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In my fluid mechanics class, I've learned that a fluid traveling in a pipe will generate a force when the exit area is smaller then the entrance area. Suppose a pipe is attached to a car that will use the kinetic energy of the air passing over the vehicle. Assuming that the car is moving at a constant velocity and the pipe is straight and the exit area is $\frac{1}{4}$ the size as the frontal area, is it feasible for a moving vehicle to generate enough force from the air to reduce energy use?

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closed as off-topic by sammy gerbil, Jon Custer, Kyle Kanos, peterh, Yashas Feb 28 '17 at 12:59

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  • $\begingroup$ Have you considered the drag if the funnel is to be of any use? $\endgroup$ – user146020 Feb 23 '17 at 23:13
  • $\begingroup$ youtube.com/watch?v=81CX-Vw3JpU $\endgroup$ – Count Iblis Feb 23 '17 at 23:23
  • $\begingroup$ Only aGerman Wikipedia available and it's about locomotives: de.wikipedia.org/wiki/Pressluftlokomotive It's the other way but anyway:-) $\endgroup$ – HolgerFiedler Feb 24 '17 at 5:28
  • $\begingroup$ The jet engines use this method I believe. $\endgroup$ – Yashas Feb 24 '17 at 6:44
  • $\begingroup$ @HolgerFiedler - your article talks about locomotives that carry compressed air as their energy source - good for use in coal mines and other areas where an internal combustion engine is undesirable / dangerous. That is not what this question is about. $\endgroup$ – Floris Feb 26 '17 at 20:21
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It sounds like you are asking for a way to get energy (or power) "for free". The answer is, no, that is not possible. Adding any pipe, of any shape, to your car will have exactly zero benefit in the best possible case, which would be when we ignore friction. Taking into account viscous drag, you will in fact lose power.

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  • $\begingroup$ I think the OP is asking about if possible to "reduce energy use", not free energy. $\endgroup$ – Shing Feb 24 '17 at 0:50
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    $\begingroup$ "Reducing energy use" in this situation amounts to exactly what I said: free energy. It's impossible in the same way and for the same reason a perpetuum mobile is impossible. It would violate energy conservation. Specifically, neglecting friction, the total force generated by such a pipe would be zero. $\endgroup$ – Pirx Feb 24 '17 at 2:59
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I just learned about air pressure, and to my knowledge you can't really generate enough force form just air pressure to power a full-sized SUV going at a constant velocity. However, if you did create some sort of high and low pressure system using the engine and exhaust pipe, you could make the car move, but the system would have to be on a huge scale to generate enough power to make a vehicle go forward.

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Yes, I believe this would be possible. You are basically saying - if I take the air hitting the front of the car, can I compress it in a funnel shaped tube, then extract power from it, and will this lower my drag.

A few things to consider:

  1. The tunnel through the car takes space; if you don't need the space, you could have made a smaller car...
  2. As the air flows faster, it will experience more drag. Whether you can come out "net ahead" is uncertain, unless you design this carefully
  3. The jet engine of some planes does exactly what you are describing (although they are also used to generate thrust by adding additional energy, the air flow path is similar to the one you are suggesting).

But note that there is no "net free energy" from something like this. What you are effectively doing is reducing the front-facing area of your car; there might be other, more efficient ways to achieve that. For example, the drag coefficient of a shape can vary quite a bit. If you assume you have quadratic drag of the form

$$F_{drag}=\frac12 \rho v^2 A C_D$$

Reducing the drag coefficient $C_D$ gives you a pretty large effect on the total drag. Drag coefficients of cars vary a lot (see this table. A Hummer H2 has a drag coefficient of 0.57, while a Toyota Prius has a factor of 0.25, and Tu Delft's Ecorunner V has a stunning 0.05 (image from https://en.wikipedia.org/wiki/Eco-Runner_Team_Delft#/media/File:Eco-Runner_5.jpg).

enter image description here

I expect that working on the external shape will give you greater efficiency gains because you can control the total air flow around the vehicle - reducing the area that the air flows over instead of increasing it.

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