Landau-Ruelle-Takens Turbulence

[RWebb]

Holy Revolution, Batman!


Shear flows undergo a sudden transition from laminar to turbulent motion as the velocity increases, and the onset of turbulence radically changes transport efficiency and mixing properties. Even for the well-studied case of pipe flow, it has not been possible to determine at what Reynolds number the motion will be either persistently turbulent or ultimately laminar. We show that in pipes, turbulence that is transient at low Reynolds numbers becomes sustained at a distinct critical point. Through extensive experiments and computer simulations, we were able to identify and characterize the processes ultimately responsible for sustaining turbulence. In contrast to the classical Landau-Ruelle-Takens view that turbulence arises from an increase in the temporal complexity of fluid motion, here, spatial proliferation of chaotic domains is the decisive process and intrinsic to the nature of fluid turbulence.

Science 333(6039): 192-196.
DOI: 10.1126/science.1203223

Avila, et al.

[notfarnow]

agreed, but sometimes a fellah just wants a smoke after supper, know what I mean?

[Flieger]

Wish I knew what that last sentence was talking about. I was intrigued to see in a film how a smooth bell mouth could push the onset of turblence into very high Reynolds numbers. I heard it was due to the instabilities of the incoming fluid. That is, if it enters the fluid nice and smoothly without a large stagnation point it would not start tumbling.

[RANDY P]

Thank god for Stall warning sensors and wind tunnel testing.

It's no longer a surprise that way.

rjp

[Superman]

The airspeed of a laden swallow is still not settled?

[Dantilla]

African, or European?

[djmcmath]

Originally Posted by Flieger

Wish I knew what that last sentence was talking about.

Crikey, I can identify a Reynolds number in a sheet of otherwise innocent figures, but can someone translate the rest of that paragraph into English? How will this change life for us mere mortals?

Dan

[Eric Coffey]

Originally Posted by RANDY P

Thank god for Stall warning sensors and wind tunnel testing.

It's no longer a surprise that way.

rjp

Yep, now if we can just get TAWS/TCAS in ALL GA aircraft (and make it affordable)...
That may have prevented the tragedy in AJ this week, which was likely a CFIT incident.

[pwd72s]

I don't know what you're talking about. I don't have a pic of a bunny with a pancake on it's head either.

[widgeon13]

Stalling.

[RANDY P]

Bernoulli's principle as well.

[RANDY P]

Originally Posted by Eric Coffey

Yep, now if we can just get TAWS/TCAS in ALL GA aircraft (and make it affordable)...
That may have prevented the tragedy in AJ this week, which was likely a CFIT incident.

That's gotta suck. they found bodies scattered all over the landscape.

[widgeon13]

Unfortunately, most CFIT is caused by pilot error.

[wdfifteen]

I discovered that when I was working out the maximum velocity of an African swallow carrying a coconut, but I didn't think it was important.

[wdfifteen]

Originally Posted by Flieger

I was intrigued to see in a film how a smooth bell mouth could push the onset of turblence into very high Reynolds numbers. I heard it was due to the instabilities of the incoming fluid. That is, if it enters the fluid nice and smoothly without a large stagnation point it would not start tumbling.

Just goes to show you should never let a scientist write porn.

[RWebb]

ok, I'll have a whack at the last sentence Fleiger - even tho it has been many long decades since I messed with this stuff (and then only to use it, not to do original research in the field)

Originally Posted by RWebb

... In contrast to the classical Landau-Ruelle-Takens view that turbulence arises from an increase in the temporal complexity of fluid motion, here, spatial proliferation of chaotic domains is the decisive process and intrinsic to the nature of fluid turbulence.

For a long time, people thought that turbulence was initiated by changes over time (short periods of time, but time) in the motion of the fluid. This was worked out by those 3 guys (not to be confused by the 5 guys (hamburger chain) and thus called L-R-T).

Avila and his budddies found that the initiation of turbulence (in their systems) is not so much a time domain phenomenon, but is caused by little areas of chaotic flow spreading out in space. Of course, it takes time to do that... BUT!


And, as we all know!, when fluid motion starts out laminar, it often winds up turbulent (just like women) -- Osborne Reynolds was the first major dude to tell ya, as Steely Dan put it...


Capiche?

[Joe Bob]

Faraday Cage Effect.....was discovered by Ben Franklin, just sayin'.....

Faraday cage - Wikipedia, the free encyclopedia

[Flieger]

This is like cosmology. We can see how the turbulence grows and behaves once it has come into existence, but what made it start in the first place? Random fluctuations in the molecular motion of the fluid. Laminar flow on the molecular scale still contains random Brownian motion.

It seems that these guys have a theory like the "inflation" after the big bang, not a theory of why the big bang happened in the first place. And what was there before the big bang for that matter.

There will be a Reynolds number (so a point along a wing) where flow becomes turbulent. This is time independent (well it does depend on velocity, so...) but for an individual fluid "particle" as it flows it could either be expressed in time or space since it is velocity dependent and so time and space are related variables.

[RWebb]

when are you going to take a fluid flow course? you can figure this all out and explain it here

[Flieger]

I am taking Fluid Mechanics 1 right now but turbulence is still not completely understood. Perhaps Fluid Mechanics 2 next quarter will explain more?

This course covers turbulent flow properties but not really what causes turbulence other than instabilities in the fluid as it enters a pipe which could be due to an abrupt opening. Turbulent flow is a little hard to define scientifically. It is a "I know it when I see it" thing. The Reynolds number definition is not fool proof, either. It is most accurate for saying that below and Re of 2000 the flow is laminar. Where turbulence sets in depends not only on Reynolds number but also factors that do not factor into the Reynolds number such as surface roughness and entry conditons- like a smooth bell mouth vs. an abrupt, thick walled pipe.

All fluid (and solids too) have random motion on the molecular scale. It must be a bit like certain types of rouge waves. They used some sort of Quantum Mechanics principles to show that in that distinctively random world of quantum mechanics where everything is possible (just maybe improbable) certain waves would begin taking energy from neighboring waves and several moderate waves would turn into one huge wave.

Perhaps a similar thing is going on here with the random fluid motion. Even static fluid has Brownian motion so there is always a certain random instability which can grow, even if the fluid is not swirling around in the first place due to windy conditions an aircraft is flying through or whatever.

This is a field that has great potential for growth. That randomness is very pesky. It would be nice to find a model that works every time to predict turbulence and then we can stop relying on average quantities.

Then there is the task of relating Navier-Stokes equations to turbulent flow. I am not sure whether average velocities can be used for that or if the equations are invalid due to some sort of thermodynamic effects (more and random energy dissipated due to turbulence). The Navier-Stokes only work for Newtonian, Incompressible fluids anyway so transsonic airflow is out.