this post was submitted on 29 Jul 2023
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Is it really stealing any more wind than say, a tall building though?
*than
Thank you
A tall building does not stop the wind. The wind moves around it. Think about it from a conservation of energy perspective: if the building stole the energy from the wind, that energy has to end up somewhere. The building isn't converting that much of the energy into some other form (electricity, heat, etc.) so the energy must still be present in the wind. (Some small amount is converted to heat through friction when the wind hits the building, but it is tiny.)
It also transfers a non trivial amount of energy into the ground which I guess is kind of it's job if you think about it.
In effect, it's turning the energy it stole back into heat somewhere too, when the energy is used. Thermodynamics always wins.
A portion of it yes as the material flexes. But it's not transforming it all to heat, most of it gets transferred through the structure and the building foundations into the ground. That's my understanding at least, I'm not really sure how converting all the energy to heat would work but maybe I'm not understanding your point.
So, conservation of energy requires that if kinetic energy is removed from the wind, it must be converted to another form. If a building removes energy from the wind (through friction, flexing, transferring vibrations into the ground, etc.), that energy total must be equal to the kinetic energy lost by the wind. Thermodynamics says that any conversion of energy from one form to another results in some of that energy being converted to heat. So there's some direct heating caused by the process of the wind hitting a building. That heat is largely heating of the air itself. After all, all those molecules had to collide with one another when they ran into the building.
The noise, vibrations, and other forms of energy conversion will all also end up as heat eventually, as they're absorbed by the materials. So some indirect heating as well. In the ground, this is basically vibrations due a shuddering building being dampened by the material and turned into heat.
Now clearly on a windy day a building does not start significantly self-heating due to the wind hitting it. So the total amount of energy absorbed by a building cannot be that substantial. So, if one side of the equation (energy absorbed by building and converted into heat) is very small, then the energy removed from the wind must also be very small. Mostly the wind just changes direction to move around it.
Close, but not quite. One cubic meter of air at atmospheric conditions is about 40 moles, or 575 grams. Let's say 600 g to be generous.
Wind moving at 250 km/h (cat. 5 hurricane) contains about 1.5 kJ of kinetic energy per cubic meter.
If all that energy is used to heat the same air, it's temperature is increased by roughly... 3.5 K.
Now consider the fact that kinetic energy scales as the square of velocity: for a normal windy (say 15 m/s wind), we only get a temperature increase of 0.16 K, which is practically immeasurable.
For these calculations I didn't even consider the building, which has a massive specific heat capacity compared to air.
In summary: Because the thermal energy required to increase temperatures appreciably is of a completely different order of magnitude than the amount of kinetic energy in the wind, the fact that a building isn't heating up is not a solid argument that only a small portion of the kinetic energy in the wind is being lost when it hits the building. In fact, even if all the energy was lost, you wouldn't even notice the temperature change, unless you were in a cat. 5 hurricane, in which case the building is probably already gone.
I see what you mean but I don't think your base assumption is correct. Energy can be transferred as well as converted to other forms, which is what happens to the vast majority of the energy removed from the wind in the case of it hitting the building It gets transferred to the ground, not converted.
If the wind hitting something converted all the kinetic energy to other forms rather than transferring most of it, the sail wouldn't work for driving a ship for instance. Again, maybe I'm not catching what you mean here so I'm sorry if that's the case. It's true everything will eventually become heat, as in the heat death of the universe, but looking at the example of the building resisting the wind in isolation only a tiny fraction of energy ends up as heat rather than being directly transferred. I'm no physicist but that's my understanding.