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Is fluid dynamics a true physics subject?

  • Thread starter MagnetoBLI
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But surely this means all solutions that are true for initial condition changes are over simplified and provide an approximate solution in reality?
What happens with turbulence and chaotic systems in general is that if your initial conditions change a little bit, then your final result becomes random. One way of thinking about this is imagine a particle near a turbulent field. If you change the initial conditions slightly, that particle could end up *anywhere*. If you care where all of the particles end up, you are stuffed, because where an individual particle ends up is random.

What this says is that the "reductionist" approach wouldn't work, you have to try something else.

You usually don't care about tracking every particle. What you care about are "generalized quantities" (i.e. if I put this shape in this gas, what's the drag coefficient). There are a number of techniques for figuring that out.

Is it that fluids are so complex that this over-simplification is more pronounced when initial conditions are altered?
It's not that fluids are particular complex, it's just undergraduate physics classes are deliberately oversimplified. The types of systems that you learn about in undergraduate physics are *deliberately* selected not to show complex behavior, and this is in part because engineers *deliberately* build systems that minimize weird behavior. So what happens is that physics undergraduates learn a "cookbook" of tools that work for certain rather simple systems. The mistake is thinking that you can use the cookbook for everything.
 
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Is anything truly random or is it just effectively random to today's computing power? But yes I understand what your saying about real systems being unique.
 
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The comment about 'similarity variables' is what I was most confused about; I knew relatively large scale properties were known but not the small scale properties, so that's interesting to hear.
The thing about similarity variables is that they let you show that one physical situation is basically the same as another, so you can run an experiment in one situation and scale it up to another.

For example, turbulence is controlled by the Reynolds number. So I can run water through a tube and get some numbers, and then apply them to some other physical situation. As long as the Reynolds number is the same, the behavior will be similar.

The problem is that there are some situations where you can't run an experiment. I can put a wing in a wind tunnel, but I can't drop gas down a black hole.

I was in general trying to target this issue; that properties are well understood and that focus is now on the phenomena/processes (collective behavior of properties and energy etc...)
I don't think that the bulk properties are well understood. Rather they are well defined.

However, I am intrigued by phenomena also. Out of interest, are there many other general solutions (rather than particular solutions) being devised or potential for existence for other fluid phenomena than turbulence?
One thing about this is that most of this is experimental/observationally driven. You see something, and you try to model it. Our understanding of the equations aren't good enough so that you can;t take the equations and *predict* some complex behavior.

You talk water, you run it through a tube. At some critical number, it turns turbulent. Now suppose you are a space alien that doesn't know about water and tubes. Someone gives you the NS equations. Looking at those equations, with the math techniques that we have, that space alien wouldn't be able to figure out that AHHH, at X situation, I see turbulence.

This matters for things like black holes and neutron stars. With GR, magnetic fields, and all sorts of quark phenonmenon, I'm sure that "something weird" happens.

Most of what I've read are particular solutions mainly focusing on, as you said, building up to something bigger, and there must be infinite particular solutions i.e. less pure feeling. I always thought more complex was not always better.
What usually happens in astrophysics is that people create particular approximations. For example, gravity works with GR, but GR is too hard to build bridges. However, by making some assumptions end up with Newtonian gravity which you can work with. Same thing happens in astrophysics. You find that with certain situations, magentic field lines get trapped with fluid, and you end up with the MHD approximation.
 
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Cheers for the replies, very useful. How does one go about choosing an area of fluid dynamics for a career? There seems to be select groups within uni departments that focus on specific areas (although maybe some overlap), presumably due to the time required to understand each area properly, and from reading over the next year (all I have until application time) I will barely scratch the surface.
 
This has been an interesting thread to read but I have to echo MagnetoBLI's view of the field. When you look at the literature it seems to, largely, be phenomenology (when you do this, this happens).

If a problem needs to be solved either a simple, soluble case is used, or it's modelled using CFD (yes, this can be an interesting challenge). Yes, it's led to new theories but there seems to be few fundamental ideas (NS equations describe everything, but are too complicated) that are used.

For example, take a simple problem of mixing milk in my tea. If I stir how long will it take to mix, if I stir twice how will that change the time? As far as I'm aware the answer to this question would be highly specific with no 'general principles'. Which is why it doesn't 'feel' like physics. This isn't necessarily a criticism, though.
 
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Cheers for the replies, very useful. How does one go about choosing an area of fluid dynamics for a career? There seems to be select groups within uni departments that focus on specific areas (although maybe some overlap), presumably due to the time required to understand each area properly, and from reading over the next year (all I have until application time) I will barely scratch the surface.
Usually someone specializes in a particular department that uses CFD in a certain way. There are some university centers for CFD and numerical programming.

One reason that it's subject focuses is that different departments look at different types of problems and those call for different techniques. For example aeronautical engineering typically looks at problems in which the fluid does do not weird things (i.e you have to worry about antimatter appearing near your airplane) but you have very complex shapes.

In astrophysics you *do* have to worry about antimatter and tau neutrinos popup out in your fluid, but you have simple boundary conditions and shapes (i.e. stars are more or less spheres). What happens is that his why AE using finite element methods whereas astrophysicists use finite element models.

Then there are the CFD simulations of the entire universe.

http://www.mpa-garching.mpg.de/gadget/

where your grids are 100s kiloparsecs and your timesteps are tens of thousands of years.
 

chiro

Science Advisor
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I don't want to hi-jack this thread, but I was wondering if anyone can give some resources for CFD, in particular in relation to your own specific sub-area considering your own issues if you have them handy or in memory.

Getting a small insight into these intracacies has been very enlightening.
 
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Reasonableman: Yes, it's led to new theories but there seems to be few fundamental ideas (NS equations describe everything, but are too complicated) that are used.
From the below two quotes it seems that fundamental ideas in the current age of physics are based upon processes rather than intrinsic properties and that new found particles are systems of the same substance such as emergence phenomena. Is this the same with other ares of physics?

Robert Laughlin's Nobel Prize lecture on solid state physics mentions that his students feel betrayed when he tells them that fits to experiment/deductive techniques are perfectly valid , as the students are trained to think in reductionist terms and think non-amenable things are unimportant. He explains that 'emergent phenomena' can be described as a particle that has it's own characteristics and properties - like an eddy, which I thought was interesting.

Boneh3ad:
Experimental physics relies on observation of phenomena under highly controlled and specific conditions in the hopes of learning something that can later be broadened to a greater number of conditions until you have eventually described the more general phenomenon
And

Boneh3ad:
Even theoretical physics follows this pattern where most if not all the fundamental properties are known and it is a matter of proving our theories about those properties.
 
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AlephZero

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The types of systems that you learn about in undergraduate physics are *deliberately* selected not to show complex behavior, and this is in part because engineers *deliberately* build systems that minimize weird behavior.
You can't minimize weird behaviour in a rational way until you can predict something about it. Otherwise you may be dancing blindfold on the edge of a cilff, and of course there are plenty of examples in engineering (some from thousands of years ago) where engineering cliff-edges were discovered by falling off them.

I agrre that predicting the "details" (whatever that means) of a turbulent flow is probably not useful, but predicting the onset of turbulence, or the boundaries of turbulent regions in a flow pattern, would be a useful start. In mechanical engineering people are using nonlinear dynamics models to track how solutions depend on system parameters, to predict unstable or chaotic behavour, locate multiple solutions to the response of a system for the "same" conditions, etc. It hasn't yet been reduced to a black box method that anybody can use wthout understanding it, but it works in real-world situations, and on models with 200,000 DOF not two. But AFAIK, CFD hasn't got to that point yet.

As a simple CFD example, think about flow through a uniform pipe. The laminar flow solution of the NS equations is well known, and mathematicallly that solution exists for all Reynolds numbers. But experiment says there is some critical value Re* such that if Re > Re* this flow pattern doesn't physically happen. It would be very useful to have a way to compute the value of Re* from a simulation of the flow. And it might even be even more useful in the long term if somebody comes up with insight into why doing that calculation based on a continuum model of the fluid (i.e. NS) is impossible!

Probably doing "only" that wouln't earn you a Millennium Prize, but you have to start somewhere...
 
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From the below two quotes it seems that fundamental ideas in the current age of physics are based upon processes rather than intrinsic properties and that new found particles are systems of the same substance such as emergence phenomena. Is this the same with other ares of physics?
This is getting close to philosophy and one thing that I've found is that different people in physics can do physics basic on radically different philosophical foundations. I'd very strongly disagree with Reasonableman's quote, but I think its probably because we are looking at the world through fundamentally different philosophical lenses.

You can't answer the question of "is fluid dynamics a true physics subject?' without answering the question of "what is physics?" and that turns out to be a surprisingly difficult question to answer.

Robert Laughlin's Nobel Prize lecture on solid state physics mentions that his students feel betrayed when he tells them that fits to experiment/deductive techniques are perfectly valid , as the students are trained to think in reductionist terms and think non-amenable things are unimportant.
This gets to some pretty deep questions like "what are physicists trying to do?" It's quite fascinating philosophy, which gets you into sociology and history (so why *are* students trained to think in reductionist terms?)

One thing that is true in my case is that my thinking about physics comes from a Chinese Confucian background which gives a much different flavor them someone that comes from a Platonic reductionist background.

He explains that 'emergent phenomena' can be described as a particle that has it's own characteristics and properties - like an eddy, which I thought was interesting.
For that matter in quantum field theory, "particles" are fourier transforms of fields. Things like the Higgs particle came out of solid state physics.
 
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An interesting quote from reading up on Confucius:

"To learn and from time to time to apply what one has learned, isn't that a pleasure? ... Learning without thought is labor lost; thought without learning is perilous. (Confucius, Analects)."

Maybe I'll get a good book on Physics Philosophy...for weekend reading only of course!
 
I'd just like to clarify that I'm not saying fluid dynamics is 'broken' but just wanted to support MagnetoBLI's initial impression (which has, subsequently been given more weight by Laughlin) that, currently, fluid dynamics does not conform to the 'physics' paradigm that is learnt/taught in universities (specifically undergraduate).

I do wonder if, perhaps, this was what physics used to be like. Maybe before quantum physics was formalised scientists thought the same way about quantum phenomena, similarly for other breakthroughs. If this is the case, then physicists should be happiest when this is the state of affairs as it offers the greatest opportunities.
 

Astronuc

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Are fundamental discoveries made in fluid dynamics currently, or is it just particular solutions based upon known fundamentals? From reviewing journal literature, it appears that either application based problems have been studied (engineering), or fluids under certain conditions that form a collective behavior (engineering-science). However I believe, much like plasma physics, collective behaviors themselves are not by definition fundamental and are understood through the application of known physics; therefore it is not pure physics.

Any thoughts on the matter?

Thanks.
A lot of the literature address applications, but there is plenty of theoretical work as well.

There is a huge effort in modeling and simulation (CFD) as experimental work - laser doppler anemometry and X-ray/gamma tomography for flow characterization. Both go together to better understand the physics of fluids from individual atoms/molecules to the ensemble. It's physics. Multi-phase flow is particularly complex and challenging, particularly at the transition from single to two-phase, and particularly where one has to model chemical species in the flow.

At the undergrad level, it's pretty simple theory. One only gets into the heavy stuff in grad school. And it's not just fluid dynamics, but conjugate heat transfer and fluid-structure interation. The ultimate goal is to develop an understanding of the fundamental physics and produce tools that provide a predictive capability. That requires a more mechanistic approach and less empiricism.

I'd just like to clarify that I'm not saying fluid dynamics is 'broken' but just wanted to support MagnetoBLI's initial impression (which has, subsequently been given more weight by Laughlin) that, currently, fluid dynamics does not conform to the 'physics' paradigm that is learnt/taught in universities (specifically undergraduate).

I do wonder if, perhaps, this was what physics used to be like. Maybe before quantum physics was formalised scientists thought the same way about quantum phenomena, similarly for other breakthroughs. If this is the case, then physicists should be happiest when this is the state of affairs as it offers the greatest opportunities.
What does one mean by 'physics' paradigm? Ensemble behavior is pretty fundamental - and rather difficult to get right on multiple scales.

I don't want to hi-jack this thread, but I was wondering if anyone can give some resources for CFD, in particular in relation to your own specific sub-area considering your own issues if you have them handy or in memory.

Getting a small insight into these intracacies has been very enlightening.
http://www.symscape.com/blog/origins-of-the-commercial-cfd-industry

POSSIBILITIES OF SIMULATION OF FLUID FLOWS USING THE MODERN CFD SOFTWARE TOOLS
http://arxiv.org/ftp/physics/papers/0409/0409104.pdf
Alexey N. Kochevsky
Research Scientist, Department of Applied Fluid Mechanics,
Sumy State University,
Rimsky-Korsakov str., 2, 40007, Sumy, Ukraine

http://www3.imperial.ac.uk/pgprospectus/facultiesanddepartments/aeronautics/research/aerofluiddynamics

http://www.epcc.ed.ac.uk/msc/programme-information/guest-lectures/2010-2011/fluidity/ [Broken]


Akshai Runchal, PhD, ACRi, Inc.,
The Emergence of CFD at Imperial College: A Personal Perspective

A Lifetime of Turbulence & CFD: Frank H. Harlow, PhD,

B. L. Smith, Thermal-Hydraulics Laboratory, Nuclear Energy and Safety Department, PSI, CH-5232
Technical Meeting on Application of CFD for NPP Design and Safety Analysis
CFD Software Packages
Commercial
CFX originally developed by AEA Technology, Harwell, UK
- acquired by ANSYS Inc. in 2003
FLUENT originally developed by Creare Inc., USA, Sheffield Univ., UK and FDI, Chicago, USA
- acquired by ANSYS Inc. in 2006

STAR-CD originally developed at Imperial College, London, then by Computational Dynamics Ltd,
STAR-CCM+ marketed by the CD-ADAPCO group, France

PHOENICS originally developed at Imperial College, London, then by CHAM Ltd

Freeware
OpenFOAM originally developed at Imperial College, London, then by Nabla Ltd, but then made freely
available by OpenCFD in 2004. Unique feature: source code access (written in C++)

Others
NPHASE-CFD developed at Rensselaer Polytechnic Institute, Albany, NY
TRIO-U developed at CEA Grenoble, France
ACE-CFD+ marketed by ESI Group, France
There's still a lot work to do and challenges to overcome.

Fully integrated/coupled multi-physics is the grail.
 
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chiro

Science Advisor
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Thank you very much Astronuc: mucho gracias.
 
What does one mean by 'physics' paradigm? Ensemble behavior is pretty fundamental - and rather difficult to get right on multiple scales.
I will concede this is not well defined and is subjective, but the best explanation I can give is there are some fundamental laws or postulates that are applied to each problem. For example with statistical or quantum mechanics you learn some very powerful techniques that can be used on many problems. This does not seem to be the case for fluid dynamics.
 
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Sorry Reasonableman, would you mind elaborating on your previous statement? How do they differ in there laws? I haven't studied these subjects, are you studying a physics currently? Cheers
 
To elaborate, in Fluid Mechanics the fundamental principles are conservation of momentum, mass and energy. The Navier-Stokes equations are derived from these principles, however they are usually too complex to solve. So, to reach solutions, further assumptions are made such as incompressibility, zero viscosity, etc. (the wikipedia page on fluid dynamics lists many of these). As a result there are a large number of sub-fields dealing with the particular assumptions in use, furthermore insight/solutions from one area, usually, do not help another area.

For statistical mechanics the fundamental idea is that if you can enumerate the possible states of a system you will be able to make predictions about what state you system will be in. This fundamental principle provides insight and allows a wide range of problems to be solved.

So the difference is, in Stat. Mech. some principles are learnt, these can then be applied to a range of problems (I believe this can be called a reductionist approach). In Fluid Dynamics, the basic principles do not provide much insight into solutions. I feel, in my education, I was taught a reductionist approach.

I am not currently at a university being taught physics (I completed a BSc 8 years ago), however I would say I am studying physics as it is (part of) my job!
 
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I believe I am a reductionist by nature and this may be why I find engineering frustrating as rule-of-thumb/well defined methods are regularly used in problems. I believe that fluid dynamics relies heavily on emergentism and particular examples, therefore it's going to take some getting used to this approach. I wonder if a reductionist can be satisfied by this approach taken in fluid dynamics research..
 

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