# What hold galaxies together?

Discussion in 'Hydrodynamics and Aerodynamics' started by Sailor Al, Aug 3, 2022 at 9:12 PM.

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### Sailor AlSenior Member

When reviewing my paper on aerodynamic force from sail, which relies on the compressibility of air to explain the phenomenon of aerodynamic force, I was perplexed by two phenomena of air.
1. When considering tornadoes, cyclones and the rotation of weather highs and lows, air appears to behave like an incompressible fluid.
2. When considering smoke rings and the eddies in smoke from a chimney, air also appears to behave like an incompressible fluid.
This led me to think that the behaviour of natural phenomena can be explained using different principles depending on the size of their dimensions.
1. Mechanics of objects.
1. At "normal" sizes, Newton's Laws explain the motion of objects at "normal" speeds.
2. At the tiny dimension of atoms, Einstein's equation predicts the behaviour of neutrons, protons and electrons.
On a very large scale, it explains the perturbations of the orbit of Mercury.
2. The behaviour of light.
1. At "normal" sizes, light behaves like a water wave with interference, reflection and refraction.
2. At a tiny scale, light behaves like a photon, an object that has mass, position and speed.
3. The behaviour of air.
1. At the "normal" scale of aircraft wings, yacht sails, wind turbines, it behaves like a compressible fluid.
2. At the large scale of weather, cyclones and tornadoes, it behaves like an incompressible fluid.
3. At the low speed of smoke rings and swirls, once again, it behaves like an incompressible fluid.
As we know, modern physics views space as a 4-dimensional space-time continuum.

If you stretch or compress some of the dimensions, it seems you have to account for physical phenomena using different paradigms.

Stretch the distance dimensions of air, you get tornadoes and cyclones whose behaviour can be explained and forecasted using the principles of incompressible fluids and fluid dynamics.

Compress the time dimension, and you get smoke rings and smoke eddies whose behaviour can also be explained using the principles of incompressible fluids and fluid dynamics.

However at "normal" dimensions of distance and time, you have to use the principles of classical thermodynamics to explain the phenomenon of the aerodynamic force on sails and aircraft wings operating as compressible fluids.

This led me to the question of what holds galaxies together? This then led me to consider the Milky Way galaxy as a vortex of incompressible fluid, quintillions of times larger than the biggest known vortex, ( Jupiter's Red Spot), and then the paradoxes of the Big Bang, Super Massive Black Holes, Dark Matter and Dark Energy all become resolved in my new article

I know this is the Hydrodynamics and Aerodynamics forum, but this idea arose from the consideration of the aerodynamics of sail and I don't know of a forum on cosmology, and I'm keen to get some views on the subject, so this is where I have started.

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### rnlockSenior Member

Gravity.

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### Sailor AlSenior Member

Nope. If gravity did it, there would be no need to invent Dark Matter.

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### latestarterSenior Member

It is still gravity or curved space.
Because there was insufficient visible matter to produce the level of gravity observed, dark matter was speculated as making up the difference.

According to NASA the cosmos is made up from:-

Matter 5%
Dark matter 27%
Dark Energy 68%

I find all this fascinating but there are times I think the scientists are in a conspiracy to come up with weird ideas to see how gullible the rest of us are.

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### Sailor AlSenior Member

Nope. If they could account for it by using gravity and/or curved space, they wouldn't have had to invent Dark Matter.
Correct, Dark Matter has been invented to account for the discrepancy. The fact that the missing 27% of the mass of galaxies is unexplained is the paradox. SVT resolves that paradox by showing there's no need for Dark Matter.
But seriously, no. They just haven't solved the problem, like the 19th Century scientists who invented ether as a medium through which light waves passed.

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### rnlockSenior Member

What reason is there to think dark matter doesn't exist? If it explains what's observed, and is useful for predictions, it's a viable theory. Lots of things you can't see affect what happens. I can't see the voltage in an electrical outlet, though in some circumstances I can feel it. I can't see radiation, either, but it can kill. There's nothing in G(m1)(m2)/r^2 that says we have to be able to see the mass. (I realize that the formula I gave is the Newtonian approximation.)

What's SVT? If it's something to do with vortices, you still have to explain what holds the vortices together, since normally stuff spinning like that will fly apart unless something's holding it together.

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### Sailor AlSenior Member

Here's the link from the original post to the explanation. I think you'll find it answers your question.
Swept Volume Theory, explained in this link which I think will answer the rest of your question.

.

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### latestarterSenior Member

Pardon my scepticism but using a well known search engine it produced pages of cosmology forums.

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### rnlockSenior Member

It's not exactly wrong, just another way of looking at the elephant. However, in the appendix, there are false assertions.

For instance, Bernoulli's equation doesn't magically turn off because air is compressible, particularly since that compressing is NEGLIGIBLE at sailboat speeds. On an airliner, that's another story, but then you just have to make the math a bit more complicated, not drop Bernoulli's equation. Fluid won't go anywhere without a pressure gradient. You can't pull on it, though I suppose viscosity complicates things a little.

As far as there being no experimental evidence of a velocity gradient, absence of evidence is not evidence of absence. I'm sure if you actually spend some time looking, you will find some laser doppler anemometer studies showing what the velocity gradient is. I used to know someone who used to do this kind of experiment in a water tunnel. And don't tell me that water is imcompressible, because that's just an approximation. It really is compressible, but can be ignored at low speeds, just as it can be ignored at even lower speeds in air. Come to think of it, I've seen some smoke tunnel film where they turned the smoke on and off, and you could see the gradient, as the interruptions, starting as vertical bars, got slanted.

The explanation for the velocity gradient is worked out from f=ma. The air won't accelerate without forces to make it do so. If you work out the math, either you have a pressure gradient, or you get a vacuum behind the airfoil, which you know can't be real. For the air to get behind that foil, it has to be pushed.

I can't understand your point in your answer to question 2. There definitely IS motion going on.

On question 3, I haven't read those particular sources, but just because you don't understand something, doesn't mean it's wrong. Since I haven't read those particular sources, I can't say that they make no mistakes, but I've studied other stuff on aerodynamics.

On question 4, I don't really see how your vague rephrasing of stuff we already know is useful. Assuming I understood it correctly. If I didn't, it could just be wrong.

On question 5, you'd better hope you don't find yourself pushing SVT to Mark Drela. He's a smart guy. Please provide a quote that supports your assertion, rather than just putting words in his mouth. Be careful, or he'll downgrade your problem set severely.

On question 6, see above.

On question 7, please provide the evidence that there isn't the velocity gradient everyone says is there, and tell me how the air can get around to the back side of the sail without velocity gradients. I guess you'd say that the flow visualization I've seen is just a part of the vast conspiracy.

On question 8, separation and turbulence are NOT the same thing at all. Furthermore, if the pressure doesn't change when flow separates, why do airplanes suddenly drop when they stall?

I don't know if I understand question 9 entirely. However, if air didn't go around the sail as it swept through, there'd be a vacuum behind the sail.

Questin 10, 1: That's absurd. Are you saying that, say, gaff rigged schooners couldn't go upwind, and then on January 1st, 1900, they could? Even now, 19th century designs can go upwind. Fore and aft rigs have been around for a long time. Also, if boats only went downwind, why did 19th century, and earlier boat designs use daggerboards, keels, etc.? Are you claiming this boat from 1891 wasn't meant to go upwind? https://herreshoff.info/Images_Vessels/S00412_Dilemma_by_Kathy_Bray.gif
If it didn't need to go upwind, why did they put a centerboard in this catboat from 1870?

10.2 That's just absurd. I guess you don't believe in evolution, since Darwin, being a natural philosopher, didn't really sail to the Galapogos in the 1830's, and it's all a fraud. When Lilienthal was flying in the 1890's, he couldn't have possibly thought about what was holding him up. Nor did Ben Franklin, or earlier kite fliers, think about what was holding their kites up. The story of scientists aboard the Endeavor in the 1760's, observing the transit of Venus, is another one of those false narratives generated by the conspiracy. It's amazing how powerful these people are, and how clairvoyant.

10.3 Powered airplanes may not have been invented until the 20th century, but clever people had been thinking about them for centuries. Actually, I take that back. Alphonse PĂ©naud had rubber powered airplanes, helicopters and ornithopters in the 1870's. Leonardo was dead by 1520, but we know he sketched an ornithopter and a helicopter, however unworkable. People have been watching birds as long as there have been people. Lilienthal was flying gliders in the 1890's. Cayley's man carrying glider flew in the 1850's. There were others.

Question 11: I'm sure many aeronautical engineers would be very interested to know that CFD doesn't apply to wings. I guess it's just coincidence that CFD results from competent engineers tend to be correct. Also that CFD results for airfoils tend to be fairly close to reality.

Question 12: If wings are so different from sails, can you explain single surface hang gliders, some of which have shapes very much like sails. How about single surface kites? Or single surface ultralights, like some of the ones from Quicksilver? Or the single surface wings on some of my model airplanes?
Also, what about boats that sail perfectly well with wingsails? I guess you haven't been watching the America's Cup races for quite some time. Or C-class catamarans for decades longer.

Ultralight aircraft can fly at 28 mph. The Carbon Dragon sailplane can fly at 22. The Gossmer Condor could fly at a slow walk.

I myself have built a model airplane that flies at SLOWER than walking speed. Then there's F1D, which is actually faster than it was years ago, before a minimum weight was imposed.

After watching the following, tell me again how aircraft only fly at 2 degrees angle of attack. (It's ok if you leave the sound off.)

https://youtu.be/eFgr90LobAQ?t=190

13. Youj're wrong about lift. Lift can be in any direction, depending on the direction the air is moving at relative to the aircraft. By definition, lift is a force perpendicular to the airflow. Gravity doesn't have anything to do with it, except it might dictate where we might decide to direct the lift. If you're going to write about technical issues, please use the same definitions for words that everyone else does.

14. You can claim that vortices are not part of the explanation, but that doesn't mean vortices don't greatly affect lift. They can't hear. Maybe if they could, they'd behave.

15. Streamlines are a "construct" until you introduce a little bit of smoke, and then you can see them. I'll grant that "flow fields" are a bit of an abstraction, but they describe something real. You'd probably run out of time if you had to describe the motion of every air molecule, but the field gives you their overall motion. If something is moving through the air, then there is fluid flow. Unless Scotty is beaming up the air just as it reaches the surface of the object, and beams it back just behind the object. But you could argue that was motion as well, at least for objects of finite thickness. And, of course, Scotty isn't real.

Suggest you check your "facts" a little more closely so as to increase your credibility.

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### Doug HalseySenior Member

I'm afraid his credibility is long gone and exposure to any number of facts will not bring it back.

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### Sailor AlSenior Member

Thanks, I'll redouble my efforts to fine one.
And thanks for your efforts. Great questions offer an opportunity to reply in detail.
At the scale of sailboat sizes and apparent wind speeds, air behaves like an ideal gas. It's deviation from an ideal gas is primarily due to its viscosity (really small) and density (also really small), so is really small. I don't have the numbers, but if it was as much as 5% I would be surprised.
Agrees, but combine that with the absence of a supporting theory supports the evidence of absence.
Yes, if you consider the air moving over the sail, the air is in motion. Air is a gas, Newton's laws don't mention gas, they apply to "bodies"
It's equally valid to say that the sail is moving in the air. Now, the motion is the sail. Newton's laws apply to the sail, but we're now looking at the behaviour of the (stationary) air.
OK, then you don't need to worry about Q3 . That question was directed to those who have and choose to object based on those sources.
Sorry, I just don't understand your objection.
I have chosen not to as he in invested in his new book. SVT blows it out of the water. I don't think he would impartial.
Ah, maybe I have misquoted the SciAm article. It does say "Drela himself concedes that his explanation is unsatisfactory in some ways." and I think I have drawn an unwarranted conclusion. My apologies to Prof Drela. The point is that he considered the theory but abandoned it.
You'll have to be more specific please.
All I can say is I have not found any experimental evidence, and believe me, I have searched! And philosophically it's impossible to prove a negative.
I have one suspicion of a hint that I have drawn from Babinsky's video that I paste below.

As evidence for the absence of wind gradient it is flawed because of a whole bunch of reasons including
• the experiment was aimed at debunking the "equal transit time" theory
• It gives no information on the scales of size and speed
• the AoA is higher than that of a cruising aircraft
• There is very little space between the foil and the upper and lower surface of the chamber: there could be significant ground effect
• etc., etc.
However it does provide food for thought.
Yes, everybody does say it's there, but , and this is an opinion and not part of the theory) they do it to support the use of Bernoulli.

I will try to reply to the remainder of your very long and thoughtful post in a separate post.

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### Sailor AlSenior Member

Once again, shooting the messenger.

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### gonzoSenior Member

Do you have any experimental data to back the 5% claim:

The relative motion of the air and the sail is what we look at. You are misunderstanding the basics of a frame of reference. The choice of frame of reference is driven for which makes the math easier.

Velocity gradient is measured by simple experiments. In fact, one of the easiest is to go outside on a windy day and hold an anemometer at ground level and then at different heights.

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### Sailor AlSenior Member

I didn't say they were. I assumed that flow separation occurs where the flow changes from turbulent at the boundary layer to laminar beyond. If I'm wrong, I stand corrected.
That is an explanation from fluid dynamics which is not my topic.
As a pilot, I can assure you planes don't "suddenly drop" when they stall. They increasingly lose altitude as the stall develops. A stall can develop from increasing the AoA and/or reducing power. Both have the effect of reducing airspeed and altitude. As the airspeed decays, the aircraft transitions from controlled flight into a spin. The loss of altitude is dangerous when close to the ground (typically in the landing approach) . A stall at altitude is not dangerous. The recovery from the spin is to manage the rotation with power and rudder and then recover from the dive with elevator and power. It's no more dangerous than taking off or landing. It's just another manoeuvre the pilot is trained to master. (apologies for the lecture)
Not quite. There would be a vacuum behind the sail is air didn't fill it. The air doesn't come from in front of the sail, it fills in mainly from behind:

No, I'm not. The same point, however, is still true (not absurd) had I been more pedantic by saying "The vast majority of European commercial and naval ships, being square rigged, only sailed downwind." Maybe I'll make that edit. And remember, this are only responses to objections.
Sure I do. I just don't get your point.
Sorry, I'm getting a bit crabby.
OK, OK, I only added these as supporting evidence. The popular press wasn't reporting about powered flight until then!
I agree. I'm sure they will be very interested, and annoyed!
It was a generalisation! You have to agree that MOST wind are different fro MOST sails. Just picking some exceptions only disproves that if those exceptions overwhelmed the generalisation.
And anyhow, I was saying that despite those differences "Wings do, however, generate a similar aerodynamic force, and I believe the Swept Volume theory also explains the aerodynamic force from wings.". All you are doing is supporting my claim.
Aha! that an interesting one.
When talking aviation, lift is the force that opposes gravity vertically down
When talking aeronautics, lift is perpendicular to the airflow, which, in level flight is also vertical, but not when climbing or turning, when it is some degrees off vertical.
It's hard to "use the same definitions for words" for lift when there are at least two definitions that, as you correctly point out, can be quite different in practice.
It's just another reason I say that lift is a mathematical construct usually resolved perpendicular to the ground or perpendicular to the airflow.
It's of little use to the sailor because we are looking to increase the resolution parallel to the direction of travel (not like lift which is perpendicular to the direction of travel) and reducing the leeway/heel component which is perpendicular to the direction of travel.

[running out of space in this post.. to be continued]

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### Sailor AlSenior Member

Yes, that's a tricky one.
For argument's sake, let's put a device in the chimney stack that breaks the smoke up into lots of separate puffs, say a second apart.
When you are standing on the ground or in a moving car you still see the smoke as a streamline.
But, what do you see if you are looking out from your hot air balloon as it passes close to the chimney? You don't see a streamline. You see a series of apparently stationary smoke puffs .
No stream lines.
As I said: " Stream-lines and flow fields are constructs of fluid dynamics. In the frame of reference of the apparent wind, the sail is moving in still air: there is no fluid flow!"
No streamlines.
It's all to do with your frame of reference.
I think my facts are pretty good.
I'm trying to gain credibility by presenting "facts".
How am I doing?

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