
To Predict Turbulence, Count the Puffs (2014) - dnetesn
http://nautil.us/issue/25/water/to-predict-turbulence-just-count-the-puffs-rp
======
btrettel
Fluid dynamicist here. While I have not read this article in full, it's worth
noting that there is no reason to believe the critical Reynolds number for
pipe flow is a constant. There are well known experiments which have produced
laminar pipe flow at a Reynolds number of about 100,000, 50 times higher than
what this piece claims is possible. This is done through careful vibration
isolation among other techniques, to eliminate as many potential disturbances
in the flow as possible. This idea is very old. Reynolds himself was able to
produce laminar flow at a Reynolds number of about 13,000. See here for a
review: [https://www.annualreviews.org/doi/10.1146/annurev-
fluid-1221...](https://www.annualreviews.org/doi/10.1146/annurev-
fluid-122109-160652)

The suggestion that these "puffs" are produced and die at the same rate a
particular Renolds number needs to assume a certain production rate, which is
going to vary from setup to setup. Consequently there is no constant
transition Reynolds number for all setups. I think the birth and death rate
idea is okay, but I suspect the popular level writer oversimplified the
result. I have not read the original paper but recall a talk that I think was
by Hof on the same subject, and I recall that they are aware of this.

(Also, if it was unclear, this critical Reynolds number only applies to long
pipes. Any other changes can affect this greatly. You can have laminar flows
at Reynolds numbers of 10^5 or higher in some applications. I see this mistake
more often than I would like...)

------
knolan
My PhD is in transitional flows, in particular the transition from laminar to
turbulent boundary layers under elevated freestream turbulence — so flows like
that over the roof of your car, but you also see them on gas turbine blades as
well as subsonic aircraft wings.

We typically refer to these puffs as turbulent spots. They manifest in many
ways. Lots of researchers generate them artificially with a wall disturbance
however they can occur all by themselves. A boundary layer velocity profile
can become unstable vis T-S waves [0] and via a complex mechanism generate
turbulent spots. They grow spatially until the boundary layer is saturated. On
the other hand freestream turbulence can buffer the outer part of the boundary
layer resulting in a streaky flow near the wall. These streaks can interact
locally and generate spots of their own bypassing the T-S process entirely.
However after the onset the growth of spots to saturate the boundary layer is
remarkably similar. You can download an enormous DNS database of just a single
flow case here if you like. [1]

[0]
[https://en.m.wikipedia.org/wiki/Tollmien–Schlichting_wave](https://en.m.wikipedia.org/wiki/Tollmien–Schlichting_wave)

[1]
[http://turbulence.pha.jhu.edu/Transition_bl.aspx](http://turbulence.pha.jhu.edu/Transition_bl.aspx)

