New_Scientist_May_27_2017

JAIME SALDARRIAGA/REUTERS
turbulent flow you produce breaks down
into smaller and smaller streams, eventually
fading into nothing.
But is that true under all circumstances?
Turbulence is one of the trickiest unsolved
problems in physics. When a thick fluid like
honey flows, we know where we are. But with
less-viscous fluids like water, the movements
can become unpredictable. We have equations
that describe this turbulent flow, but they
can’t be solved exactly, meaning that we’re
not certain they capture what is happening.
In 1963, Carl Gibson, an oceanographer at
the University of California, San Diego, looked
at turbulence in wind tunnels, water tunnels
and tidal channels and became convinced
that the data didn’t fit the expected pattern.
Instead, he points to the existence of “fossil
turbulence”, an idea first mentioned in the
1950s by George Gamow, a Russian physicist
living in the US.
Gamow was talking of galaxies being a kind
of snapshot of the turbulent gas flows in the
young cosmos. But Gibson has extended the
idea: “Turbulence always starts with eddies
forming at small scale and cascades to larger
scales,” he says.
This is the reverse of the accepted
understanding of turbulence – that energy
dissipates from large to small scales, says
Colm-cille Caulfield who studies turbulence at
the University of Cambridge. But this is a net
Not just nukes: drug gangs use home-made
“narcosubs” to evade detection
effect and recent computer modelling studies
now suggest “backscattering” of a small
amount of energy to larger scales is possible.
Gibson’s idea is that those spinning whorls
of turbulence you can create with your hands
in the bath will combine to produce larger and
larger whorls over time. And he does have a
little evidence to support these controversial
thoughts.
In 2002, he joined an environmental
research project studying the impacts of a
waste water pipe in Mamala Bay, Hawaii. The
team deployed a series of sensors to measure
water velocity in three dimensions and Gibson
used this to map the turbulence. He says the
sensors picked up patterns of turbulence that
increased in size as they rose to the surface.
He then analysed satellite photographs
of the sea, looking for tiny changes in the
brightness of the surface. Despite the pipe
being 70 metres underwater, he reported
in a 2005 paper that those anomalies were
there, and that they matched the patterns
of turbulence recorded by the sensors.
Signs were visible on the surface as far as
12 kilometres away from the pipe.
This means fossil turbulence is real, says
Gibson, and submarines could be tracked
via their wakes. True, subs typically cruise
at a depth of about 300 metres, but Gibson
insists his extrapolation should hold.
“The fossil turbulence remnants from a
submarine persist for many days,” he says.
“The Russians have understood this from
the beginning, but have considered all their
progress to be important state secrets.”
Recently there have been hints that the US
Navy is also waking up to wakes. It funds firms
to carry out research through a government
scheme called Small Business Innovation
Research. Records from the scheme show that
the Navy contracted a firm called Cortana
Corporation in Falls Church, Virginia, to
develop models of various ways a sub might
be detected (see “How to track a sub”, left).
One of them is how underwater currents and
turbulence might manifest themselves on the
ocean surface.
The company declined an interview with
New Scientist. But military analysts are also
“If fossil turbulence is real,
submarines could be tracked
via their wakes ”
sounding the alarm. “The Russian interests
in this field are real,” says Polmar. “Some
of their accomplishments are impressive,
and a concern to US and British officials.”
Even if he’s wrong, the worries over
submarine tracking aren’t over. Keir Lieber at
Georgetown University’s Center for Security
Studies in Washington DC points out that
a number of technologies, including
underwater drones, have evolved to the point
at which submarine undetectability is no
longer a given.
This casts a shadow over the NATO efforts
to create stealth submarines that are invisible
to sonar. If Polmar and Gibson are right, then
perhaps more focus should be on finding
ways of reducing the wake that submarines
create. That, it seems, is what the Russians
have done. “Many Russian submarines
have vortex attenuators on their screws and
small vortex-unwinding propellers,” says
independent naval analyst Jacob Gunnarson
in Williamsburg, Virginia.
It’s enough to give us pause over UK
government plans to upgrade its nuclear
deterrent. The planned four new Dreadnought
class subs will cost upwards of £31 billion.
A government document released in 2016
says: “It is unlikely there will be any radical
technological breakthrough which might
diminish materially the current advantages
of the submarine or make the oceans
transparent.” That statement now looks just
a tiny bit less certain. ■
David Hambling is a freelance writer based in London
27 May 2017 | NewScientist | 39