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  • You're on an airplane when you feel a sudden jolt.

  • Outside your window nothing seems to be happening,

  • yet the plane continues to rattle you and your fellow passengers as it passes through turbulent air in the atmosphere.

  • Although it may not comfort you to hear it,

  • this phenomenon is one of the prevailing mysteries of physics.

  • After more than a century of studying turbulence,

  • we've only come up with a few answers for how it works and affects the world around us.

  • And yet, turbulence is ubiquitous, springing up in virtually any system that has moving fluids.

  • That includes the airflow in your respiratory tract.

  • The blood moving through your arteries.

  • And the coffee in your cup, as you stir it.

  • Clouds are governed by turbulence,

  • as are waves crashing along the shore and the gusts of plasma in our sun.

  • Understanding precisely how this phenomenon works would have a bearing on so many aspects of our lives.

  • Here's what we do know.

  • Liquids and gases usually have two types of motion:

  • a laminar flow, which is stable and smooth;

  • and a turbulent flow, which is composed of seemingly unorganized swirls.

  • Imagine an incense stick.

  • The laminar flow of unruffled smoke at the base is steady and easy to predict.

  • Closer to the top, however,

  • the smoke accelerates, becomes unstable,

  • and the pattern of movement changes to something chaotic.

  • That's turbulence in action,

  • and turbulent flows have certain characteristics in common.

  • Firstly, turbulence is always chaotic.

  • That's different from being random.

  • Rather, this means that turbulence is very sensitive to disruptions.

  • A little nudge one way or the other will eventually turn into completely different results.

  • That makes it nearly impossible to predict what will happen,

  • even with a lot of information about the current state of a system.

  • Another important characteristic of turbulence is the different scales of motion that these flows display.

  • Turbulent flows have many differently-sized whirls called eddies, which are like vortices of different sizes and shapes.

  • All those differently-sized eddies interact with each other,

  • breaking up to become smaller and smaller

  • until all that movement is transformed into heat,

  • in a process called theenergy cascade."

  • So that's how we recognize turbulence

  • but why does it happen?

  • In every flowing liquid or gas there are two opposing forces:

  • inertia and viscosity.

  • Inertia is the tendency of fluids to keep moving,

  • which causes instability.

  • Viscosity works against disruption,

  • making the flow laminar instead.

  • In thick fluids such as honey,

  • viscosity almost always wins.

  • Less viscous substances like water or air are more prone to inertia,

  • which creates instabilities that develop into turbulence.

  • We measure where a flow falls on that spectrum

  • with something called the Reynolds number,

  • which is the ratio between a flow's inertia and its viscosity.

  • The higher the Reynolds number,

  • the more likely it is that turbulence will occur.

  • Honey being poured into a cup, for example,

  • has a Reynolds number of about 1.

  • The same set up with water has a Reynolds number that's closer to 10,000.

  • The Reynolds number is useful for understanding simple scenarios,

  • but it's ineffective in many situations.

  • For example, the motion of the atmosphere is significantly influenced

  • by factors including gravity and the earth's rotation.

  • Or take relatively simple things like the drag on buildings and cars.

  • We can model those thanks to many experiments and empirical evidence.

  • But physicists want to be able to predict them through physical laws and equations

  • as well as we can model the orbits of planets or electromagnetic fields.

  • Most scientists think that getting there will rely on statistics and increased computing power.

  • Extremely high-speed computer simulations of turbulent flows

  • could help us identify patterns that could lead to a theory

  • that organizes and unifies predictions across different situations.

  • Other scientists think that the phenomenon is so complex

  • that such a full-fledged theory isn't ever going to be possible.

  • Hopefully we'll reach a breakthrough,

  • because a true understanding of turbulence could have huge positive impacts.

  • That would include more efficient wind farms;

  • the ability to better prepare for catastrophic weather events;

  • or even the power to manipulate hurricanes away.

  • And, of course, smoother rides for millions of airline passengers.

  • Despite how difficult it is to explain turbulence mathematically,

  • Vincent Van Gogh was able to capture it with a sounding accuracy

  • in his iconic painting "The Starry Night".

  • Watch this video to learn more about the surprising man behind his masterpiece.

You're on an airplane when you feel a sudden jolt.

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B2 US TED-Ed turbulence reynolds number reynolds viscosity inertia

Turbulence: one of the great unsolved mysteries of physics - Tomas Chor

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    Amy.Lin posted on 2019/04/16
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