Will air-flow creation promote even greater air flow in existing wind?
January 25, 2015 11:16 PM   Subscribe

Say there is a flat plane/field and there is a consistent amount of wind in one direction across it across a wide area, at speed x mph. If a device that creates air movement is placed on the surface of that field, and creates air movement with the existing wind but just slightly faster (x+n same direction), will the wind in front of the air-mover travel faster than the air-mover's speed (x+n+b)?

(I hesitate to use 'wind generator' since 'generator' is more often used to describe a device that takes energy away from the wind, rather than contributing to the flow of air)

Here's an MS Paint version of what I mean for the setup, pre-question.

Math is not my strong suit; a yes or no, with some elaboration, would suffice :-3

My thinking is that the surrounding air movement might angle/slope slightly inward to the faster moving air moved by the mover and make the air in front of the air-mover travel faster than the actual output of the air-mover's +n, for an x+n+b speed ahead of the x+n mover.

Might be a good science fair project for someone with kids in school.. I don't really have any way to test it..
posted by Quarter Pincher to Science & Nature (6 answers total)

There's a long answer based on computational fluid dynamics, but here's a much shorter one based on common sense:

If that's the way wind worked, then the arrangement (device + space in front of it) would act in its own right as a "device" to create air movement (x+n+b), and the air in front of that would move even faster, and this effect would increase without limit. All you'd need in order to generate an infinitely fast wind would be a fan in a field, which is clearly absurd.
posted by flabdablet at 11:50 PM on January 25, 2015 [1 favorite]

Best answer: Agreeing with flabdablet. The air moving past the sides of the device would slow the air being accelerated by the device. There'd be turbulence at the edges of the device outlet and you'd end up with a gradient, higher velocity in the center of the device's stream, slowing toward the edges and all of that faster air spreading out and and slowing and finally merging indistinguishably with the rest of the flow.

Another way to think about it: Imagine you are running on one of those moving people-movers you see at some airports. If you reach out and drag your hand along the railing, which is "moving" but slower than your run, then the drag from touching the railing would slow you down, not speed you up.
posted by jon1270 at 3:58 AM on January 26, 2015

This photo doesn't answer your question but gave me insight to the thermodynamics of a similar set-up. Here the wind turbines are removing energy rather then adding to it. The contrails show the air temperature dropped after passing thru the turbine. I was surprised by the extent of the wake.
posted by tinker at 8:27 AM on January 26, 2015

In sailboat racing, it's accepted that the disturbance caused by a boat's sails extends downwind about 7 times the height of the mast. I would expect about that the effect of your wind generator to have a similarly limited effect.

BTW, it will have an effect upwind as well.
posted by SemiSalt at 2:00 PM on January 26, 2015

Response by poster: I can accept "No" but the parallels to justify it don't really work for me.

@flabdablet the 'absurd' illustration doesn't make sense to me in perhaps a different way that you intended -- the device adds energy in the premise, rather than taking away. Suggesting that the device removes energy, in any way, contradicts the premise. Energy is only added, in a limited amount. Energy 1 + Energy 2 does not equal Energy infinity.

The device need not have a moving blade, but perhaps instead an extremely efficient aerodynamic exterior relative to the energy moving against it.

Also suggesting that an absence of movement could cause a fan to turn is similarly contradicting, in that the mover adds energy rather than removing it. A fan that is in the middle of a still air field, adding energy to it, does move air faster than the surrounding still air, else the fan would not even be able to turn.

@jon1270 - The 'horizontal escalator' device you describe doesn't really work either, because the hand rail is at a fixed and relatively-immobile speed (in that you could not drag it even if you tried, without an obscene amount of force atypical to a runner to remove it from its mechanism.

To use that particular device as an illustration, consider this version:

There is an area crowded with people all running in the same direction who cannot be tripped in any way, and are attempting to crowd in as close as possible to fit as many people into the space as possible. There is a people mover with no hand rail (surface of the floor only) that starts in the middle of the crowd, moving in the same direction as the crowd, and people who get on this mover are able to move faster than the surrounding crowd without tripping anyone. If the people on the mover are only using the same amount of energy to run forward as the crowd is, the people on the mover move faster than the crowd and free up space at the beginning of the mover faster than space is freed from people just normally running, like a vacuum cleaner.

I can see the answer of "No." to my question taking the form of whether it would be possible for people at the sides of the mover to be able to run faster than the people on the mover and still only exert the same amount of energy as the mover's runners are.

@SemiSalt - Riding on the coat-tails of the sailboat concept -- consider an illustration of what I mean to be an ideally aerodynamic bullet train moving through still air, that could draw more air than the amount of air that it displaces, and eject that air out its back at a speed faster than the train itself is moving, with a braking system that prevents the expelled air from propelling the train faster as a result.

@tinker - Are the disturbances a result of the energy being drawn away from it, or due to the physical object merely displacing the air without drawing useful energy from them? How much displacement might there be if the blades were moving perfectly in speed with the air to the point that just as much energy were expelled as was taken?
posted by Quarter Pincher at 6:39 AM on February 21, 2015

the device adds energy in the premise, rather than taking away.

Quite so.

Consider the device as a "black box" of unknown construction. All you know about it is that (a) you put a certain amount of energy in (b) you get an increase in downstream windspeed out.

My point is that if there is some physical principle that results in an increase in downstream windspeed provoking a further increase in windspeed further downstream, perhaps via entraining some of the surrounding air in some fashion, then you can just draw your conceptual black box a little bigger so that it includes all of the region surrounding the original device where airflow speed is measurably affected by that device.

Having done that, you now have a new device that (a) you put a certain amount of energy in and (b) you get an increase in downstream windspeed out.

And then you can do the same thing again: draw your conceptual black box a little wider still, defining a third "device" that's even more effective than your second one. And again, and again, and again.

Now, it's observationally verifiable that this model does not work. If you put a domestic pedestal fan at the upwind end of a runway, you do not see a howling tempest at the downwind end.

Therefore, the single assumption upon which the model is built (that an air-pushing device can entrain surrounding wind in such a way as to make air further downstream move faster than air immediately downstream) must be false.
posted by flabdablet at 7:05 AM on February 21, 2015

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