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| PlainName:
Blackbird doesn't need accelerating. When the prop is 'turned on' Blackbird is already at speed. Don't you take in anything I tell you, even several times? Further, even if acceleration had to be taken into account, the longer it takes the more power you acquire from the wind. It's just going to be slower, not impossible. |
| electrodacus:
--- Quote from: dunkemhigh on July 09, 2022, 08:35:15 pm ---Blackbird doesn't need accelerating. When the prop is 'turned on' Blackbird is already at speed. Don't you take in anything I tell you, even several times? Further, even if acceleration had to be taken into account, the longer it takes the more power you acquire from the wind. It's just going to be slower, not impossible. --- End quote --- Black bird can self start it is just slower (takes longer) than pushing it and in that case the people that push help charge the energy storage faster (create that pressure differential). Not quite sure you even know what acceleration means. In any case I spent 5 minutes or so to give you a proper reply including the math and you 10 second response has no value (pun intended as you provided no equations or numbers). But you have a false strong intuition that wind somehow can power a vehicle driving in same direction at higher than wind speed. |
| Naej:
--- Quote from: electrodacus on July 09, 2022, 03:15:10 pm --- --- Quote from: Naej on July 09, 2022, 09:49:22 am ---Yes the propeller is powered by the wheels. Yes it is silly. Many things in physics are silly yet correct. Yes air applies a force on the car, which pushes it. Amazing, no? By the way, you didn't tell what was the engine providing 3MW to a wind turbine so that it can stay at the same place. :-X --- End quote --- Wheels are not a power source (they are not radioactive and not being burned to extract energy from them). Wheels are an intermediary but power to any part of the (wind powered vehicle) can only be provided by air particles hitting the vehicle body (any part of the body including the propeller blades). When air particles hit the direct downwind vehicle from the back (wind speed higher than vehicle speed) the wind power is available to vehicle to accelerate in the desired direction and or store that. When vehicle speed equal wind speed there is zero available wind power to vehicle and when vehicle exceeds wind speed the vehicle will now hit the air particles so wind power available to vehicle will be negative meaning will slow down the vehicle unless as it is the case here vehicle also has access to earlier stored pressure differential that will be used to cover vehicle friction losses including air drag. --- End quote --- When vehicle speed equal wind speed there is plenty available wind power. You just need to power your propeller with your wheels, and get more force out of it than you took from the wheels so that your wheels turn faster. The air is accelerated by the propeller. So in the Earth reference frame, wind decreased behind the car: the kinetic energy of the wind is now in the car. That's how it works and if you can't understand you should read about Newton laws and stop thinking in terms of energy until you understand them. The amount of power available to a wind powered vehicle (any design) is 1/2*mass flow*wind speed^2. If the vehicle seed is twice the wind speed, you get mass flow=density*area*wind speed. Your computations with the wind turbine are quite funny. What if there is wind not for 1ms, but for 1 week? What if I replace the wind turbine by a water one? There you go: take a dam (1 million ton of concrete, say), and it produces 1 GW for 10 years. What is the speed of the dam? |
| PlainName:
Not my problem if you spent 5 mins thinking up rubbish. Watch the video again. Blackbird accelerates up to near wind speed and then the prop is unfeathered. And, again, none of the demos started at zero - all of them start at speed. And, as I just said, it may take 7Wh or whatever to accelerate however many kg to some arbitrary speed, but all the time the wind is blowing it is applying power. If it takes longer to get to speed that just means the wind power has been applied over more time, so 7Wh or 100Wh is meaningless unless you specify the time limit for allow the wind to blow. It could give you 1W over 7 hours, for instance. Or 2W for 7 hours while you nick 1W for something else. |
| electrodacus:
--- Quote from: Naej on July 09, 2022, 09:05:54 pm --- When vehicle speed equal wind speed there is plenty available wind power. You just need to power your propeller with your wheels, and get more force out of it than you took from the wheels so that your wheels turn faster. The air is accelerated by the propeller. So in the Earth reference frame, wind decreased behind the car: the kinetic energy of the wind is now in the car. That's how it works and if you can't understand you should read about Newton laws and stop thinking in terms of energy until you understand them. The amount of power available to a wind powered vehicle (any design) is 1/2*mass flow*wind speed^2. If the vehicle seed is twice the wind speed, you get mass flow=density*area*wind speed. Your computations with the wind turbine are quite funny. What if there is wind not for 1ms, but for 1 week? What if I replace the wind turbine by a water one? There you go: take a dam (1 million ton of concrete, say), and it produces 1 GW for 10 years. What is the speed of the dam? --- End quote --- How much wind power do you think is available when vehicle speed equal wind speed and what is the equation describing that ? What you say sounds exactly like "generator supplies the motor and the motor supplies the generator" not only that but there is also plenty of extra energy. Power and energy is all you need to use the forces and speed (voltage and current) are not relevant when you try to do a energy balance calculation. Do you agree that a typical propeller is just around 70% efficient for propulsion in air while a wheel is well over 90 to 95% efficient. So when you are at wind speed what is the difference between using the energy generate at the back wheels in to the front wheel/wheels instead of putting it in a less efficient propeller ? If you take 1000W from the wheels for 1ms that will be 1Ws when vehicle is at wind speed vehicle kinetic energy will be reduced by this exact amount 1Ws as there is no wind power available. Then when you take this 1Ws and put in to the 70% efficient propeller the kinetic energy of the vehicle increases by 0.7Ws so vehicle ends up with a deficit of 0.3Ws meaning vehicle has lower speed after you do this. --- Quote ---The amount of power available to a wind powered vehicle (any design) is 1/2*mass flow*wind speed^2. If the vehicle seed is twice the wind speed, you get mass flow=density*area*wind speed. --- End quote --- We are making progress as now you say that wind power available to vehicle is the same as I always say but just with the equation presented a bit different You wrote: 0.5 * (air density * area * wind speed) * wind speed2 That is correct and the same with 0.5 * air density * area * wind speed3 All you need to clarify now is that wind speed in this equation is wind speed relative to vehicle and not to the ground. The wind speed relative to vehicle = (wind speed relative to ground - vehicle speed relative to ground) and now you will have the equation I provided you with many times. Now when vehicle is at 2x the wind speed you have 0.5 * air density * area * (wind speed - (2 * wind speed))3 = 0.5 * air density * area * (-wind speed)3 Notice the negative sign in front of wind speed meaning negative power relative to vehicle meaning vehicle will be slowed down / decelerated at that rate and not accelerated. The vehicle hits the air not the air hits the vehicle when vehicle speed is 2x wind speed. If you replace air (compressible fluid) with water (incompressible fluid) you can no longer store energy in pressure differential thus you can no longer demonstrate 2x or 3x fluid flow speed for that vehicle. |
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