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Mess with your minds: A wind powered craft going faster than a tail wind speed.
bdunham7:
--- Quote from: electrodacus on December 16, 2021, 08:34:03 pm ---That will not be directly downwind it will be at an angle and it will get to another location compared to someone that went direct down wind.
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Then how about the case that I stated where the boat makes one turn at the halfway point and arrives exactly 20 miles downwind in less than an hour?
electrodacus:
--- Quote from: Kleinstein on December 16, 2021, 08:41:04 pm ---At a vehicle speed much smaller than the air speed it does not matter if one subtracts the vehicle speed. At zero vehicle speed your (wrong) formular gives the full power theoretical power in the wind, and this is not the power actually captured. A sail is not at all 100% efficient in converting the energy ! Going against the wind actually needs extra power, while the could provide power if used in a different way. At zero speed the efficency is zero.
When the vehicle moves at the speed of the wind it does not matter if w-v or v-w is used, both would be zero. So even if the sign is wrong it would not make a difference at that point. When using the correct formular for the power, there is (w-v)² * v and in the square the sign makes no difference.
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That is potential power if you want so it will not do any work but as soon as vehicle moves it will do work.
Obviously at vehicle speed zero w-v or v-w will make no difference other than the sign of potential power showing direction in witch the power can be used if you start moving.
The sign is important as it will show if vehicle accelerates or decelerates and w-v is correct as it will show vehicle can accelerate when vehicle speed is below wind speed directly downwind while using v-w will mean vehicle decelerates while vehicle speed is below wind speed and that will not match any real experiment.
The reason Derek decided to use v-w was to show there is available wind power when vehicle is above wind speed but he needed to use a wrong formula to match his wrong understanding of how the vehicle actually works. Vehicle is not powered by wind directly when vehicle above wind speed (that will be impossible) but it is powered by stored energy.
While that energy that was stored is still wind energy since it is stored energy it will get used up by all the vehicle losses so any real vehicle will slow down after that is used up.
Vehicle is in touch with only two mediums and those have a relative speed of wind speed - ground speed so basically wind speed since that is referenced to ground.
Vehicle weight 100kg vehicle sail area 1m^2 only allowed to drive directly downwind
Relative to ground 0m/s
Wind speed 10m/s
Vehicle speed 0m/s
Potential wind energy in this above mentioned conditions will be
0.5 * 100kg * (10)^2 = 5000Ws = 1.39Wh and this is the max kinetic energy an ideal vehicle can get to since potential wind energy will decrease and vehicle kinetic energy will increase.
How come nobody answered the time it will take such a vehicle to get to half the wind speed so 5m/s ?
To be able to correctly answer this question you will need to understand the relation of vehicle speed to available wind power.
I'm sort of an expert in this because I work (my hobby also) in renewable energy storage so I investigated all types of energy storage available and also designed my own wind turbine many years ago. I ended up not using the wind turbine as solar PV made way more economic sense even with relatively good wind resources at my location.
I have designed my own net zero energy house and so both electricity for appliances and heating are supplied by PV solar with energy storage in LiFePO4 and thermal storage in thermal mass (LiFePO4 cost amortisation is around 20cent/kWh while thermal storage is just 1cent/kWh).
I'm not an expert in all area of physics but I have large amounts of experience in energy storage of any type and renewable energy generation.
Kleinstein:
--- Quote from: electrodacus on December 16, 2021, 09:09:19 pm ---That is potential power if you want so it will not do any work but as soon as vehicle moves it will do work.
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Some ( a small fraction when at low speed) of the power would be used if going with the wind, none of this power would be used when moving against the wind. It is wrong to assume that whole available power would be transferred to the vehicle when moving with the wind - only a small fration ( vehicle speed / wind speed) is.
--- Quote from: electrodacus on December 16, 2021, 09:09:19 pm ---Obviously at vehicle speed zero w-v or v-w will make no difference other than the sign of potential power showing direction in witch the power can be used if you start moving.
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How do you define a direction for the power ? power is a scalar value, not a vector so it has no direction. The sign shows wether a system takes up power or gives up power, but this not a direction of geometric sense. A direction in space is a feature of the force.
--- Quote from: electrodacus on December 16, 2021, 09:09:19 pm ---The sign is important as it will show if vehicle accelerates or decelerates and w-v is correct as it will show vehicle can accelerate when vehicle speed is below wind speed directly downwind while using v-w will mean vehicle decelerates while vehicle speed is below wind speed and that will not match any real experiment.
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The importance of the sign is correct, but with the (w-v)³ formular this gets you the wrong result (allways positve) near zero. This is because the fomular is for the theoretical possible power, but not the actual transfered power or power needed to push the vehicle. The formula for the actual power under the condition ( for W>V) , goes with (w-v)²*v gives the right change in sign at aroung v=0 and thus include the transition between against and with the wind. This formular was never meant to also include the case v > w. To also incluse the reversal of the wind direction at the vehicle it needs an extra factor for the direction. So the form for the full range is more like (w-v)2 * v * (w-v) / |w-v| for the direction of the drag force.
Anyway for the calculation of the vehicle with the prop we do not really care about a vehicle with only a sail. For the ideal case with a 100% efficient prop the formular for the wind power would come up, as the inverse as the power at least needed for the prop to drive the vehicle. A real prop would need more power, but can still be good enough.
The case of the backbird vehicle is different from the case of driving against the wind with the wheels driven by the prop / fan. An important point is that the prop is driving the vehicle forward, while the wheels provide the power. One may get a wrong idea here when using the analog to the sailboats. So I consider the sail-boat part rather confusing for understanding the version with the prop.
As the prop is used for propulsion and not to generate the power directly, we actually do not care about how much power the prop in theory could produce. For the way that I find easiest to understand I don't need that formula and I also don't need complications with storred energy. There are ways to make it work with storred energy, but there are also ways to make it work without. The picture is relatively simple and does not need a prop with ideal or even good efficiency. I have shown this before, so don't repeat here. The main point that electrodacus tends to get wrong is that the bances of forces decides which way the vehicle can accelerate. There is no such thing as a balance of driving and breaking power - especially not if the efficiency is not 100%.
bdunham7:
--- Quote from: electrodacus on December 16, 2021, 09:09:19 pm ---How come nobody answered the time it will take such a vehicle to get to half the wind speed so 5m/s ?
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Because it depends on the mass of the vehicle, which can be made arbitrarily small for theoretical calculations. Some aerogel and carbon fiber if you like. It doesn't affect the end result, only how long it takes to get there.
--- Quote ---I'm not an expert in all area of physics but I have large amounts of experience in energy storage of any type and renewable energy generation.
--- End quote ---
Let me guess--you're a completely self-taught iconoclast. Your constant muddling of terms and concepts is a dead giveaway. The very basic error in your understanding of mechanics is actually not an uncommon one for beginning physics students, however your intransigence and resistance to the methods and examples typically used to overcome those issues is truly astounding. I suspect you believe that conventional views on physics are somehow 'wrong' and you have your own personal way of thinking about it that is 'right'. That would be OK, sometimes I think academics and others can be rigid and pedantic about certain issues that don't matter in nature, but when you come up with a clearly wrong answer, verifiable through experiment, it is time to reexamine your views and methods. You've actually made some very good models and drawings that quite nicely disprove your points, but then you post them and claim just the opposite.
So here are some things you've gotten very wrong.
1) Conservation of energy by itself is not a law. You need a lot more definitions and conditions--which are not met in most of your examples and ruminations--before you can use it.
2) 'Stick-slip hysteresis' and all other strange phenomena you refer to are probably real physical effects (if we could figure out exactly what you mean) but can be made arbitrarily small in a theoretical model--and if you think they must be included, you need to quantify them.
3) A sailboat cannot use 'stored kinetic energy' to travel faster than the wind for 45 minutes. A large oil tanker would have trouble with that, a sailboat will coast to a stop within a few hundred yards (with a buttered teflon hull) and within a few inches (on anything I've sailed on).
4. The Blackbird can indeed travel straight downwind indefinitely at faster than wind speed.
Edit: If anyone thinks they can convince Electrodacus to admit his errors, read this and the subsequent replies first....good luck!
https://www.eevblog.com/forum/chat/mess-with-your-minds-a-wind-powered-craft-going-faster-than-a-tail-wind-speed/msg3665644/#msg3665644
thm_w:
--- Quote from: electrodacus on December 16, 2021, 01:52:12 am ---People on a motorcycle are shielded by the motorcycle body there is no 0.5m^2 of human exposed to those speeds.
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What? Even skydiving where your whole body is exposed does not need shielding at 240km/h, just goggles:
"A stable, freefly, head down position has a terminal speed of around 240-290 km/h (around 150-180 mph). Further minimizing body drag and streamlining the body position allows the skydiver to reach higher speeds in the vicinity of 480 km/h (300 mph)."
--- Quote from: BrianHG on December 16, 2021, 08:31:25 am ---The resistance drop on under inflated tires tends towards an exponential curve, not linear and it also depends on the weight of the driver. I used to be 250lb, an 80psi tire was mushed close to the rip and believe me, I was adding a good additional 50-75 watts just to maintain 25km/h compared to 120psi tires at that weight.
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It may have felt like 75W, but even a really poor performing tire at 40psi is sub 30W total: https://www.bicyclerollingresistance.com/cx-gravel-reviews/challenge-grifo-pro
If its near the rim we are probably talking ~15psi, severely under-inflated.
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