All language subtitles for Turbulence one of the great unsolved mysteries of physics - Tomás Chor.eng
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1
00:00:06,710 --> 00:00:10,550
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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00:00:13,260 --> 00:00:17,320
yet the plane continues to rattle
you and your fellow passengers
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00:00:17,320 --> 00:00:21,460
as it passes through turbulent air
in the atmosphere.
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00:00:21,460 --> 00:00:23,929
Although it may not comfort
you to hear it,
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00:00:23,929 --> 00:00:28,339
this phenomenon is one of the
prevailing mysteries of physics.
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00:00:28,339 --> 00:00:31,353
After more than a century
of studying turbulence,
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00:00:31,353 --> 00:00:34,473
we’ve only come up with a few
answers for how it works
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00:00:34,473 --> 00:00:36,823
and affects the world around us.
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00:00:36,823 --> 00:00:39,283
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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00:00:47,313 --> 00:00:49,663
The blood moving through your arteries.
14
00:00:49,663 --> 00:00:53,073
And the coffee in your cup,
as you stir it.
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00:00:53,073 --> 00:00:55,313
Clouds are governed by turbulence,
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00:00:55,313 --> 00:01:00,893
as are waves crashing along the shore
and the gusts of plasma in our sun.
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00:01:00,893 --> 00:01:04,043
Understanding precisely how this
phenomenon works
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00:01:04,043 --> 00:01:07,933
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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00:01:12,813 --> 00:01:15,933
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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00:01:21,253 --> 00:01:23,723
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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00:02:05,094 --> 00:02:09,814
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,
50
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which causes instability.
51
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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.
55
00:03:04,687 --> 00:03:09,687
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,
59
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which is the ratio between a flow’s
inertia and its viscosity.
60
00:03:24,230 --> 00:03:25,940
The higher the Reynolds number,
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00:03:25,940 --> 00:03:29,200
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.
70
00:04:00,024 --> 00:04:03,886
We can model those thanks to many
experiments and empirical evidence.
71
00:04:03,886 --> 00:04:08,686
But physicists want to be able to predict
them through physical laws and equations
72
00:04:08,690 --> 00:04:13,790
as well as we can model the orbits
of planets or electromagnetic fields.
73
00:04:13,790 --> 00:04:17,935
Most scientists think that getting there
will rely on statistics
74
00:04:17,935 --> 00:04:20,049
and increased computing power.
75
00:04:20,049 --> 00:04:23,869
Extremely high-speed computer simulations
of turbulent flows
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00:04:23,869 --> 00:04:27,869
could help us identify patterns that could
lead to a theory
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00:04:27,869 --> 00:04:33,479
that organizes and unifies predictions
across different situations.
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00:04:33,479 --> 00:04:37,291
Other scientists think that the phenomenon
is so complex
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00:04:37,291 --> 00:04:41,951
that such a full-fledged theory
isn’t ever going to be possible.
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00:04:41,951 --> 00:04:43,775
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.7314
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