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profloater
2019-Jan-03, 12:16 PM
Spin is observed in liquids, vapours and gasses. It can be induced into any flow and in real fluids may be inevitable but that perhaps in my question.
In an ideal gas or vapour molecules must pass each other more often than they collide. This will cause local spin either by charge effects or gravity. Local spin can either cancel or reinforce within the volume and we see this in the atmosphere when circumstances allow us to see it, more often it is invisible. In mass observation spin accelerates and causes loss of pressure (Bernoulli) which of course changes the flow and this is a major phenomenon. Tornadoes, huge cyclones and anticyclones. But does it occur at a local level unseen in a volume of gas? The same is true of liquids but they are more closely bound molecules, although the dynamic viscosity (not featured in ideal fluids) of water is less than of air. I suspect spin is ubiquitous, is that so?

grant hutchison
2019-Jan-03, 12:28 PM
I think vorticity is maybe going to be a useful search term to help you explore this topic.

Grant Hutchison

profloater
2019-Jan-03, 01:06 PM
I think vorticity is maybe going to be a useful search term to help you explore this topic.

Grant Hutchison

Indeed thank you for that reminder, vorticity played a big role in aeronautics in my engineering notes, although mainly in one plane. I can explain that I was prompted by another thread to think about it in a gas volume without surfaces. Or a long way from surfaces since there is a stability. (I did not intend that one plane pun but now I quite like it!) The idea of stars as a non colliding gas also prompts me to think about the origins and development of vorticity out there coming from mathematical randomness. It seems to me to be a micro macro phenomenon.

Ken G
2019-Jan-03, 02:35 PM
The idea of stars as a non colliding gas also prompts me to think about the origins and development of vorticity out there coming from mathematical randomness. It seems to me to be a micro macro phenomenon.Yes, you are delving into much more advanced topics about gas behavior, topics that are at the frontier of research. In addition to "vorticity", which appears after you have made the "fluid approximation" (averaging over small volumes of lots of particles and invoked concepts like local density and local flow speed), you can consider the larger area of "turbulence." Then at the fluid level, you have "incompressible turbulence" (where the flow speeds are so much less than the sound speed that induced density changes are very small) and "compressible turbulence" (which involves high flow speeds and even includes shocks). Then you get into the micro/macro distinction you are referring to, where the fluid approximation can be regarded as macroturbulence, but you can also have microturbulence-- velocity correlations on scales smaller than the collisional mean-free-path between particles, so you have to explicitly include the effects of collisions in your treatment of the turbulence (which is a lot harder!). It also raises the spectre of dissipation-- when the correlation length decays to zero via the subdivision of turbulent eddies, until you just call it heat rather than turbulence.

There are a lot of interesting fluid theorems involving vorticity, depending on what simplifying assumptions you want to make. It's not simple stuff, and goes beyond anything I know much about. But turbulence is a crucial issue in airplane flight, since what a plane leaves behind is a turbulent wake, and the existence of that turbulence is the main reason airplanes (and boats) experience drag. I've also heard it claimed, and debated, that it is essential to creating lift, which creates a kind of inevitable connection between lift and drag.

profloater
2019-Jan-03, 02:54 PM
Yes, you are delving into much more advanced topics about gas behavior, topics that are at the frontier of research. In addition to "vorticity", which appears after you have made the "fluid approximation" (averaging over small volumes of lots of particles and invoked concepts like local density and local flow speed), you can consider the larger area of "turbulence." Then at the fluid level, you have "incompressible turbulence" (where the flow speeds are so much less than the sound speed that induced density changes are very small) and "compressible turbulence" (which involves high flow speeds and even includes shocks). Then you get into the micro/macro distinction you are referring to, where the fluid approximation can be regarded as macroturbulence, but you can also have microturbulence-- velocity correlations on scales smaller than the collisional mean-free-path between particles, so you have to explicitly include the effects of collisions in your treatment of the turbulence (which is a lot harder!). It also raises the spectre of dissipation-- when the correlation length decays to zero via the subdivision of turbulent eddies, until you just call it heat rather than turbulence.

There are a lot of interesting fluid theorems involving vorticity, depending on what simplifying assumptions you want to make. It's not simple stuff, and goes beyond anything I know much about. But turbulence is a crucial issue in airplane flight, since what a plane leaves behind is a turbulent wake, and the existence of that turbulence is the main reason airplanes (and boats) experience drag. I've also heard it claimed, and debated, that it is essential to creating lift, which creates a kind of inevitable connection between lift and drag.

AHA, thank you, I am facing a challenge from 50 years ago. In my final exams there was just one question I knew to avoid, it was thermal flow across a turbulent (or indeed laminar) boundary layer using Bessel functions. Ironically several times later I had to encounter these in various projects from evaporation of ponds and lakes to maximising heat transfer in flash boiling heat exchangers. Fortunately I only had to use the convoluted empirical equations for the relevant Reynolds number because theory was always too complicated. But I have not visited those areas for twenty years and ideas may have moved on. I was going to move to boundary layers which are fascinating because all real flow has them just as all real collisions are mediated by electrons. But I will be content with "it's complicated" and stick with engineer's empirical results and their crazy formulae. Engineers have to find solutions while the science drags behind.:p

profloater
2019-Jan-03, 03:00 PM
..... I've also heard it claimed, and debated, that it is essential to creating lift, which creates a kind of inevitable connection between lift and drag.

We were taught the various theories of lift, vorticity, pressure gradient due to path length of flow but the winner was, I think, simple momentum shift, the lift comes form the momentum of the air pushed downwards by the angle of incidence and the rest is all to do with boundary layer separation, oops there I go again. I think an ideal fluid would still give lift for that reason, but no skin drag.

Ken G
2019-Jan-03, 03:00 PM
Yup, the "it's complicated" approach is the one I take as well. The only interesting thing I know about boundary layers is that the most appropriate boundary condition is the most surprising one-- when a fluid flows along a surface, it is better to use the "stick" rather than "slip" boundary condition-- by that I mean, the fluid flow velocity is zero right next to the boundary, and increases smoothly with distance until you reach the transverse flow speed. This fact greatly reduces drag and dissipation, because the viscosity is smoothed out over a wider region. It's the reason golf balls have dimples and baseballs have laces-- these help achieve a wider boundary layer and reduce drag to the point that you can hit the balls much farther. It's wonderfully counterintuitive that roughing up the surface of something makes it experience much less drag! In effect, air slides more easily past air than it does past a smooth surface. (This is also the reason a fan blade gathers dust. And to blow dust off an old book, it requires a pretty thick layer-- if you want to dust your keyboard, you'll need a very high speed wind indeed.)

profloater
2019-Jan-03, 03:08 PM
Yup, the "it's complicated" approach is the one I take as well. The only interesting thing I know about boundary layers is that the most appropriate boundary condition is the most surprising one-- when a fluid flows along a surface, it is better to use the "stick" rather than "slip" boundary condition-- by that I mean, the fluid flow velocity is zero right next to the boundary, and increases smoothly with distance until you reach the transverse flow speed. This fact greatly reduces drag and dissipation, because the viscosity is smoothed out over a wider region. It's the reason golf balls have dimples and baseballs have laces-- these help achieve a wider boundary layer and reduce drag to the point that you can hit the balls much farther. It's wonderfully counterintuitive that roughing up the surface of something makes it experience much less drag! In effect, air slides more easily past air than it does past a smooth surface.

Yes indeed, those separated boundary layers again! It is the counter intuitive fact that right on the surface the flow is zero is the fascination. And it's easy to demonstrate and that layer grows as you move along the surface so that rivets back from the leading edge are completely covered in boundary layer as you fly at hundreds of mph. Because of that the top layer of air on a wing is truly stuck, so it is only the deflected momentum you have to worry about in calculating. The marvel of the boundary layer remains complicated and exam questions about it still best avoided IMO!

Ken G
2019-Jan-03, 04:04 PM
Although be aware, sometimes the exam question on the difficult topic is actually easier-- if the topic is too difficult to ask about in depth, the question might actually be easy to answer by virtue of being necessarily superficial! It sounds like the Bessel functions might be the temperature profile, so if you only knew what kind of equation they solve (diffusion equations in cylindrical symmetry), you're in like Flynn.

profloater
2019-Jan-03, 06:05 PM
Although be aware, sometimes the exam question on the difficult topic is actually easier-- if the topic is too difficult to ask about in depth, the question might actually be easy to answer by virtue of being necessarily superficial! It sounds like the Bessel functions might be the temperature profile, so if you only knew what kind of equation they solve (diffusion equations in cylindrical symmetry), you're in like Flynn.
My approach was to work through old papers, giving a probability of recurrence as well as the format. It served me well but the progress through the Bessel functions defeated me and I kind of thought i would never need it. That proved correct but unlike some i had to use my other degree notes for many years in engineering practice. I worked on Concorde and even dealing with supersonic flows the empirical data beat theory.
I agree that if you know the subject, the tough question might get more marks.

Len Moran
2019-Jan-03, 09:35 PM
I worked on Concorde and even dealing with supersonic flows the empirical data beat theory.


Just a little side post - how very interesting that you worked on Concorde - I recently just happened to come across a TV documentary that I hadn't seen before that charted the life of the plane from its inception to the tragedy and it's last flying days following a safety revamp - it made for fascinating viewing. I find the whole enterprise of Concorde to be so interesting - it really was a technological marvel and a massive project, well ahead of the passenger supersonic game being played in the US and Russia. And yet, it was a commercial flop, an aspect incidentally that was well explained in the documentary. But still, from a technological point of view, I still think of it as a marvel, I'm envious to know that you were involved in the project!

Ken G
2019-Jan-03, 10:06 PM
I worked on Concorde and even dealing with supersonic flows the empirical data beat theory. Yeah, sadly to make something work in the real world, you need the real world, not the idealizations.

Ken G
2019-Jan-03, 10:09 PM
[QUOTE=Len Moran;2472247And yet, it was a commercial flop, an aspect incidentally that was well explained in the documentary. [/QUOTE]Yes I saw that also, it really made the point clearly that sometimes whether or not something will succeed is really a function of economic factors that are hard to anticipate, moreso than the technology itself. As I recall, the documentary basically said that opposite to "too big to fail," the Concorde was more like "too small to succeed."

profloater
2019-Jan-04, 10:04 AM
Yes I saw that also, it really made the point clearly that sometimes whether or not something will succeed is really a function of economic factors that are hard to anticipate, moreso than the technology itself. As I recall, the documentary basically said that opposite to "too big to fail," the Concorde was more like "too small to succeed."
When Boeing pulled out, within weeks the FAA banned supersonic flying overland, we knew then it would be too small, so many routes lost. One anecdote, i was on a team fighting to reduce the weight, i devised a self rolling window blind that would have saved tens of kilos when we grasped at grams but it failed the smoker test. People in those days stubbed out their gaspers on the window blinds.!
Then the airlines insisted on steel cutlery and real glasses , a huge weight penalty. But in relation to gas flow the management of the lift change at supersonic transition, by pumping fuel, and using fuel to absorb friction heat were great achievements.

Ken G
2019-Jan-04, 02:44 PM
Not to mention the nose that doesn't let the pilot see forward!

profloater
2019-Jan-04, 03:52 PM
Not to mention the nose that doesn't let the pilot see forward!
Right, it wasn’t in the original design, it’s for the high incidence landing and there was to be a camera and screen, but the pilots insisted, Trubshaw that is, and i could find a few anecdotes about him! But i thought it actually made the personality in the end.

profloater
2019-Jan-04, 03:57 PM
Just a little side post - how very interesting that you worked on Concorde - I recently just happened to come across a TV documentary that I hadn't seen before that charted the life of the plane from its inception to the tragedy and it's last flying days following a safety revamp - it made for fascinating viewing. I find the whole enterprise of Concorde to be so interesting - it really was a technological marvel and a massive project, well ahead of the passenger supersonic game being played in the US and Russia. And yet, it was a commercial flop, an aspect incidentally that was well explained in the documentary. But still, from a technological point of view, I still think of it as a marvel, I'm envious to know that you were involved in the project!
Concorde made a good trading profit with high premium fares it was usually full. But it could never repay the investment. If allowed to fly over land at mach 2.2 it would have made money until fuel sensitivity would kill it. I heard the bang often over Wales and it was not too bad:)

publiusr
2019-Jan-04, 08:17 PM
I think vorticity is maybe going to be a useful search term to help you explore this topic.

Grant Hutchison

Vortices can be small indeed
http://www.spacedaily.com/reports/Simulations_show_swirling_rings_whirlpool_like_str ucture_in_subatomic_soup_999.html
http://www.spacedaily.com/reports/Vortex_laser_offers_hope_for_Moores_Law_999.html