General > General Technical Chat
Mess with your minds: A wind powered craft going faster than a tail wind speed.
electrodacus:
--- Quote from: bdunham7 on December 17, 2021, 07:49:30 pm ---
--- Quote from: electrodacus on December 17, 2021, 06:44:39 pm ---Fair points so I took a look on what pressure will you have on a flat 1m^2 wall in 10m/s wind.
So I will say this is a good explanation of what happens as there will be super high air density just next to the wall but I think my way of calculating what happens is way easier and accurate.
Maybe I'm just biased but I'm confident my results are correct as they match any real world experiment.
--- End quote ---
You appear to have realized that your energy transfer model would require the initial acceleration to be very, very high--diverging to infinity as the speed is zero. And so it won't work.
Keeping in mind that the Blackbird is not a wall or a sail, you are going to have to stipulate what the equations or formula or calculator or whatever-have-you are going to produce for the force on the sail, if we're using sails to demonstrate something. I thought we had that all worked out with formulas that you and IanB agreed on and the result was 60N at zero vehicle speed and 53N at 1m/s vehicle speed. If you want to change that, fine--but justify it somehow.
And perhaps this wouldn't be so hard to verify experimentally, and perhaps someone else already has and such results can be googled.
--- End quote ---
Have you looked at the link I provided. The same very high pressure the closer you are to the sail / wall.
To me all seems to line up and make sense both calculations if done properly will have the same result I provided. For me the one involving power and energy works better (simpler).
Blackbird propeller fan will act as a wall with variable area.
There will be a significant pressure differential on upwind and downwind of the disc shape created by the blade swept area.
Kleinstein:
--- Quote from: electrodacus on December 17, 2021, 07:57:14 pm ---
To me all seems to line up and make sense both calculations if done properly will have the same result I provided. For me the one involving power and energy works better (simpler).
--- End quote ---
The calculation with only energy and avoiding forces at all costs gets quite complicated and error prone if there are elements (like a sail) that don't have 100% efficiency. It may look easier to calculate with energy only, but I am afraid this is only an easy way to the wrong result. A sail is not 100% energy efficient - with low speed it is actually rather low efficiency.
Even if you think you got the energy based calculation done, please also have a look at the way with forces, without taking the result from the other calculation for granted. The force based calculation is not that complicated and the way it is usually done.
electrodacus:
--- Quote from: Kleinstein on December 17, 2021, 09:06:38 pm ---
--- Quote from: electrodacus on December 17, 2021, 07:57:14 pm ---
To me all seems to line up and make sense both calculations if done properly will have the same result I provided. For me the one involving power and energy works better (simpler).
--- End quote ---
The calculation with only energy and avoiding forces at all costs gets quite complicated and error prone if there are elements (like a sail) that don't have 100% efficiency. It may look easier to calculate with energy only, but I am afraid this is only an easy way to the wrong result. A sail is not 100% energy efficient - with low speed it is actually rather low efficiency.
Even if you think you got the energy based calculation done, please also have a look at the way with forces, without taking the result from the other calculation for granted. The force based calculation is not that complicated and the way it is usually done.
--- End quote ---
All this calculations are done for ideal case so to get the best case and show that even with best case (ideal) no vehicle can exceed wind speed direct downwind without energy storage.
But in terms of efficiency (converting wind power to kinetic energy) the sail is by far the most efficient.
Propeller may peak at around 70% efficiency so nowhere near a sail and the way wind interacts with any vehicle will be as an equivalent sail. It needs a surface area in order to push the vehicle.
That is why is just impossible for wind to power a vehicle that is driving directly downwind at or above wind speed.
This seems to me so obvious that it is frustrating to see people trying to find way a vehicle can still be powered by wind when no air molecules can push forward the vehicle. Above wind speed there is an apparent wind but the direction opposed the vehicle travel direction thus it can not be used to accelerate the vehicle.
Same applies for a direct upwind vehicle and while fairly different as wind power is always available it requires energy storage as the wind direction opposes the vehicle travel direction. And for direct upwind I even provided video of a toy vehicle showing exactly how energy is stored and released multiple times a second.
So despite all tests being fully supported by my theory (not actually mine as I have not come up with anything new) people seems to just prefer to "believe" the current wrong explanation.
The current explanation is not even close to describing what is seen in the test results and the claim is in violation of energy conservation.
When you take any energy from the wheel while above wind speed all that is taken out of the vehicle kinetic energy and you can only put back at best of fraction of that back as propeller is at best 70% efficient but even if ideal 100% efficient it will not be able to accelerate.
Kleinstein:
--- Quote from: electrodacus on December 17, 2021, 09:19:57 pm ---All this calculations are done for ideal case so to get the best case and show that even with best case (ideal) no vehicle can exceed wind speed direct downwind without energy storage.
But in terms of efficiency (converting wind power to kinetic energy) the sail is by far the most efficient.
.....
--- End quote ---
We have spend quite some time with the low speed case to show you that the formula provided is wrong. They produce a diverging force / acceleration (e.g. accelaration to 0.1 m/s in less than 1 ms) near zero speed and are thus obviously wrong. :horse:
A sail is not the most efficient way to harness the wind. It is actually very low effciency at low speed. Assuming 100% efficiency for the sail is one cause of getting the rediclulous fast acceleration. At zero vehicle speed the sail has zero energy efficiency - that is a very simple fact. A Wind turbine has a efficiency better than zero and is thus higher effciency than the sail (proven here for the case when the windmill is not moving).
With just a sail in a 1 D world (straight downwind) one can not reach a speed higher than the wind.
However the sail is not the only option and with a more intelligent way (e.g. the prop drive like in the Blackbird) it is possible to get more energy from the wind than the sail and move faster than the wind.
electrodacus:
--- Quote from: Kleinstein on December 17, 2021, 09:46:06 pm ---
--- Quote from: electrodacus on December 17, 2021, 09:19:57 pm ---All this calculations are done for ideal case so to get the best case and show that even with best case (ideal) no vehicle can exceed wind speed direct downwind without energy storage.
But in terms of efficiency (converting wind power to kinetic energy) the sail is by far the most efficient.
.....
--- End quote ---
We have spend quite some time with the low speed case to show you that the formula provided is wrong. They produce a diverging force / acceleration (e.g. accelaration to 0.1 m/s in less than 1 ms) near zero speed and are thus obviously wrong. :horse:
A sail is not the most efficient way to harness the wind. It is actually very low effciency at low speed. Assuming 100% efficiency for the sail is one cause of getting the rediclulous fast acceleration. At zero vehicle speed the sail has zero energy efficiency - that is a very simple fact. A Wind turbine has a efficiency better than zero and is thus higher effciency than the sail (proven here for the case when the windmill is not moving).
With just a sail in a 1 D world (straight downwind) one can not reach a speed higher than the wind.
However the sail is not the only option and with a more intelligent way (e.g. the prop drive like in the Blackbird) it is possible to get more energy from the wind than the sail and move faster than the wind.
--- End quote ---
Please see this link https://eurocodeapplied.com/design/en1991/wind-pressure-freestanding-wall
Play with some numbers and you will understand why it is not wrong to have high acceleration rates at the start.
The way pressure varies at the back of a sail / wall is exponential and that is perfectly matching that decreasing rate of acceleration shown using power and energy. Not to mention they perfectly match any experimental test.
You just think sail is low efficiency because you use incorrect formula. The reality is that you can not have a more efficient wind power conversion to kinetic energy than a sail.
Also my point is that no vehicle can exceed wind speed directly downwind unless it has an energy storage device.
That energy storage device will be charged during the initial acceleration phase thus a sail vehicle will accelerate much faster than an equivalent area blackbird vehicle since Blackbird is not only less efficient but it needs to charge energy in to the storage device so that latter can exceed wind speed.
Not considering energy storage will get you in this sort of wrong conclusion that will violate the conservation of energy (even if you do not get that is the case).
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