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You’re on an airplane
when you feel a sudden jolt.
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Outside your window nothing
seems to be happening,
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yet the plane continues to rattle
you and your fellow passengers
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as it passes through turbulent air
in the atmosphere.
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Although it may not comfort
you to hear it,
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this phenomenon is one of the
prevailing mysteries of physics.
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After more than a century
of studying turbulence,
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we’ve only come up with a few
answers for how it works
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and affects the world around us.
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And yet, turbulence is ubiquitous,
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springing up in virtually any system
that has moving fluids.
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That includes the airflow
in your respiratory tract.
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The blood moving through your arteries.
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And the coffee in your cup,
as you stir it.
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Clouds are governed by turbulence,
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as are waves crashing along the shore
and the gusts of plasma in our sun.
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Understanding precisely how this
phenomenon works
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would have a bearing on so many
aspects of our lives.
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Here’s what we do know.
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Liquids and gases usually have
two types of motion:
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a laminar flow, which is stable
and smooth;
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and a turbulent flow, which is composed
of seemingly unorganized swirls.
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Imagine an incense stick.
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The laminar flow of unruffled smoke
at the base is steady and easy to predict.
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Closer to the top, however,
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the smoke accelerates, becomes unstable,
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and the pattern of movement changes
to something chaotic.
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That’s turbulence in action,
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and turbulent flows have certain
characteristics in common.
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Firstly, turbulence is always chaotic.
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That’s different from being random.
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Rather, this means that turbulence
is very sensitive to disruptions.
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A little nudge one way or the other
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will eventually turn into
completely different results.
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That makes it nearly impossible
to predict what will happen,
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even with a lot of information
about the current state of a system.
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Another important characteristic of
turbulence
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is the different scales of motion
that these flows display.
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Turbulent flows have many
differently-sized whirls called eddies,
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which are like vortices of
different sizes and shapes.
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All those differently-sized eddies
interact with each other,
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breaking up to become smaller and smaller
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until all that movement is
transformed into heat,
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in a process called the “energy cascade."
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So that’s how we recognize turbulence–
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but why does it happen?
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In every flowing liquid or gas there
are two opposing forces:
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inertia and viscosity.
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Inertia is the tendency of fluids
to keep moving,
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which causes instability.
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Viscosity works against disruption,
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making the flow laminar instead.
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In thick fluids such as honey,
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viscosity almost always wins.
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Less viscous substances like water or air
are more prone to inertia,
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which creates instabilities that
develop into turbulence.
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We measure where a flow falls
on that spectrum
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with something called the Reynolds number,
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which is the ratio between a flow’s
inertia and its viscosity.
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The higher the Reynolds number,
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the more likely it is that
turbulence will occur.
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Honey being poured into a cup,
for example,
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has a Reynolds number of about 1.
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The same set up with water has a Reynolds
number that’s closer to 10,000.
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The Reynolds number is useful for
understanding simple scenarios,
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but it’s ineffective in many situations.
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For example, the motion of the atmosphere
is significantly influenced
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by factors including gravity and the
earth’s rotation.
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Or take relatively simple things
like the drag on buildings and cars.
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We can model those thanks to many
experiments and empirical evidence.
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But physicists want to be able to predict
them through physical laws and equations
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as well as we can model the orbits
of planets or electromagnetic fields.
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Most scientists think that getting there
will rely on statistics
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and increased computing power.
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Extremely high-speed computer simulations
of turbulent flows
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could help us identify patterns that could
lead to a theory
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that organizes and unifies predictions
across different situations.
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Other scientists think that the phenomenon
is so complex
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that such a full-fledged theory
isn’t ever going to be possible.
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Hopefully we’ll reach a breakthrough,
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because a true understanding of turbulence
could have huge positive impacts.
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That would include more
efficient wind farms;
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the ability to better prepare for
catastrophic weather events;
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or even the power to manipulate
hurricanes away.
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And, of course, smoother rides
for millions of airline passengers.