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| Mess with your minds: A wind powered craft going faster than a tail wind speed. |
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| electrodacus:
--- Quote from: bdunham7 on December 19, 2021, 01:23:29 am --- Mea culpa, I forgot to put in 'area'. But you've dodged the actual question. Since all of the energy in the first question represents the total wind energy available for that particular area and the second equation represents only part of that energy, where does the rest of it go? --- End quote --- The equation is for wind power available to vehicle not wind energy. Wind power will be stored over time as kinetic energy and pressure differential in the case of direct downwind blackbird. If there was just kinetic energy like it is the case with a sail vehicle then that vehicle could not exceed wind speed unless it was driving at an angle and not directly down wind. You all seems to be confused on what power and energy is so most of your questions do not make sense. I will rather talk with GPT-3 :) |
| electrodacus:
--- Quote from: IanB on December 19, 2021, 01:40:51 am ---It's really necessary to work with power, rather than energy. For a vehicle to travel at any steady speed, v, the power it requires is the power required to overcome the frictional resistance--the rolling resistance on the ground and the drag from the surrounding air. Let's suppose that a vehicle wants to travel at a speed, v, that is faster than the wind, then it will require some motive power, Pv. If a vehicle can obtain at least that much power from the wind, using a turbine or other mechanism, then it can travel at that speed. It becomes an engineering problem of how to design that mechanism, that will necessarily include such items as turbines, fans, gears, shafts, even possibly a motor-generator combination. There is no violation of conservation of energy implied, since clearly the wind has unlimited power available (if you want more power, just use more area for your fan/turbine). To be clear, sailors know this, since they routinely sail downwind faster than the wind, as noted by my example earlier in the thread. If the wind speed is 20 mph due west, then racing yachts can travel 20 miles due west at an average speed greater than 20 mph. And they can do this indefinitely. Energy storage cannot be claimed for a boat that is travelling for an hour or more at these speeds. (Specifically, if point B is 20 miles due west of point A, and the wind is blowing at 20 mph due west, then a boat can travel from A to B in less than 1 hour.) --- End quote --- Yes an ideal vehicle will need to power to maintain speed as there is no fiction loss. You just forget that wind power is available to vehicle only as long as a direct down wind vehicle speed is lower than wind speed. You have unlimited energy to use as long as you stay below wind speed. As soon as you exceed wind speed there is no more wind power available so to do that in the first place you need some earlier stored energy. The way they will travel with a sail faster than wind will be to drive at an angle to the wind that way you still have access to wind power since vehicle while faster than wind is not faster direct down wind (it is a geometry thing) so boat is faster but wind / air particles can still reach the boat to push as boat is still slower in the direction of the wind. Then the boat once it charged energy as kinetic energy can turn strait down wind and if friction losses are low it can drive for some amount of time direct down wind faster than wind while the speed will continually drop and of course so will the kinetic energy. |
| electrodacus:
--- Quote from: Brumby on December 19, 2021, 01:38:39 am --- Using this, you have 5.5W spinning a propeller. This propeller creates a thrust force against an air mass which is stationary with respect to the vehicle. How can a non-zero force NOT create a change in velocity? --- End quote --- Yes but those 5.5W will not create more than 5.5W at the propeller output. So the kinetic energy / speed reduction to get those 5.5W will require the same 5.5W just to be where you started from in ideal case. If you have no wind power available to help or no stored energy then you can just not accelerate above wind speed. Since wind power is not available above wind speed for a vehicle traveling directly down wind then energy storage is the only option or of course some external energy source maybe solar if you want. |
| IanB:
--- Quote from: electrodacus on December 19, 2021, 02:22:52 am ---...as boat is still slower in the direction of the wind. --- End quote --- No, the boat is faster than the wind in the direction of the wind. That is the whole point of how the boat can travel 20 mph downwind in less time than the wind takes to cover the same distance. (If you disagree with this, you will have to tell the sailors that what they are doing is impossible. They will be very interested to hear that.) |
| bdunham7:
--- Quote from: electrodacus on December 19, 2021, 02:05:59 am ---The equation is for wind power available to vehicle not wind energy. Wind power will be stored over time as kinetic energy and pressure differential in the case of direct downwind blackbird. If there was just kinetic energy like it is the case with a sail vehicle then that vehicle could not exceed wind speed unless it was driving at an angle and not directly down wind. You all seems to be confused on what power and energy is so most of your questions do not make sense. I will rather talk with GPT-3 :) --- End quote --- OK, you're right. It's turtles all the way down. |
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