Every time I think of this story, it gives me some strange feeling – something is basically wrong. I just don’t know what :-)

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Turbulence in pipes smoothes out with time, study finds

Date2008-11-25

illustration of this article
Scientists in Germany and the Netherlands have shown that, contrary to
popular belief, the state of turbulence seen in liquids and gases when
they flow at high speeds is not permanent. The study demonstrates with
unprecedented precision that in pipe flows, turbulence disappears with
time. The results are published in the journal Physical Review Letters.

In fluid dynamics, turbulence describes a state of chaotic,
random changes. Non-turbulent or smooth flow is called ‘laminar’ flow.
As speed increases, a transition is made at some point from laminar to
turbulent flow. This transition is to some extent governed by the inner
forces of the molecules involved: when the forces of acceleration are
greater than the inner forces that hold molecules together, unsteady
vortices appear, eddies are formed and drag fluctuates. Speed is a
major factor in turbulence, but object size and viscosity also play a
major role.

Scientists until now have assumed that once
turbulence is achieved, it will remain constant so long as the velocity
is maintained. However, a study led by Dr Björn Hof of the Max Planck
Institute for Dynamics and Self-Organization in Göttingen, Germany, has
found evidence to the contrary.

Turbulence reduces energy
efficiency in equipment such as ducts, pipes and gas turbines.
Accordingly, the reduction of turbulence is an area of intense study.
‘Turbulent flow consumes more energy than laminar flow,’ said Dr Hof.
‘In many applications such as oil pipelines they are therefore
bothersome.’

The team created eddies in water flowing through
one-metre-long glass pipes of varying widths, using a constant pressure
head. A constant resistance was added to avoid fluctuations due to
drag, and temperature was kept strictly constant to avoid changes in
viscosity. They then followed the movement of ‘turbulence puffs’ down
the pipe and calculated the probability that the puff would arrive at
the exit, or smooth out. The velocity of the puffs was measured using
laser Doppler anenometry. The researchers were then able to interpret
the ‘decay rate’ of turbulence with unprecedented accuracy.

‘In
order to discern whether turbulence is stable or only has an extremely
long lifetime, our measurements had to be very exact,’ explained Dr
Hof. ‘Our measurements show that every turbulent flow in a pipe will
inevitably become laminar.’ The transition is by no means swift –
depending on the geometry of the pipe, it may take years – but it is in
fact inevitable.

A complete, exact description of turbulence
is one of the outstanding challenges in the physical sciences. The
current study represents an advance in this area, although it does not
address the nature of turbulence outside of pipes. Building on their
new knowledge, the scientists believe that it may be possible to
shorten the duration of turbulence within pipes, which could result in
significant energy savings.

Contact person:For more information, please visit:

Physical Review Letters
http://prl.aps.org/

Max Planck Institute for Dynamics and Self-Organization
http://www.mpg.de

Remarks:

Category:Miscellaneous

Data Source Provider:Max Planck Institute; Physical Review Letters

Document of reference:Hof
B., et al. (2008). Repeller or attractor? Selecting the dynamical model
for the onset of turbulence in pipe flow. Physical Review Letters.
Published online 21 November; DOI: 10.1103/PhysRevLett.101.214501.

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