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Newton's third law problem.
Nominal Animal:
--- Quote from: electrodacus on November 23, 2022, 11:07:58 pm ---Elastic collisions is what you have between the air particles and the vehicle and that is how kinetic energy is transferred from the air molecules to the vehicle.
--- End quote ---
Only if the vehicle is basically a stiff box, and has no mechanism at all.
Consider autogyro. If what you claimed was correct, it could not fly. Yet, it does. Lift is provided by freely rotating rotor blades, due to a pressure differential between different sides of the blade. The rotation of the blades is caused by perpendicular airflow below the rotor, generated by a fore or aft propeller engine.
Things like airfoils (aeroplane wings, wings in vertical wind turbines, etc.) are not elastic collisions, as the pressure varies around the airflow. Air is a compressible fluid, and has properties which can be exploited in a mechanism, for example via by conversion between different forms of energy (which means it behaves non-elastically). Linear momentum is not conserved, but kinetic energy is, if and only if we include temperature (thermal energy) as part of the kinetic energy.
A particular example of conversion between different forms of energy in a compressible fluid (air) is de Laval nozzle in supersonic engines, converting thermal energy to kinetic energy.
The maximum speed a vehicle can attain depends on the losses. To simplify, the power (energy transferred or converted per unit time) used matches the power lost, when the vehicle travels at its maximum velocity. However, the losses can be made arbitrarily small. (As an example, if you dimple your car's surface like a golfball, you'll reduce aerodynamic drag significantly, and increase fuel efficiency, as demonstrated famously by MythBusters.) So, it is just a question of how to harvest energy from the wind, using a platform traveling downwind at velocities below and above the windspeed. It is just an engineering problem, really, hiding inside a scenario that seems counterintuitive to most people.
electrodacus:
--- Quote from: Nominal Animal on November 24, 2022, 02:15:38 am ---The maximum speed a vehicle can attain depends on the losses.
--- End quote ---
Only true for a vehicle traveling perpendicular to wind direction.
For particular cases of interest here:
Direct downwind limit is wind speed.
Direct upwind limit is zero.
Both direct downwind faster than wind and direct upwind is possible using energy storage.
For direct downwind as soon as you are above wind speed there is no longer any wind power available so you can drive above that speed only for as long as the stored energy permits.
For direct upwind wind the same is true is just that due to fast cycles of charge and discharge effect is not visible in most cases without slow motion video.
--- Quote from: Nominal Animal on November 24, 2022, 02:15:38 am ---To simplify, the power (energy transferred or converted per unit time) used matches the power lost, when the vehicle travels at its maximum velocity. However, the losses can be made arbitrarily small. (As an example, if you dimple your car's surface like a golfball, you'll reduce aerodynamic drag significantly, and increase fuel efficiency, as demonstrated famously by MythBusters.) So, it is just a question of how to harvest energy from the wind, using a platform traveling downwind at velocities below and above the windspeed. It is just an engineering problem, really, hiding inside a scenario that seems counterintuitive to most people.
--- End quote ---
The only way the vehicle is powered by wind is by air particles colliding and providing their kinetic energy to vehicle.
So to get a real world example we can take a Tesla model 3
First google search say Cd = 0.23 and frontal area 2.22m2
So if vehicle is stationary with the front facing the wind (so upwind) and wind speed is say a constant 30m/s (maybe a wind tunnel or something that can provide this perfectly constant air flow).
Then vehicle will experience a wind power trying to push vehicle backwards of
Pw = 0.5 * 1.2 * 0.23 * 2.22 * 303 = 8272W
So this is both the power that wind wants to accelerate the vehicle backwards (as front faces the wind) so the max theoretical available wind power and the power the vehicle will need if it wants to stay in place without the brakes engaged.
So if the vehicle wants to drive at 1m/s upwind it will need the following amount of power
P = 0.5 * 1.2 * 0.23 * 2.22 * (30+1)3 = 9127W
Hope you see that it is impossible for a vehicle even ideal one with no friction losses to drive upwind at any speed as power out can not be higher than power in.
fourfathom:
--- Quote from: electrodacus on November 24, 2022, 02:49:35 am ---[...]
Direct upwind limit is zero.
Both direct downwind faster than wind and direct upwind is possible using energy storage.
For direct downwind as soon as you are above wind speed there is no longer any wind power available so you can drive above that speed only for as long as the stored energy permits.
For direct upwind wind the same is true is just that due to fast cycles of charge and discharge effect is not visible in most cases without slow motion video.
Hope you see that it is impossible for a vehicle even ideal one with no friction losses to drive upwind at any speed as power out can not be higher than power in.
--- End quote ---
And with this you have completely lost the plot. Direct upwind is possible, has been demonstrated in the real world, and requires no energy storage, no "charge and discharge cycle". You have not learned a thing from these tedious discussions.
electrodacus:
--- Quote from: fourfathom on November 24, 2022, 03:18:35 am ---And with this you have completely lost the plot. Direct upwind is possible, has been demonstrated in the real world, and requires no energy storage, no "charge and discharge cycle". You have not learned a thing from these tedious discussions.
--- End quote ---
Of course direct upwind is possible using energy storage.
The explanation of how it works is wrong nobody is denying the experiments.
IanB:
--- Quote from: electrodacus on November 24, 2022, 03:21:10 am ---Of course direct upwind is possible using energy storage.
The explanation of how it works is wrong nobody is denying the experiments.
--- End quote ---
And if you average the upwind movement over thousands or millions of cycles, then at the limit it becomes continuous upwind movement, which can last forever, without stopping.
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