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Mess with your minds: A wind powered craft going faster than a tail wind speed.

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PlainName:

--- Quote ---It does not need to be interpreted to observe the simple fact that it demonstrates, which is that the pressure in front of the fan is lower than the ambient pressure.
--- End quote ---

Once again, I don't think it is showing what you (and the chap in the video) think it is. Blow across a pipe and it will suck the air out.

Last time I raised this issue you didn't catch on for whatever reason, so here's a video I just ran off to demonstrate:



That's an ally pipe with a plastic bag cellotaped on the end. The vacuum cleaner nozzle is actually the output of my vacuum, so blowing instead of sucking. (FYI it's a DeWalt DC500 so not particularly strong, but I think we can agree that it is higher pressure than ambient and absolutely not a vacuum.) As you can see, blowing air across the pipe sucks air out of the bag, giving the exact same effect as the chap in your video.

How does he, and you, know that he is not recreating this situation?

PlainName:

--- Quote ---I will like to show the below diagram but hopefully that will not create even more confusion for you.
--- End quote ---

Well it has! What's the blue wheel, the green thing, etc? Is the green thing touching the blue wheel? What do the arrows mean? It would be less confusing if you told us what the things represent instead of leaving us to guess (and then tell us we're wrong about our conclusions based on those quesses)  :-//

IanB:

--- Quote from: dunkemhigh on August 30, 2021, 09:20:09 pm ---
--- Quote ---It does not need to be interpreted to observe the simple fact that it demonstrates, which is that the pressure in front of the fan is lower than the ambient pressure.
--- End quote ---

Once again, I don't think it is showing what you (and the chap in the video) think it is. Blow across a pipe and it will suck the air out.
--- End quote ---
Oh, absolutely it is showing what I think it is.


--- Quote ---Last time I raised this issue you didn't catch on for whatever reason, so here's a video I just ran off to demonstrate:
--- End quote ---
I did catch on, and I gave the explanation.


--- Quote ---That's an ally pipe with a plastic bag cellotaped on the end. The vacuum cleaner nozzle is actually the output of my vacuum, so blowing instead of sucking.
--- End quote ---
That's excellent that you did the experiment. Far better than drawing pictures and just imagining what will happen.


--- Quote ---(FYI it's a DeWalt DC500 so not particularly strong, but I think we can agree that it is higher pressure than ambient and absolutely not a vacuum.)
--- End quote ---
No, I can't agree there, as you will see. It is actually a vacuum.


--- Quote ---As you can see, blowing air across the pipe sucks air out of the bag, giving the exact same effect as the chap in your video.

How does he, and you, know that he is not recreating this situation?

--- End quote ---
Yes, I can see what you see, and here is the explanation.

Firstly, there is no such thing as "suction". Therefore, the pipe cannot suck air out of the bag. When we talk about suction this is just loose and imprecise terminology.

Gases (like air) consist of particles that are all pushing each other apart and trying to spread out. In the absence of overriding gravitational or accelerating fields, gases will spread out to evenly fill any container. The gas pushing against the walls of the container manifests as pressure, which can be measured. There is no such thing as "suction" because gases are pushing apart and trying to spread out, not pulling together. To have "suction" requires a "pull", which gases do not have.

Next, in the absence of gravity or acceleration, gases will flow from regions of higher pressure to regions of lower pressure. So we see, and can deduce, that the air flows out of the bag and through the pipe because the pressure inside the bag is higher than the pressure at the end of the pipe. The pressure in the bag is ambient pressure, and the pressure at the end of the pipe in the vacuum exhaust stream is less than ambient.

A follow on question is to ask, how, exactly, does a vacuum cleaner work? What basically happens here is that the motor creates low pressure inside the canister by "throwing" air out through the exhaust nozzle at high speed (it is a centrifugal blower). It works by catching air in the rotating blades, spinning it round really fast and throwing it out, much like you might get thrown off a roundabout if it is spinning fast and you let go.

By creating low pressure inside the canister, the high pressure atmosphere now pushes in through the vacuum nozzle to fill the void, and that is why the nozzle "sucks". But also, the low pressure air inside the canister is the same air the the blower is ejecting. This is partly why the stream of air coming out is at lower pressure than the surrounding air.

There are more technical details of course, but this is the gist of it.

bdunham7:

--- Quote from: IanB on August 30, 2021, 11:48:33 pm --- But also, the low pressure air inside the canister is the same air the the blower is ejecting. This is partly why the stream of air coming out is at lower pressure than the surrounding air.

--- End quote ---

Are you saying the air within the nozzle, before it gets out, is below ambient atmospheric pressure?  And does it follow that a vacuum cleaner is incapable of generating a pressure from it's nozzle when you connect to the out port?

IanB:

--- Quote from: bdunham7 on August 31, 2021, 01:48:28 am ---
--- Quote from: IanB on August 30, 2021, 11:48:33 pm --- But also, the low pressure air inside the canister is the same air the the blower is ejecting. This is partly why the stream of air coming out is at lower pressure than the surrounding air.

--- End quote ---

Are you saying the air within the nozzle, before it gets out, is below ambient atmospheric pressure?
--- End quote ---
Yes, most likely. The air stream can only increase in pressure as it slows down and mixes with the surrounding air, so the pressure inside the nozzle will be at its lowest point. In fluid mechanics, the place where the flow is fastest and the stream is narrowest is called the vena contracta. This is also the point where the pressure is lowest.


--- Quote ---And does it follow that a vacuum cleaner is incapable of generating a pressure from it's nozzle when you connect to the out port?
--- End quote ---
No, it does not follow. A flowing fluid obeys the energy conservation law, and each element of fluid has two components to its energy: its pressure energy and its kinetic energy. The fast flowing fluid has higher kinetic energy, which is why the pressure decreases as it goes faster (the total energy must remain the same). However, if you capture that fast flowing fluid inside a container, some of the kinetic energy can be converted back to pressure energy, and this will cause the pressure to rise. Since the vacuum blower has given a lot of kinetic energy to the fast flowing air, if you slow down that air you can get back a higher pressure than it started with. This is the principle of operation of centrifugal compressors.

You don't get back all the kinetic energy though. Some of it is dissipated as waste heat through turbulence, but in a well designed system you can get 80% or more of the kinetic energy back as a pressure increase.

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