Turbulence: one of the great unsolved mysteries of physics – Tomás Chor

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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 the “energy 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.

 

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