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 21, 2021, 11:09:06 pm ---
--- Quote from: electrodacus on December 21, 2021, 08:26:43 pm ---As soon as you start moving those now less than 600W will be split between vehicle acceleration and brake friction (lost as heat).
--- End quote ---
Is the car (and the wind through the car) not still pushing on the earth?
--- End quote ---
Think about the wind turbine (normal fixed to the ground) and then think that the more efficient that turbine is the less the tower and ground needs to deal with.
In the case of a cart with a sail you can see the stationary part as the vehicle having the same mass as the entire earth as they move together.
If there are no brakes then vehicle will accelerate based on the vehicle weight minus the friction losses that can include a friction brake.
If there is no friction all power will be available to cart to accelerate so nothing will be transferred to the ground.
A sail vehicle is just the most efficient way to convert wind power to kinetic energy. If you take the ideal case with no friction then it 100% of available power will be converted to kinetic energy. Still even this ideal vehicle can not exceed wind speed not because is not as efficient as it gets but because it has no energy storage thus there is no way for it to accelerate above the speed of the thing that is pushing it.
IanB:
--- Quote from: electrodacus on December 21, 2021, 11:10:13 pm ---When I mentioned external power I was excluding the two belts / treadmills.
--- End quote ---
OK, no belts or treadmills. They are excluded.
--- Quote ---You need to be able to demonstrate that using the energy from road treadmill (generator wheel) and applying that to Motor wheel you are able to advance forward (meaning accelerate from left to right).
--- End quote ---
It seems that won't be possible, as the treadmills are excluded. If the road treadmill is excluded it cannot supply energy.
--- Quote ---You will not be able to do that without adding energy storage.
--- End quote ---
Energy storage from where? The treadmills are excluded, so they cannot supply any energy to store. There is no other source of energy in the system.
--- Quote ---I do not think I can continue to argue here as your level of understanding is way below what is required. Not sure if this is a failing of the education system or a limitation of most humans brain (or maybe a combination of both).
Even if I setup an experiment and show conclusively that my theory is correct all you will do is defend the new theory without understanding what you are defending.
--- End quote ---
It's certainly true that your brain is different from most humans. You are exceptional. Congratulations!
IanB:
--- Quote from: Kleinstein on December 21, 2021, 08:54:27 pm ---The mechanical power from a sail is transferered as force times vehicle speed.
--- End quote ---
The sail situation is actually rather interesting.
Take a square sail, directly downwind. We can analyze the system like this:
Bernoulli's principle in stationary coordinates:
$$P_a + \frac{1}{2} \rho v_w^2 = P_s + \frac{1}{2} \rho v_s^2$$
Shifting to the reference frame of the sail:
$$P_a + \frac{1}{2} \rho (v_w - v_s)^2 = P_s$$
Therefore, the pressure difference across the sail is:
$$P_s - P_a = \frac{1}{2} \rho (v_w - v_s)^2$$
And the power transferred to the sail is:
$$W_s = (P_s - P_a) A v_s = \frac{1}{2} \rho (v_w - v_s)^2 A v_s$$
If we plot this, putting the relative vehicle speed on the horizontal axis as a fraction of wind speed, and the fraction of available wind energy captured by the sail on the vertical axis, we get a graph like this:
If we do the analysis by differentiating and finding the maximum, we find that the maximum power transfer occurs when:
$$v_s = \frac{1}{3} v_w$$
And under these conditions, the power transferred to the sail is:
$$v_s = \frac{2}{27}\rho Av_w^3$$
Therefore the sail only captures at most 15% of the available power from the wind, which is far worse than the Betz limit for a wind turbine of about 59%.
Brumby:
--- Quote from: electrodacus on December 21, 2021, 06:38:24 pm ---I do not see how else you can call the pressure differential created by the propeller other than a form of energy storage.
--- End quote ---
I would not call it energy storage - but I see why you are saying that.
--- Quote ---There is loop but it is as follow (proportions are just an example).
Wind power available to vehicle will be split say in two equal parts. One half accelerates the vehicle directly thus ends up as kinetic energy the other half will be taken from wheels and sent to propeller witch then increases the pressure differential
--- End quote ---
I'm not sure I agree with the proportions but, as you say, they are just to illustrate the example. The important part here is they represent two non-zero values.
--- Quote ---(you can look at this as increasing the apparent wind speed relative to vehicle).
--- End quote ---
You have just given the answer as to why the Blackbird could be capable of exceeding wind speed.
While ever the Blackbird is moving, the propeller will always produce this pressure (also known as thrust) which means the "apparent wind speed" (as you have called it) which the Blackbird experiences will always be greater than the actual wind speed.
All your claims about the Blackbird's abilities (or lack thereof) are 100% correct if you were to use this "apparent wind speed". It could never exceed the "apparent wind speed" for any length of time, if at all.
--- Quote ---... ... ...
For blackbird since it takes in this example half of the available power and stores that as pressure differential basically increasing the available potential energy it allows the blackbird to exceed wind speed for some limited amount of time as when above wind speed direct downwind there is no longer any wind power available to vehicle and it is starting to use the energy stored as pressure differential but it will continue to use just half of the power provided by the pressure differential to accelerate (increase kinetic energy) and then the other half it will put back in to increasing the pressure differential.
--- End quote ---
Again, I'm not enthusiastic about some of the terminology and I'd be cautious about the proportions used - but as a qualitative description, that's not too bad.
--- Quote ---Obviously since only half is put back the overall pressure differential will drop and vehicle acceleration rate will continue to drop
--- End quote ---
(Again, proportions) - But this is not unexpected. Acceleration will tend to zero.
--- Quote --- until there is not enough
--- End quote ---
As in not enough "pressure differential" (as you call it) to provide acceleration? I can agree with that. There will be a point where everything balances out. This will be the "steady state" situation I was asking about earlier.
--- Quote ---and it will need to start to slow down.
--- End quote ---
Now this statement is where (as I see it) you have made your error.
When acceleration drops to zero, it does not mean the Blackbird slows down. It means the Blackbird's velocity does not change. As long as the wind blows the same, it will continue at that same speed.
As long as the Blackbird is moving, it is continually producing (not storing) this "pressure differential" as you call it and the "apparent wind speed" will be greater than the actual wind speed.
How this translates into the Blackbird's final velocity is very dependent on the proportions you mentioned. These proportions and the mechanisms that determine them need to be properly worked out.
eti:
“Mess with your minds”? 🤨
No, not really, in fact not at all. It’s pretty obvious that the thing which everyone expects to work one way, is gonna work the opposite way. Let’s face it - one could spend months going down countless rabbit holes watching YouTube videos, but there’s only 24hrs in the day, and so what how a land sail works!
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