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Veritasium wrong about faster than wind direct down wind.
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fourfathom:

--- Quote from: electrodacus on December 11, 2021, 11:37:58 pm ---
--- Quote from: fourfathom on December 11, 2021, 10:54:11 pm ---Do we agree that when the wind is blowing that there is energy available to a stationary device?

--- End quote ---
There is a potential energy available yes that will be 0.5 * vehicle mass * (wind speed)^2
That potential energy can be converted in to kinetic energy
This is true for a vehicle with no energy storage device like a sail type vehicle.
This type of vehicle can drive directly down wind up to wind speed (depends on friction losses) and can not drive directly upwind at any speed.
--- End quote ---
So why doesn't this sail-type vehicle not have the compressed-air energy storage you claim powers the propeller-type vehicle?

And what is the time-constant of this energy storage?  You've been talking about air compression behind the propeller.  How much energy is stored and how quickly does it bleed off into the surrounding medium?  I think this storage, if any appreciable, lasts for milliseconds -- and the vehicle accelerates beyond windspeed much longer than that.


--- Quote from: electrodacus on December 11, 2021, 11:37:58 pm ---
--- Quote from: fourfathom on December 11, 2021, 10:54:11 pm ---Does this change if the device is moving?  There is still a speed difference between the wind and the ground, regardless of the speed and direction of the device.
In the case where the device is moving directly downwind at windspeed, there is zero apparent wind (a sailing term, wind relative to the boat), but there is groundspeed, and the available energy still exists.
--- End quote ---
If vehicle is not moving there is no power available just a static force.
--- End quote ---
There is potential energy, and it can be tapped.


--- Quote from: electrodacus on December 11, 2021, 11:37:58 pm ---Power available to a vehicle moving directly down wind is this:   0.5 * air density * area * (w-v)^3  where w - wind speed and v- vehicle speed
From this you see that vehicle speed can be any as long as it is between zero and wind speed since when vehicle is at wind speed no wind power is available.
--- End quote ---
Perhaps this holds with a flat sail.  But a spinning propeller is not a flat sail.


--- Quote from: electrodacus on December 11, 2021, 11:37:58 pm ---
--- Quote from: fourfathom on December 11, 2021, 10:54:11 pm ---Accept this, and there is no need for energy storage, "slip-stick hysteresis", or any of that stuff.  All that remains is to devise a method of extracting energy from the air/ground speed difference.  This has been demonstrated.

--- End quote ---

If you want to exceed wind speed directly down wind then you need an energy storage device so that you can store energy while below wind speed and then use the stored energy to exceed wind speed.
Yes what was demonstrated was a vehicle with an energy storage device (pressure differential) and that can exceed wind speed. The test was incomplete else it will have showed how vehicle will start to slow down below wind speed as stored energy was used up.

It is fairly frustrating to see this disregard for energy conservation on an electronics forum.

If you have electrical knowledge think about this:

You have a 10V 3A CC-CV lab power supply and you can not output higher voltage or current using a device that has no energy storage device same for a negative voltage relative to ground/GND
You will need to use either inductors or capacitors or both in order to build a DC-DC boost converter.   Using just resistor dividers you can get any voltage as long as it is between 0V and 10V to get more or less (negative) you need an energy storage device.

--- End quote ---

This is a flawed comparison.

Sure there is some sort of energy storage in a switching regulator, or even in a transformer.  But these are not "energy storage" devices, and can run continuously: 10V/1A input, 5V/2A output (assuming no losses).  The energy storage you are claiming is more like a battery being depleted, and it doesn't apply in the DDWFTTW case.  Your analogy is flawed, your models are flawed, and your equations are flawed.


electrodacus:

--- Quote from: fourfathom on December 12, 2021, 12:27:35 am ---So why doesn't this sail-type vehicle not have the compressed-air energy storage you claim powers the propeller-type vehicle?

And what is the time-constant of this energy storage?  You've been talking about air compression behind the propeller.  How much energy is stored and how quickly does it bleed off into the surrounding medium?  I think this storage, if any appreciable, lasts for milliseconds -- and the vehicle accelerates beyond windspeed much longer than that.
 
--- End quote ---

It has that but it is not useful as you can not add more to it. You can imagine you push a vehicle but instead of pushing directly you add a spring between you and the vehicle. Still vehicle you are pushing can not exceed your speed.
For blackbird is like it can compress the spring from the other side so that when it is at wind speed the spring is still compressed so it has energy to continue to accelerate the vehicle is like there is still a wind speed from the back relative to vehicle.
Spring analogy may not be perfect but for a sail type vehicle when sail speed equal wind speed the spring is fully decompressed no more energy.




--- Quote from: fourfathom on December 12, 2021, 12:27:35 am ---There is potential energy, and it can be tapped.

--- End quote ---

Yes there is potential energy exactly 0.5 * vehicle mass * (wind speed)^2
And this can be converted to kinetic energy as soon as you remove the breaks or rise the sail or remove whatever it keeps you from moving.
When all potential energy was transformed in kinetic energy (ideal case) your sail vehicle will now have same speed as the wind and the remaining air potential energy is zero while vehicle kinetic energy is 0.5 * vehicle mass * (vehicle speed)^2

In the mean time you can stay at say a quarter of the wind speed and capture as much energy as you want to fry eggs or heat the air or anything you want but your speed will remain constant if you use all that available power at that speed and so one thing you can do is store that energy as much as you want in a battery and then you can use that stored energy to accelerate way above the wind speed for a limited amount of time.




--- Quote from: fourfathom on December 12, 2021, 12:27:35 am ---Perhaps this holds with a flat sail.  But a spinning propeller is not a flat sail.
 
--- End quote ---

It is not and is more complicated to calculate area that will vary from just blade area when is not spinning up to swept area of the propeller when power output of the propeller matches the wind power.
As you put power (over time energy) in to propeller that results in increased pressure differential so as far as the vehicle is concerned it will look like higher apparent wind speed.
 


--- Quote from: fourfathom on December 12, 2021, 12:27:35 am ---This is a flawed comparison.

Sure there is some sort of energy storage in a switching regulator, or even in a transformer.  But these are not "energy storage" devices, and can run continuously: 10V/1A input, 5V/2A output (assuming no losses).  The energy storage you are claiming is more like a battery being depleted, and it doesn't apply in the DDWFTTW case.  Your analogy is flawed, your models are flawed, and your equations are flawed.

--- End quote ---

As any analogy it may have some limitations but for the direct upwind version the DC-DC converter analogy works perfectly well other than frequency of charge discharge is at least kHz for a DC-DC and it is just a few Hz for the direct upwind.
For the direct down wind an inductor is the best analogy but it will be a massive inductor .
The same way an inductor stores energy in the magnetic field around it and as long as current is constant that field will remain constant and is similar to a propeller fan runing at constant speed.
if you reduce the current energy from the magnetic field will flow in to your circuit similar how pressure differential will drop but in that time it will push the vehicle.
The propeller has another type of energy storage similar to a capacitor but that alone can not help with exceeding the wind speed (unless gear change is allowed or propeller pitch change) that other type of energy storage will be called a flywheel and definition is just two words (spinning mass).
While you may not think at a normal capacitor as a large capacity energy storage device there are massive flywheel that can store many kWh of energy.

To get some sense of the numbers.
During blackbird record test a total of 6Wh of energy was used probably about that amount was stored in the pressure differential (it was fairly close to start slowing down when it got to 30mph even if wind peak at 15mph and it helped multiple times).
I do not have exact weight from Rick but my rough estimations get around 2Wh or so stored in the propeller flywheel type energy storage so all this numbers are not that large considering the size of that propeller 5.3m diameter (huge 20m^ swept area).
Your regular inductor or capacitors are nowhere near that massive :) 
 
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