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Mess with your minds: A wind powered craft going faster than a tail wind speed.

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bdunham7:

--- Quote from: IanB on August 31, 2021, 02:01:35 am ---(the total energy must remain the same).

This is the principle of operation of centrifugal compressors.

--- End quote ---

I'm not doubting your assertion, as I haven't tested it but I guess it's plausible for some nozzles anyway.  But I'm not sure why you assert that the energy must remain the same--and I'm familiar with Bernoulli's experiments and equations.  The vacuum cleaner is a centrifugal compressor and it imparts a great deal of energy to the air.  I would have thought it would have positive (above atmospheric) pressure from the outlet of the motor to the nozzle.  Obviously I see why a narrowed nozzle will have a lower pressure than in the hose--that's straight out of a high school physics class--but I don't see why it has to be lower than atmospheric.  I can (and have) easily reproduce the vacuum experiment using an air compressor and a nozzle, but I'm completely sure that the pressure is higher than atmospheric right up to the nozzle.

IanB:
Obviously the compressor or blower itself increases the energy of the air, so when the air goes through the compressor the conservation of energy principle includes (some of) the power output from the motor. But after the air leaves the compressor, you can then apply the conservation of energy principle downstream from there.

So the air inside the machine, before it gets to the outlet nozzle, might have a higher pressure than atmospheric, depending on how fast it is moving at that point. But inside the nozzle, when the air is flowing very fast, that is when the pressure is going to be lower.

However, vacuum cleaners do not tend to have compressors inside them, they have something more like a blower. The difference is that a compressor is designed mainly to raise a smaller flow of air to a high pressure, whereas a blower is designed mostly to move a large volume of air without increasing the pressure all that much.

In short, your question about the pressure of the air inside the nozzle depends on where you measure it. I interpreted your question to be about the pressure inside the nozzle itself, rather than before the nozzle. On the other hand, vacuum cleaners are all about achieving a clean, unobstructed flow path for the air without too many obstacles in the way (there are enough dust filters there already without adding any more resistance). So it is a reasonable assumption that the pressure is not very high on the outlet side of a vacuum cleaner, unless you block up the outlet nozzle.

electrodacus:

--- Quote from: dunkemhigh on August 30, 2021, 09:26:01 pm ---Well it has! What's the blue wheel, the green thing, etc? Is the green thing touching the blue wheel? What do the arrows mean? It would be less confusing if you told us what the things represent instead of leaving us to guess (and then tell us we're wrong about our conclusions based on those quesses)  :-//

--- End quote ---

The diagram was made for Derek to explain to him that a a wheel touching a plain on that top or bottom is not the same thing.
I think the analogy I made for him was a car put in drive (froward) but the car had only the top of the wheels touching a surface that they can drive on and in that case vehicle will move in reverse.

The diagram represents a simplified version of the device he shows in his video that device has 4 small wheels and a large wheel driven by the small wheels.
I only used a small wheel and a large wheel for simplicity.
In figure a) it is about the same thing as he tried to demonstrate a proof for higher than wind speed travel powered only by the wind but in fact what he demonstrated was vehicle powered by wind traveling at lower speed than wind speed.
His claim was green bar (lumber in his experiment) was the wind so input source for vehicle and floor was the road that vehicle traveled on.
My interpretation of a) (there can only be one correct interpretation) is that floor is the input source (wind equivalent) the small wheel is the generator wheel driving the large wheel than then travels on the lumber in opposite direction to floor (wind) direction.
The arrows represent relative speed so two arrows on the red bar (vehicle body) shows a speed 2x larger than that of the green bar (lumber) but since wind is the floor that moves much faster comparing to vehicle that travels on the lumber and actual lumber also moves in that same direction as vehicle.

Then b) is taking that lumber and touching the bottom of the wheel so you can imagine the large wheel as wider than small wheel and so there is space to run a lumber under the wheel to power the vehicle. In this scenario the green bar lumber will represent the input (so wind) and it will move 2x faster than vehicle in the same direction and so this will represent the vehicle traveling in same direction as the wind.

The idea was that for you should show why flipping the vehicle 180 degree so that Motor wheel touches the ground with what was before the top of the wheel makes a huge difference in what the vehicle represents and it will no longer be the same vehicle shown in my diagram.

If you look at forces acting on the vehicle in my diagram you will see a breaking force on the generator pointing from right to left and then a opposing force from the Motor wheel pointing from left to right and since power for the motor is only provided by the Generator wheel one of the wheels will slip or if not possible then treadmill will be locked and unable to move.  What is impossible is for this vehicle to travel from left to right.
   
On the flipped vehicle where motor wheel is to the right of the generator wheel the forces are also opposing but instead of pointing to each other they point away from each other that means that generator is powered both by propeller but also by motor wheel when vehicle travels from left to right witch is very possible in this configuration since this is a completely different vehicle just because it was flipped.

You maybe see why I did not wanted to make an explanation as it is long and people usually do not have the patience to read what I write.

ejeffrey:

--- Quote from: electrodacus on August 30, 2021, 08:05:44 pm ---
--- Quote from: IanB on August 30, 2021, 07:37:52 pm ---It does not need to be interpreted to observe the simple fact that it demonstrates, which is that the pressure in front of the fan is lower than the ambient pressure.

The fact is that fans and propellers do not compress air. What they do is accelerate the air passing through them. This acceleration generates a reaction force that drives the propeller forwards.

Unfortunately, if we show you the experiment where the cart in your diagram moves from left to right, you will dispute that too, and will claim that experiment also is being misinterpreted. So it's not worth our time showing you results that you will refuse to believe.

--- End quote ---

I almost feel like I'm talking with flat earthers and so nothing I can say will convince you. The best I can suggest is you do the experiment and see that what happens is exactly what I predict.
So have a vehicle as shown in my diagram (not flipped) and observe that whatever you do in therms of gear ratio the vehicle will move from right to left.
The flipped vehicle is completely different due to the way the forces act on the vehicle are different and that vehicle can travel in both directions based on gear ratio. That flipped vehicle represents the analog of the propeller vehicle where propeller is the generator (not a fan) and so it drives against the wind direction.
What you can observe on that one if you want is that it can not exceed the wind speed meaning vehicle relative to stationary ground can not travel faster than the speed of the treadmill.
So the vehicle in my diagram is very different from the vehicle that was demonstrated where vehicle was flipped.
I will like to show the below diagram but hopefully that will not create even more confusion for you. That shows what happens when you flip a vehicle and top of the wheel will touch the surface instead of the bottom of the wheel.
This diagram was to explain Derek what his wheel vehicle with the floor and lumber represents.

 


In any case just build the vehicle in my original diagram and experience the fact that vehicle will always move from right to left. This will be analog to getting a flat earther in to space to see for himself that earth is an approximately spherical object.

--- End quote ---

Here you go.  The setup is a little janky, but clearly demonstrates the effect.  The right wheel rotates ~2.5 times for every rotation of the left wheel.  I don't have a treadmill so I used a piece of paper which I slide back and forth under the right wheel to push on it.  I can push with the paper "treadmill" in either direction and the cart moves in the opposite direction: towards the application of force.  This is due to the gear ratio, and the formula which I posted above that you ignored and claimed had to be wrong without describing why.  No slippage is occurring on the wheels or the belt.  It works at very low speeds or moderately fast speeds, if I go faster it obviously starts to slip.

Apparently the eevblog forum won't allow video embeds regardless of size, so sorry about the imgur link:

https://imgur.com/a/lTqAFg6

Brumby:
We are still talking about "motor wheels" and "generator wheels"....??

:palm:

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