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| Mess with your minds: A wind powered craft going faster than a tail wind speed. |
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| electrodacus:
Black bird is hit by an air particle. What will happen there ? Stop thinking that a vehicle has a brake. |
| gnuarm:
--- Quote from: electrodacus on December 28, 2021, 08:57:02 pm ---The equation for force is correct and includes wind speed - vehicle speed the equation for power will also need to include both. Just imagine air as a solid hitting the vehicle. Can you see why there power available and not just force will include the speed of that solid ? This is just wrong equation 0.5 * air density * area * (wind speed - vehicle speed)² * (vehicle speed) the correct one is this for all applications 0.5 * air density * area * (wind speed - vehicle speed)² * (wind speed - vehicle speed) --- End quote --- Again, not trying to calculate the power in the wind. We are calculating the power to move the object in the wind. That power is the force times the vehicle velocity. You can dance around all you want, but the fact remains you are applying the wrong equation to this. Imagine the sail is stationary and the wind blows on it. The force on the sail is applied through linkages to the vehicle. Now the vehicle is not in the wind, only the sail. Clearly in this situation the force that must be applied to the vehicle to maintain a constant velocity is the force on the sail from the wind and the ONLY velocity that is relevant to the vehicle is the vehicle speed, so 0.5 * air density * area * (wind speed - vehicle speed)² * (vehicle speed) I've explained it, I've derived it. You are not capable of understanding it or you are just refusing to acknowledge you understand it in public. |
| fourfathom:
--- Quote from: gnuarm on December 28, 2021, 08:51:51 pm --- --- Quote from: fourfathom on December 28, 2021, 08:40:21 pm --- --- Quote from: gnuarm on December 28, 2021, 08:19:33 pm ---So the full equation for the power applied to the vehicle through the wheels is 0.5 * air density * area * (wind speed - vehicle speed)² * vehicle speed --- End quote --- Are you sure about that? When wind speed = vehicle speed, that equation still reduces to zero. --- End quote --- Yes, that is correct. When the wind speed and the vehicle speed are the same, there is no wind force on the vehicle and so no force required to maintain the vehicle speed... other than friction which we are not factoring in here. This is just about the wind forces. --- End quote --- OK. I will shut up and wait until friction is re-introduced. I want to see the situation when the vehicle speed = windspeed, and force has to be found to overcome friction. I know this works, but how the math applies to the actual real-world blades, gears, and wheels still isn't clear to me. |
| electrodacus:
--- Quote from: gnuarm on December 28, 2021, 09:12:45 pm ---Again, not trying to calculate the power in the wind. We are calculating the power to move the object in the wind. That power is the force times the vehicle velocity. You can dance around all you want, but the fact remains you are applying the wrong equation to this. Imagine the sail is stationary and the wind blows on it. The force on the sail is applied through linkages to the vehicle. Now the vehicle is not in the wind, only the sail. Clearly in this situation the force that must be applied to the vehicle to maintain a constant velocity is the force on the sail from the wind and the ONLY velocity that is relevant to the vehicle is the vehicle speed, so 0.5 * air density * area * (wind speed - vehicle speed)² * (vehicle speed) I've explained it, I've derived it. You are not capable of understanding it or you are just refusing to acknowledge you understand it in public. --- End quote --- We are trying to calculate the available wind power to the vehicle as that is all the vehicle can use since it is a wind only powered vehicle thus the most important equation to get right. The vehicle will not be stationary if there is no friction including no brake. The lowest energy state for that vehicle will be vehicle speed = wind speed. If vehicle without brakes wants to maintain zero speed relative to ground it will need to provide this power 0.5 * air density * area * (wind speed - vehicle speed)² * (wind speed - vehicle speed) And since you want vehicle speed to be zero relative to ground you have 0.5 * air density * area * (wind speed)3 This shows that even an ideal vehicle able to get all wind power can at most just use that to stay in the same place as if it will want to move upwind it will need that plus something extra. As for direct down wind the ideal vehicle can get to same speed as wind speed but not above that without having some stored energy or an external energy source other than wind. You all seems to agree that is correct for a sail vehicle but refuse to believe that is valid for any other type of vehicle including blackbird. Blackbird is not using wind power when above wind speed but it is using earlier stored wind energy and the difference is stored energy is a limited resource so a real vehicle with friction at some point will start to slow down. |
| IanB:
--- Quote from: gnuarm on December 28, 2021, 08:19:33 pm ---The equation 0.5 * air density * area * (wind speed - vehicle speed)3 may be the right equation, but for the wrong purpose. --- End quote --- There is an equation, 0.5 x (air density) x (area) x (wind speed)^3 This equation gives the flow of wind kinetic energy through the swept area of a turbine. It breaks down into three parts: 1) kinetic energy of wind per unit mass of air 2) mass flow of air per unit area 3) swept area of turbine Hence: (flow of kinetic energy) = (energy per unit mass of air) x (mass flow per unit area) x (area) If any one of those terms is zero, the available energy will be zero. If you try to apply this to a sail, you find that the mass flow of air though the sail is zero (because the sail is a wind barrier). So you cannot use this equation in this form for sails. When I brought up this point with ED and asked him to explain it, he quickly changed the subject and avoided giving an answer. |
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