Outward Expansion starts fast, then slows down. Inward Expansion speeds up. A typical Outward Expansion, say from an explosion, begins quickly, but stops. Inward Expansion, say from the nozzle of an operating Central-Vac, placed in the middle of a room, speeds up gradually, all the way to terminal speed.

Many people are unfamiliar with Inward Expansion, but if this Central-Vac evacuates the air from the center of the room, surrounding air will move in, expanding, and picking up speed, to fill the void. Am I right or wrong?

This is scarily reminiscent of an idea called "conspansion" from the "CTMU"...

I won't get further into it here, though. If interested, ask Mr. Google to help you out.

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"...wait for the ricochet."

"Inward expansion" sounds like an oxymoron.

It's generally called an "implosion".

Hehe, yeah. What he said.

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"...wait for the ricochet."

CTMU - I went there. Wow, talk about complex. No, I'm just looking for some agreement on this terribly basic observation of mine. I mean, everybody knows about this outward expansion for example, if you inflate a baloon - but doesn't it just reach a point where it's hardly expanding any more. I mean, past that point, you're just increasing the pressure, with very little expansion happening. That continues, this lack of expansion, probably right untill the balloon explodes. Can I get some agreement?

I haven't blown up a balloon lately, but I believe you're right - it's easy to blow up the balloon initially, but at a certain point you start stretching the material of the balloon too much (at which point its tensile strength kicks in). Further expansion beyond that point causes it to fail catastrophically (i.e. explode).

Best way to see for sure is to try blowing up a balloon and see what happens

Good point. Me, I call it a paradox. I say, it's paradoxical, how something can expand inwardly, but the air moving towards that Vacuum-Cleaner wasn't it Losing Pressure? And according to Boyle's Law, isn't that the same Increasing Volume? I mean, is that the Law, or isn't it?

And doesn't 'common sense' tell us that Increasing Volume is simply another way to say Expansion? Definitely, I would call it a paradox, how Inward Expansion can even happen, but my observations have shown me - Time and Time again, that it is, in fact, the reality.

Yes, but, according to Wiki, it's not the kind exampled by the sub being crushed from the outside - an implosion, of course, but one of 'fluid dynamics', by one Victor Schauberger, who said a suction from inside causes an implosion, a movement pedicentral, but not straight radially to the center, but as a vortex.

I'm just quoting Wiki, on the subject of 'implosion,' so help me. I wish everybody would go there. Read it for yourself - fluid dynamics... Victor Schauberger - it's highly interesting. Tho' it does say, his work was not generally accepted...

So yes, it's called an implosion. Only what's the opposite to 'implosion'?

It's 'exploding', isn't it? I know it's semantics, but I still like my 'inward expansion', which I hold to be the opposite of 'outward expansion'.

The two things are different, but real, to me. But 'implosion' is fine too - as long as it's from the inside a la Victor Schauberger

I think what ur describing also has to do with as you blow up a balloon, the volume increases so it takes more air to expand the radius of the balloon a given distance, but overall, the volume is increasing at the same rate; it's just spread out more, and the expansion appears to be slower (and it is, radially, but - assuming the amount of air you input remains at a constant rate - the volume increase remains about the same).

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"...wait for the ricochet."

Vacuum cleaners don't really make a vacuum, they just shift air along with a fan [though I suppose if the fan was fast enough you might get a cavitation effect (like when a boat propeller goes "too fast" in water)].

Although your central vacuum cleaner system will appear to make air speed up, that's a fairly understandable.

The cleaner is removing a given volume of air from the room (and sending it to where-ever the filter system is). The air at the nozzle will be moving at whatever speed.

The air that comes into the room to replace the shifted air, will come from all over the place. That spreads the given volume of air out over many locations - the speed of air at all those air sources can then be lower.

If you completely sealed your room, and just left a vacuum cleaner nozzle sized hole in the wall for air to get in, the air comming in that hole would be moving at the same speed as the air dissappearing down the tube.

(Ignoring the "elasticity" (due to compressibility) of the air which will have some effect.)

Of course, as the air comming through that hole enters the room, it would "spread out" into the room and go "slower", while air getting to the nozzle would appear to "speed up".

I don't think that's any different than looking at a river. At the narrows, the water "speeds up", and when the river widens again, the water "slows down".

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Well,it was just an analogy.

Yeah, well thanks for agreeing with me, anyway.

Yes, well I think, as I say, that everybody knows about this 'outward' expansion.
I'm more concerned with the inward kind. It seems to me, right now, that this is a 'desert' of scientific thinking. I mean, everywhere there's a Lowering Pressure there's Increasing Expansion - assuming Temperature and other things aren't factors.

This Increasing Expansion is important, I feel.

.Of course you're right, they don't necessarily pull 30 inches of mercury, but they do lower the pressure, and they do expand the air they take into the nozzle.

'Cavitation' is right under 'implosion' in Wiki.

Now hold on here! That air is moving at maximum speed. That air is travelling at its Terminal Velocity.

Here, air-speeds will be minimal. This air is barely moving.

That air will be Slowing Down as it leaves the hole. Whether it's one hole or many - once inside the room (the boundary for this experiment) the air will assume its minimum speed.

(Ignoring the "elasticity" (due to compressibility) of the air which will have some effect.)

Glad you can see, that once the air is in the room, and part of the experiment, it will Speed up on the way to the nozzle.

I guess so, but that's a different experiment, no? But it's interesting, isn't it, that at the 'narrows', the river Speeds Up.

You'd almost expect it to Slow Down at the restriction. To me, it's another (or maybe the same) paradox - that according to Bernoulli, this 'restricted' river should, here, Speed Up. I'm right, aren't I?

Notice also, however, that as the water 'narrows' it gets deeper, and a given volume of water needs to get through that 'pinch' in a certain amount of time, hence, it 'speeds up' (provided there's more water coming downstream from above the pinch to provide the 'motivation').

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"...wait for the ricochet."

Why is this in ATM? All I see are rather elementary questions and answers about familiar fluid-dynamics phenomena. What unorthodox theory, if any, is being proposed?

Umm...

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"...wait for the ricochet."

No.

That air is moving as fast as the fan in the vacuum cleaner can make it move.

My vacuum cleaner (regular portable thing) has a speed setting slider.

That affects the speed of the fan motor; that affects the speed of the air movement; and that affects the amount of cat-hair-sucking ability.

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Just wait a wee bit and then the vortices will come calling, and the old thread can be picked up again about clumping up etc. etc. There are still a load of unanswered questions in that thread, which is closed at this point untill astrocat reopens it again.

For the rest, astrocat should first read a book on physics, especially the part on gas dynamics etc. It can all be calculated by the simple continuity equation: div(rho v) = 0, the divergence of the product of density and velocity (a vector) for a closed system is zero (i.e. there are no sources of new stuff in the system), where the divergence operator is given by div(A) = dA_x/dx + dA_y/dy + dA_z/dz, and do not forget the chain rule for in the above case A = rho v.

And there is nothing "paradoxical" about inward expansion, but it definitely is an "oxymoron".

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善數, 不用籌策 (shàn shù, bù yòng chóu cè)
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“A good scientist has freed himself of concepts and keeps his mind open to what is”
道德經, 二十七 (dào dé jīng, 27)

Now I can see the possibility that astrocat is attempting to buy time by spending a few days going through the motions of building a consensus on a topic about which there is no real controversy, and then co-opting it for pushing a real ATM idea in another thread. I would recommend that we abstain from further participation in this one.

That's how it was done in the other thread, astrocat just kept on asking questions and making statements, but, because the topic was allegedly "so delicate" (s)he never even presented the model (s)he was working on, and only showed a lack of understanding, but claiming to be well versed in e.g. gas dynamics. But the 'expectation' of astrocat about the narrow part of the river shows that (s)he does not understand hydrodynamics and that fluids behave the same way as incompressible gasses. So, don't get your hopes up for anything significant.

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善數, 不用籌策 (shàn shù, bù yòng chóu cè)
He who is good at counting, uses no counting tools
“A good scientist has freed himself of concepts and keeps his mind open to what is”
道德經, 二十七 (dào dé jīng, 27)

The speeding up of the flow has completely nothing to do with Bernouilli's law (which states that the pressure perpendicular to a flow becomes less, see the experiment with two sheets of paper and blowing in between them and have them move together), but has all to do with the continuity equation that I described above.

Naturally, at the begining there will be a slowing down of the flow, which gets transported upstream by (shock)waves. After the steady state has set in, th continuity equation demands that in the narrow the flow speeds up, otherwise the river will overflow at the narrow part.

And the last part does not really make sense the way it is written, because it seems to imply that you expect it to slow down, but you think that Bernouilli says (dunno why you think that he says that) that it should speed up. You seem to like the term paradox, but that is mainly because you use the wrong physics.

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善數, 不用籌策 (shàn shù, bù yòng chóu cè)
He who is good at counting, uses no counting tools
“A good scientist has freed himself of concepts and keeps his mind open to what is”
道德經, 二十七 (dào dé jīng, 27)

Hornblower, I'm just trying to establish that 1) The kind of Expansion that Speeds Up is the Inward Kind and the kind that Slows Down and stops is the Outward Kind, and that 2) This Inward type of Expansion, the kind that Speeds Up, has not been studied properly. Few people, I think, even know that things can expand inwardly. It does, as I have been told, seem like an oxymoron. I call it a paradox.

Few things are more difficult to explain than a paradox, but these paradoxes - the kind that arise in Science - must, surely, be pointed out by somebody - even if that person gets chewed out for doing it!

That air is not going to move any faster. That air has reached a maximum speed. That air has achieved terminal velocity.

Set it on Maximum, and it will pull your cat-hairs in at maxcimum speed. No?

I think the 'paradox' you see is really simply an artifact of how things work, and there are a couple of things that could explain it.

For example, if you have a channel through which a given volume of air escapes at a constant rate (let's say we're using a vacuum cleaner for emptying a giant spherical balloon, and let's ignore the other effects for a bit), the apparent diameter of the surface bounding that volume will shrink at an increasing rate as the air is removed, however overall volume of the space will decrease at a constant rate because a larger percentage of the volume at any given time is being removed. This may look like the 'inward expansion' is increasing, but it's only true w/rgds to diameter of the sphere, not it's volume. It's simply the reverse of process of inflating the volume (at a given constant rate).

Another sort of 'increased speed' of 'inward expansion' can also be seen with gravity (i.e. things fall towards the center faster and faster the closer they get to the center of mass (until they hit the surface, at any rate) because the force of gravity increases the closer you come). This effect is largely what happens in a supernova when a star's core collapses.

So, what you're observing is not really ATM except perhaps in how it's being 'interpreted'.

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"...wait for the ricochet."

I tried, but nobody from BAUT would answer my e-mails. This is a very difficult concept, of course. I was studying 'implosion' in Wiki - there are a few kinds, mostly where implosion is caused from the 'outside', but the kind discussed by Victor Schauberger under 'fluid dynamics' is caused by suction, from within, and it is this kind that interests me. Now it was Victor Schauberger, not me, who said that this 'centripetal' movement was not 'radial' (straight to the center) but involved a vortex. I was simply quoting (to the best of my ability) Wiki. I feel I shouldn't be held responsible for what other people say in my thread, but this, apparently, is the case.

On gas dynamics I'm no slouch. You, obviously know more. But where in your continuity equation does it tell you the difference between outward expansion and inward expansion.

I'm jealous of your ability to come up with these continuity equations. But that doesn't make me wrong.

Not exactly sure what an oxymoron is, but it doesn't sound nice. I guess we're back to the declining pressure in Boyle's Law leading to Increasing Volume - but that to me is nothing else but Expansion - concentrating strictly on the relationship between pressure and volume.

No controversy? I don't see anyone agreeing with me. Sure, a consensus of opinion is what I'm always seeking - but it's something I rarely get.

I am so disappointed to hear you say this. I feel this inward expansion has been overlooked by scientists, but it is a genuine physical phenomenon, and as such is worthy of discussion. I am looking for places where this Speeding Up expansion of the Inward kind can be found.

I got closed down, for Pete's sake. I'm no slouch.
Well, i'll have to read that again. But incompressible gasses? Which gas can't you compress? Bring it to me - I'll show you how to do it. One way I know is to reduce its volume.

Umm, isn't a reduced volume the net effect (and definition) of compression?

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"...wait for the ricochet."

An oxymoron is a phrase that is self-contradictory. The word "expansion" means outward.