| General > General Technical Chat |
| Mess with your minds: A wind powered craft going faster than a tail wind speed. |
| << < (208/285) > >> |
| IanB:
--- Quote from: Kleinstein on December 22, 2021, 05:07:48 pm ---With a failure to recognize even such a simple point, I think I will give up on convincing electrodacus. Not sure if he does not know better or is acting as a troll just to keep the discussion running in circles, repeating his wrong claims over and over again. --- End quote --- This is, apparently, the unfortunate truth. When I did the analysis of the cart on the belts a few posts back, and showed how the cart can move against the direction of the belts with appropriate gearing, this analysis was simply rejected as contrary to the belief that such a result is impossible. |
| electrodacus:
--- Quote from: Brumby on December 22, 2021, 08:10:01 am ---Wow. Just, wow. That is just so wrong. There is no energy storage as you maintain. There can't be. --- End quote --- Is air a compressible fluid ? If so using a propeller/fan will create a pressure differential. I will no longer post the graph but you can check that here https://en.wikipedia.org/wiki/Axial_fan_design Having a pressure delta is similar to having a compressed spring so energy can be stored that way. The fact that you (all here) do not understand what energy storage is (among many other things) dose not make what I say invalid. The equations I presented and the explanation fully matches what happens in all tests that where done. On the other had you multiple wrong equation do not predict what is observed from all experimental test's. Take just Derek's formula using (vehicle speed - wind speed) in the video I critiqued and that formula will predict deceleration when below wind speed (never observed in any experiment not to mention ridiculous) and predicts ever increased rate of acceleration when vehicle is above wind speed again never seen in any test and ridiculous to even claim. Not being able to provide the most important equation of wind power available to vehicle for a wind power only vehicle means you (all) have no understanding of the subject. Seems to be more of a cultural thing that you never answer with I do not know and trow in guesses no matter how unlikely they are to be correct. Correct formula for a direct down wind vehicle in therms of available wind power is this: 0.5 * air density * area * (wind speed - vehicle speed)^3 Not only the formula is correct it also shows that without energy storage no direct down wind vehicle can exceed wind speed. It is as simple as that if you (all) where to understand physics. It was a waste of time to discus the issue with you (all) but I did try. Hopefully someone with more patience and better teaching skills will give it a try. For now I have better things to do and if I can think of a simpler way to explain this to you (all) I will maybe attempt that in the future. |
| electrodacus:
--- Quote from: Kleinstein on December 22, 2021, 05:07:48 pm --- The problem is in what the formula stands for: The (w-v)³ type formular is for the power available to a moving wind turbine (ignoring the power needed of vailable form the movement of the turbine). It is the the same as the normal ergy availabel to a wind turbine, just with a different wind speed. This is different from it is totally different from the power from a sail on the vehicle: that power is force times vehichle velocity as also shown in the calculation of IanB. So the 2 parts together may make more sense. Anyway the equation for the power a sail vehicle could use is not even relevant for the calculation that shows that the Backbird vehicle can work. With only using a formula for a sail based setup on can not calculate the prop driven vehincle, as this is something different. When you do the calculation for the simple (e.g. 1 D world directly downwind) sail driven vehicle the result is that it can not go faster than the wind. This still does not say anything about a sail driven vehicle going zig-zag. This is known to be able to go faster. I only took a more detailed look with the case of zero and low speed, to show that the 0.5 * air density * area * (wind speed - vehicle speed)^3 form is obviously wrong for a sail based vehicle. For some reason electrodacus has difficulties in accepting that he can be wrong with his equation. It is not the only point he is wrong, but I had hope he would recognize the error in a equation that is not really related (at least I don't see a good way to use it one way or the other) to the main question. With a failure to recognize even such a simple point, I think I will give up on convincing electrodacus. Not sure if he does not know better or is acting as a troll just to keep the discussion running in circles, repeating his wrong claims over and over again. The sail in the calculation is just the planar surface with wind perpendicular (down wind). So not sophisticated areodynamics included. --- End quote --- The equation 0.5 * air density * area * (wind speed - vehicle speed)^3 perfectly applies to any direct down wind vehicle no matter if it is a cart with a sail one with a wind turbine or one with a propeller/fan. The difference on the propeller/fan is that energy can be stored as pressure differential. Maybe pressure differential energy storage is a bit complicated to understand but all you should need to know is that the above mentioned equation is correct and if you can understand that you will see why energy storage is needed else wind speed can not be exceeded direct down wind and no speed can be achieved direct upwind. The type of energy storage is very different for direct down wind (pressure differential) and direct upwind (small elastic or gravitational storage and stick slip hysteresis). |
| Kleinstein:
--- Quote from: electrodacus on December 22, 2021, 06:14:14 pm ---Correct formula for a direct down wind vehicle in therms of available wind power is this: 0.5 * air density * area * (wind speed - vehicle speed)^3 Not only the formula is correct it also shows that without energy storage no direct down wind vehicle can exceed wind speed. --- End quote --- Were do you get that magic formula from ? It is not supported by the wind power PDF you linked and I have not seen it in any books. The standard formular is just the 3rd power of the wind speed, without the vehicle moving. With the idealization of the Betz limit this is OK. The moving wind turbine is a rather exotic problem and thus usually no read made tabulated solution for this problem and one has to derive it from the general principles. |
| electrodacus:
--- Quote from: Kleinstein on December 22, 2021, 07:08:37 pm --- --- Quote from: electrodacus on December 22, 2021, 06:14:14 pm ---Correct formula for a direct down wind vehicle in therms of available wind power is this: 0.5 * air density * area * (wind speed - vehicle speed)^3 Not only the formula is correct it also shows that without energy storage no direct down wind vehicle can exceed wind speed. --- End quote --- Were do you get that magic formula from ? It is not supported by the wind power PDF you linked and I have not seen it in any books. The standard formular is just the 3rd power of the wind speed, without the vehicle moving. With the idealization of the Betz limit this is OK. The moving wind turbine is a rather exotic problem and thus usually no read made tabulated solution for this problem and one has to derive it from the general principles. --- End quote --- The formula of course contains just wind speed for a stationary device like a ground mount wind turbine but since this refers to a vehicle moving directly down wind the wind speed relative to vehicle is what counts and that is wind speed minus vehicle speed. And this same formula will also apply to a wind turbine if the wind turbine is installed on a vehicle moving directly down wind. And yes is not a usual thing to install the wind turbine on a moving vehicle (it will not make sense since if you need to move the vehicle a sail will be more efficient and if you need electricity you just get that at the wheel in any wind powered vehicle). The formula is for the ideal case but applies to all wind powered vehicles and in the form I shown applies to a directly down wind vehicle or a directly upwind . Since for direct upwind the speed direction is negative (wind speed - (-vehicle speed)) you get (wind speed + vehicle speed) so you always have access to wind power but you need energy storage as direction of the wind opposes vehicle direction. You can find different ways to calculate but result can not be different since this perfectly matches what is observed in real tests. And also this formula is used if you want to know how what is the minimum power your vehicle engine or motor requires in order to overcome the drag due to wind. Most small passenger vehicles have around 2.5m^2 of frontal area and if they are aerodynamic the Coefficient of Drag may be around 0.3 thus effective frontal area 2.5m^2 * 0.3 = 0.75m^2 So say this vehicle needs to drive on highway at 120km/h about 33m/s and there is no wind. Then vehicle will need 0.5 * 1.2 * 0.75 * (33)^3 = 16.2kW just to deal with the drag. A bit more will of course be needed to deal with other friction losses internal and rolling resistance but this air drag will be the most significant. So no vehicle with those characteristics 0.75m^2 effective frontal area can ever claim it needs less than 16.2kW/120km/h = 135Wh/km |
| Navigation |
| Message Index |
| Next page |
| Previous page |