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

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Kleinstein:

--- Quote from: electrodacus on December 13, 2021, 10:03:54 pm ---
The turbine is supplied by wind power so you can not add that again.
Everything on a wind only powered vehicle is supplied by the wind power.
You can search research papers or similar and see that any direct down wind sail vehicle powered only by the wind will use this wind power formula
0.5 * air density * area * (wind speed - vehicle speed)^3

--- End quote ---

The formular would be OK for the power a turbine can generate on the moving vehincle, but this is not relevant here.

The energy added to a vehicle with sail (usually in the form of kinetic energy or energy available to generate from a genrator at it's wheels) it is the formula I gave. The power is simply force times speed.  Wind force is proportional to the square of relative velocity and the speed is the speed of the vehicle. You need the vehicle speed to get zero power from the sail when not moving at all, which is the trivial case: no speed means no power from a sail. You don't want the house to gain kinetic energy from the wind !


--- Quote from: electrodacus on December 13, 2021, 10:03:54 pm ---The passive sail is actually the most efficient and an ideal one will be 100% efficient.

--- End quote ---
No : the sail is not most efficient way to harness the wind, at least not in all cases. It is obviously not at low speed, aspecially 0 speed, as it is 0 efficiency there. It is also not the most efficient way when the vehicle speed approaches the wind speed, as there it is zero efficiency too. The vehicle in question here is shown to be more efficient, as it can still get power from the wind, even at the speed of the wind and a little faster. You don't have to believe it up front, but you also can't exclude it up front from intuition. Doing this is trying a circular argument so it is not a valid argument.


--- Quote from: electrodacus on December 13, 2021, 10:03:54 pm ---Sail boat going at an angle is a geometric problem as the sail will still have access to wind speed since it will not be driving faster than wind relative to wind direction.

--- End quote ---
That claim is not true: - by going zig zag with the wind a sail boat and especially an ice sail can go faster than a ballon, just not a straight line. Just look at the graph in the answer before yours. They get some 1.6 times the speed of the wind / ballon when doing a zig zag with a 45 degree angle and even more with a smaller angle like 20 degree.

electrodacus:

--- Quote from: Kleinstein on December 14, 2021, 12:33:22 am ---
The formular would be OK for the power a turbine can generate on the moving vehincle, but this is not relevant here.

The energy added to a vehicle with sail (usually in the form of kinetic energy or energy available to generate from a genrator at it's wheels) it is the formula I gave. The power is simply force times speed.  Wind force is proportional to the square of relative velocity and the speed is the speed of the vehicle. You need the vehicle speed to get zero power from the sail when not moving at all, which is the trivial case: no speed means no power from a sail. You don't want the house to gain kinetic energy from the wind !


--- End quote ---

OK let me try again this time with an example that should give you a better intuition of what happens.

1m^2 swept area wind turbine with 40% efficiency in a 10m/s wind speed   vs   1m^2 sail

If thery speed relative to the ground is zero then of course sail will do nothing and there will just be a static force as on any wall where the wind turbine output power will be 0.5 * 1.2 * 1 * 10^3 * 0.4 = 240W

So stationary wind turbine wins 240W vs 0W

Now install the wind turbine and the sail on a vehicle driving directly downwind at 0.1m/s, 5m/s and 9m/s

0.1m/s
wind turbine  0.5 * 1.2 * 1 * (10-0.1)^3 * 0.4 = 232.87W
sail (can install an electric generator at the wheel say that is 90% efficient)   0.5 * 1.2 * 1 * (10-0.1)^3 * 0.9 = 523.96W

5m/s
wind turbine 0.5 * 1.2 * 1 * (10-5)^3 * 0.4 = 30W
sail               0.5 * 1.2 * 1 * (10-5)^3 * 0.9 = 67.5W

9m/s
Wind turbine 0.5 * 1.2 * 1 * (10-9)^3 * 0.4 = 0.24W
sail                0.5 * 1.2 * 1 * (10-9)^3 * 0.4 = 0.54W

 

IanB:

--- Quote from: electrodacus on December 14, 2021, 01:25:10 am ---1m^2 swept area wind turbine with 40% efficiency in a 10m/s wind speed   vs   1m^2 sail

If thery speed relative to the ground is zero then of course sail will do nothing and there will just be a static force as on any wall where the wind turbine output power will be 0.5 * 1.2 * 1 * 10^3 * 0.4 = 240W

So stationary wind turbine wins 240W vs 0W

Now install the wind turbine and the sail on a vehicle driving directly downwind at 0.1m/s, 5m/s and 9m/s

0.1m/s
wind turbine  0.5 * 1.2 * 1 * (10-0.1)^3 * 0.4 = 232.87W
sail (can install an electric generator at the wheel say that is 90% efficient)   0.5 * 1.2 * 1 * (10-0.1)^3 * 0.9 = 523.96W

5m/s
wind turbine 0.5 * 1.2 * 1 * (10-5)^3 * 0.4 = 30W
sail               0.5 * 1.2 * 1 * (10-5)^3 * 0.9 = 67.5W

9m/s
Wind turbine 0.5 * 1.2 * 1 * (10-9)^3 * 0.4 = 0.24W
sail                0.5 * 1.2 * 1 * (10-9)^3 * 0.4 = 0.54W

--- End quote ---

So if, by your own admission, the sail power is zero when the vehicle is stationary at 0 m/s, how can the sail power suddenly be 524 W when the vehicle is traveling at 0.1 m/s? (Which is nearly stationary.)

Would the sail power be higher or lower when the vehicle is traveling at 0.01 m/s? How about 0.001 m/s?

Can you make a graph of sail power vs vehicle speed? What does it look like?

electrodacus:

--- Quote from: IanB on December 14, 2021, 03:12:31 am ---
--- Quote from: electrodacus on December 14, 2021, 01:25:10 am ---1m^2 swept area wind turbine with 40% efficiency in a 10m/s wind speed   vs   1m^2 sail

If thery speed relative to the ground is zero then of course sail will do nothing and there will just be a static force as on any wall where the wind turbine output power will be 0.5 * 1.2 * 1 * 10^3 * 0.4 = 240W

So stationary wind turbine wins 240W vs 0W

Now install the wind turbine and the sail on a vehicle driving directly downwind at 0.1m/s, 5m/s and 9m/s

0.1m/s
wind turbine  0.5 * 1.2 * 1 * (10-0.1)^3 * 0.4 = 232.87W
sail (can install an electric generator at the wheel say that is 90% efficient)   0.5 * 1.2 * 1 * (10-0.1)^3 * 0.9 = 523.96W

5m/s
wind turbine 0.5 * 1.2 * 1 * (10-5)^3 * 0.4 = 30W
sail               0.5 * 1.2 * 1 * (10-5)^3 * 0.9 = 67.5W

9m/s
Wind turbine 0.5 * 1.2 * 1 * (10-9)^3 * 0.4 = 0.24W
sail                0.5 * 1.2 * 1 * (10-9)^3 * 0.4 = 0.54W

--- End quote ---

So if, by your own admission, the sail power is zero when the vehicle is stationary at 0 m/s, how can the sail power suddenly be 524 W when the vehicle is traveling at 0.1 m/s? (Which is nearly stationary.)

Would the sail power be higher or lower when the vehicle is traveling at 0.01 m/s? How about 0.001 m/s?

Can you make a graph of sail power vs vehicle speed? What does it look like?

--- End quote ---

While there is zero power there is a force acting on the sail and so you can stay stationary only if you get good enough brakes and enough tire friction.

You can calculate so for 0.001m/s is 0.5 * 1.2 * 1 * (10-0.001)^3 * 0.9 = 539.8W

Yes you can see the graph in my video at about minute 2 https://youtu.be/4Hol57vTIkE?t=119  there the example is also for a 1m^2 sail and 10kg vehicle ideal case no friction and you can see vehicle speed (blue) vs time and power (green).

Kleinstein:

--- Quote from: electrodacus on December 14, 2021, 03:42:51 am ---You can calculate so for 0.001m/s is 0.5 * 1.2 * 1 * (10-0.001)^3 * 0.9 = 539.8W

--- End quote ---
With some 500W at only 0.001 m/s would require a force of 500 kN from the sail.
This value is obviously wrong (way to high) and shows that the formular used above must be wrong, not just at zero speed.


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