you would put on your car would not be efficient. It would slow down you car, that's all.
Different story though if you don't mind TO STOP or SLOW DOWN. That energy could be used and is used in e.g. electric vehicles to charge the batteries.
There is a reason why the US patent and trademark office does not require a prototype for any invention with the EXCEPTION of a WORKING prototype for a perpetual motion patent application. They were tired of having to deal with inventors and inventions that claim to have invented a machine that powers itself forever. Now they have no more applications for this. Zero. Because you cannot build such a machine.
Hope this helps.
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The truck is doing the work to push the air out of the way and creating a slipstream whether you're in it or not though, you're not slowing a truck down by tailgating it
Yes you are. You are changing the airflow and, thus, the pressure at the back of the truck. Just because you aren't touching it doesn't mean you have no effect.
The majority of the energy loss from a moving vehicle is due to turbulence, which you can't get energy from. The non-turbulent flow is actually quite small - the air just parts and comes together again, which doesn't involve much power. If you put your car behind a truck, near enough to get useful power from it, then the truck loses more power than it would if you weren't there.
Hire an under-powered transit and experiment, trying to overtake a big truck. You will soon fine that, when you are in the position where the truck is actually tailgating you and you will find that you slow down. Between you and the truck is a low pressure region, pulling you back and the truck forward.
Remember, I have already stated that a rubbish (or appropriate) design of vehicle could do what you want but they don't make them like that - it would be a total waste except when going past your energy extraction system.
Aerodynamics is not a subject which is intuitive. That is an approximate quote from Colin Chapman in his book about the Sports Car. He should know!
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Aerodynamics is not a subject which is intuitive. That is an approximate quote from Colin Chapman in his book about the Sports Car. He should know!
I plead guilty to using intuition instead of studying what the hell i'm talking about. But what about before how I was saying a passing truck can near blow you over if you're close to it, does that slow the truck down? you're not in its slipstream are you? Will a truck on a road with pedestrians (or any objects) along the side of it go slower than one without them?
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If you're close enough, then the truck will be affected. The air around the truck (effective over a region at least several truck-widths) would normally move to the side and then back, to make room for the vehicle with as small a pressure change as possible - losing no appreciable energy. The fact that you are present would change that airflow a bit and absorb some energy. It would be hard to measure such a small change in the truck's speed due to the encounter, however. A whole row of pedestrians, of a row of bollards could well affect the truck's speed if it went close enough. There would definitely be a change in pressure between the truck and an extended array of objects (a bit like the ground effect when planes are very low) but, unless there were turbulence, there would be no extra losses, necessarily.
On the other hand, you may have noticed that driving near to a paper bag in the road will often achieve little more than lifting it off the ground by a few cms. The bag may move out and back a bit, with the displaced air, but not take much energy with it - except due to a bit of turbulence.
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"Will a truck on a road with pedestrians (or any objects) along the side of it go slower than one without them?"
Intuitively speaking (=not knowing really), I would say yes. If the air movement created by the truck causes other objects to be moved, it cannot move as freely as without objects. I expect the effect is most noticeable when the objects are large and close (e.g. a tunnel) and may diminish dramatically when the distance is increased. It may also have to do with the direction those objects are moved (which has to do with the distance of the object). I would hypothesize that objects that are moved the same direction the truck moves have a higher impact on slowing down the truck. I you get sucked along, the truck is doing the work. If air was "stringy" you would see it better. Anytime the strings get stuck you slow down.
I read somewhere that a race car actually accelerates when a competitor's car behind them leaves their slipstream. My little Saturn slows down when a truck passes me. And when riding a bike you can feel quite well the pushing and sucking created by a passing truck. It was scary when my bike (a recumbent at the time) had a rear disk wheel cover. I did not like to be moved 50 cm closer anytime a larger vehicle passed. I removed the wheel cover. Too dangerous.
My intuition is based on "there is no free lunch". And while I did some truck surfing on some trips a long time ago, I came to the conclusion that I may be saving gas, but we are not.
Karsten
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I agree with all of that.
The fact is that a lot of air gets moved around when a vehicle or plane goes through it. As long as it can move out and back with no interference, only a tiny amount of this energy is lost. This a bit like wave motion, in which there may be a lot of sloshing around but no energy is dissipated. The 'wavelength' involved is very long ( because it is a very low frequency of disturbance) so it affects a large volume of air around the vehicle.
Anything in the way can disturb the normal flow and dissipate this 'reactive' energy.
There is a different situation for boats / ships traveling at high speed, which actually launch a huge bow wave, which takes a lot of energy. They don't ever get this energy back and it does a lot of work / damage when it hits a smaller craft (me!) or the shore. This effect is akin to a plane going at near and supersonic speeds in air; the shockwave carries energy away which, again, is loss. Surface waves (on the sea) travel very slowly and a very slow boat will drift on and on with very little slowing down because there is no build up of bow wave but a fast one goes faster than the water wave and breaks the 'speed barrier'.
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Your question seems to be really amazing. Lets see what could the answer for it. Is it really produce useful wind energy (http://http:).
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There is a trade off,
As for using energy from passing traffic. Low drag bodies are the best value for motor cars so they would produce relatively little power in the form of 'wind'. Anything which took substantial power from the disturbed air from passing cars would almost certainly couple itself to the cars and slow them down a bit. You can't get owt for nowt, lad.
It'd probably be cheaper to pay taxes, although with that you can't be sure that the money's going into energy.
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Michele Benn asked the Naked Scientists:
Is it possible to capture the wind that is produced from driving a automobile to power the vehicle. It has to be good for something.
What do you think?
i think scientist are now making moves to do an experiment regarding with this matter. i think it can be done. since we are intelligent.
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