Original subtitles

A portion of this video was sponsored by loda

This is like a scientist trap.

It certainly is; case in point, that is Space Station commander Chris Hadfield

What this isn't is turbulent. Nope, this is largely laminar flow.

“Did somebody say peculiar flow! ?”

no i dont

If you didn't know, Destin from smarter every day loves laminar flow,

where all the particles of the fluid move parallel to each other in organized layers or laminae

look at that it made a bubble!!!

Where I live people will roll down the window in their car when they see me in the street

and they will scream “turbulent flow” to me

That happens, that happens in Huntsville. Yeah.

Here- Here's my argument to you, Destin

nashe

Okay.

but turbulent flow if you make that effort is actually more awesome.

Um...

no.

Turbulent flow is not better than laminar. It is awesome

But it is not better than laminar flow.

Can I just say I get it

I get where Destin is coming from

I mean laminar flow is pretty and it's well behaved,

Meanwhile turbulent flow is a mess in more ways than one.

I mean, there isn't even a universally agreed-upon definition of turbulent flow.

You know it when you see it

[Laughing]

Hahaha, So that's the deal with turbulence, you know it when you see it?

Pretty much. Yeah.

So instead of a formal definition, in this video

we are going to build a checklist of characteristics of turbulent flow,

so that you know it when you see it

and the first characteristic of turbulent flow is that it is unpredictable

That's right. Turbulent flow is messy it's unpredictable.

It is literally definitionally chaotic

meaning it is sensitively dependent on initial conditions.

So if you were to change something somewhere in the fluid

well, it would completely change the final state

and that means you can't make predictions with turbulent flow

All you can do is speak about it statistically.

I mean, there are the Navier-Stokes equations which are meant to govern all fluid flow

Including turbulence, but they are notoriously difficult to solve.

In fact, there is a million-dollar prize

for anyone who can even make progress towards getting insight into these equations that would explain turbulence,

so yeah, I get it, turbulence is a mess，

laminar flow is easy to love

It's like the bell of a ball

whereas turbulent flow is kind of an ugly duckling

But in this video I want to transform that ugly duckling into a beautiful swan

I want you to see that if you make the effort

The love you can have for turbulent flow is so much deeper and richer than that superficial fling you have with laminar flow

You are looking at the motion of air in a room, which is generally turbulent

the physics girl and friends imaged a cross-section of air using a fog machine and a laser sheet

one of the defining characteristics of turbulent flow is that

it consists of many interacting swirls of fluid also called Eddies or Vortices

These eddies span a huge range of sizes

In the case of air in a room, from the micrometer scale all the way up to meters in diameter

Can you think of another physical phenomenon that exhibits structures over such a range of sizes?

But turbulence can be much larger.

The surface of the Sun is turbulent as hot plasma rises to the surface in huge convection currents.

The cell like structures here are roughly the size of Texas

Larger still are the turbulent swirls on Jupiter.

The Great Red Spot is a vortex bigger than the Earth

The rest of the planet is covered in Eddie's of all sizes

down to the limits of our ability to measure them from orbiting spacecraft

Even the dust between the stars is in turbulent motion

It makes radio sources twinkle the same way the turbulence in our atmosphere makes stars twinkle

a stunning example of this turbulent dust is the Orion Nebula: twenty four light years across

Turbulence is cosmic. In contrast, laminar flow has to be small.

This was shown experimentally in 1883.

Osborne Reynolds passed water through a glass pipe at different flow rates

and to visualize the flow, he introduced a stream of dye in the middle of the pipe

He found at low flow rates the dye remained in a steady stream: laminar flow

but as the flow rate increased the dye began to oscillate back and forth

and beyond a certain critical point, the dye became completely diffused throughout the pipe.

This was turbulent flow.

Reynolds had observed another essential characteristic of turbulence,

It is diffusive, meaning it mixes things together

Turbulent flows caused things to spread out

not only dye, but also heat or momentum

They all become distributed throughout the fluid

Reynolds found the transition to turbulence was not only dependent on the flow rate

turbulence occurred more readily in wider pipes

But less readily with more viscous fluids, things like honey

He calculated a dimensionless quantity now called the Reynolds number

Equal to the velocity of the fluid times the characteristic length, say the diameter of the pipe

Divided by the kinematic viscosity of the fluid

which you can think of as a measure of its internal friction

high Reynolds numbers result in turbulent flow

Have a look at the smoke rising from a candle flame

At first, it's laminar. But the hot gases accelerate as they rise,

and once the Reynolds number gets too big the smoke transitions to turbulence

so laminar flow only occurs at low Reynolds numbers

Which means it is limited to low speeds small sizes or viscous fluids

This is why in our everyday lives most fluid flow is turbulent

Turbulent flow is the rule.

Laminar flow is the exception.

The air flowing in and out of your lungs is turbulent,

the blood pumping through your aorta is turbulent

the Atmosphere near the surface of the earth is turbulent

as is the air flow in and around cumulus and cumulonimbus clouds

In fact modeling shows that turbulent flow plays an essential role in the formation of rain drops

so turbulence literally makes it rain.

[Thunder crashes, Rain sounds]

I'm going to create turbulence in this rheoscopic fluid

Rheoscopic just means that it shows the currents

and it does that by having these tiny particles suspended in the water

But what you notice if you look at this turbulent flow is that it gradually dies away

And that's because another characteristic of turbulence is that it's dissipative

That is it takes in energy at the largest scales at these big eddies,

and then that energy gets transferred down to smaller and smaller eddies

until on the smallest scales that energy gets dissipated to the fluid as heat

And so in order to maintain turbulence

You need a constant source of energy, something to keep generating those large eddies,

which is why we often think about turbulence around objects that move through a fluid

things like planes cars or boats.

So I want to think about the interface between an object and the fluid.

So picture fluid flowing over a flat surface

far away from the surface, the fluid isn't affected. It keeps moving with what will call its free stream velocity

But right at the surface，due to friction and adhesion

The molecules of the fluid are effectively stuck to the surface. Their velocity is zero.

The fluid next to it can flow only slowly due to friction with this stationary layer

with increasing distance from the surface，the fluids velocity increases from zero until it reaches the free stream velocity

and this region of velocity adjustment is known as a boundary layer.

In this case, it's a laminar boundary layer

To form this boundary layer, the surface is applying a force to the fluid

That means the fluid is applying an equal and opposite force on the surface

and this is known as skin friction

Now if the fluid velocity is particularly fast or if the surface is long

the boundary layer will grow and eventually transition to turbulence

in a turbulent boundary layer，the fluid swirls and mixes bringing faster flowing fluid closer to the surface

and this increases the skin friction

so turbulent boundary layers result in significantly more drag than laminar ones

and the boundary layers around planes and large ships are mostly turbulent

and skin friction accounts for the majority of the drag they experience

to make matters worse laminar boundary layers can be tripped into becoming turbulent by small obstacles or rough surfaces

in practice this means clean smooth surfaces can significantly reduce drag saving on fuel costs

If your car is really dirty, it likely gets worse gas mileage than if it were clean

This is what the Mythbusters found when they tested it.

It also explains why planes are frequently washed

So when you think about airplanes, I imagine that they would be built as smooth as possible

I think of the scene in The Aviator

where Leo says he wants all of the rivets shaved down flush

and you can see that with this plane all of these screws are are set in to the wing

and really to make the smoothest surface possible,

but then you look over here and there are these ridges that stick up out of the plane,

which seem to make no sense

I mean, why would you add roughness to the surface of the wing?

the answer is actually to induce turbulence in the flow of air over the wing

when cruising in level flight, air smoothly follows the curve of the wing

but at low speeds or higher angles of attack the airflow can separate

you can think of it as not having enough energy to follow the curve of the wing

This leads to a condition known as stall which dramatically decreases lift

Here you can see the airflow of via strings taped onto the wing

and as the plane slows the flow separates

and the strings go wild

This plane has stalled.

The way to delay flow separation and stall is by adding small fins on the wing called vortex generators

What these vortex generators do, is they actually cause turbulence

which mixes the faster flowing higher up air down closer to the surface

so you're energizing that fluid flow as it passes over the wing

and because that flow has greater energy it is able to follow the surface of the wing for longer

That means the air flow remains attached

and if you have attached airflow over the wing then you can maintain lift

so in the case of airplanes

You actually need turbulence and you induce more turbulence on the wing

in order to fly efficiently and effectively and be able to climb at higher angles of attack.

A similar principle is at work with golf balls.

The Scott found out about turbulence the hard way

because they started playing with a very smooth golfball

and it wouldn't fly as far as it would once it got sort of dimple nicked and dirty

you can see why by observing the airflow in a wind tunnel

with a smooth ball the air forms a laminar boundary layer over its surface

this leads to low skin friction, which is a good thing

But it also means the air flow separates easily

leaving a large wake of low pressure turbulent air behind the ball

and that leads to a different form of drag.

Is that a pressure difference drag?

That's right, that's a pressure drag.

So the boundary layer itself has a skin friction drag and then if it separates there's a pressure drag

And if you force that boundary layer to become turbulent

So you have mud or roughness or mix on the golf ball

then a turbulent boundary like this can get further around the golf ball before it separates

And so it reduces that wake and reduces that pressure drag.

So by reducing the pressure drag to more than your increase in this kind of drag, golf ball travels further

Yep!

Golfers started carving grooves into their golf balls

before the aerodynamics of this was fully understood

And since then dimples have found to work the best for creating a turbulent boundary layer

Dimples are very shallow compared to the diameter of the golf ball, but they have a pretty massive effect

What sort of effect are we talking?

Looking at the drag, and we call it drag coefficient

You see a really big drop almost a factor of two when the boundary layer becomes turbulent.

So having a turbulent boundary layer reduces the size of the turbulent wake

but turbulent wakes themselves are interesting and

scientists are looking for ways to harness the energy they contain

I came to Caltech to see this experiment

where the water flows around a cylinder and transitions to turbulence in its wake

The flow is visualized here using a fluorescent dye

You can see how under the right conditions

Vortices are shed by one side of the cylinder, and then the other,

alternating back and forth in a regular pattern

This is known as periodic vortex shedding

and the pattern it creates downstream is called a von Karman Vortex Street

These patterns appear all over the place, most spectacularly in images taken from space

At this scale, an Island acts as the obstacle that creates the periodic vortex shedding

and the vortex street is made visible by patterns in the clouds

These patterns can even be seen from ground level

Obviously this phenomenon is not strictly turbulent because it follows a predictable pattern

but it is part of the transition to turbulence

and these scientists are looking for ways to harness the energy in these vortex structures

One experiment showed that if you put a dead fish in the wake of an object

it will actually swim upstream

This suggests fish can take advantage of turbulent water to swim more efficiently

It's just one way that animals have adapted to live in a turbulent world

So to sum up, turbulence is everywhere,

it's inside you around you from the smallest scales up to the largest structures in the universe

and it's useful for flying airplanes, forming raindrops,

making golf balls fly further, and helping fish, dead or alive, swim upstream

In contrast, laminar flow is small, superficial, it's a toy

That's why it's most notable use is in decorative fountains

It appeals to your desire for order, but the world like turbulence is messy

That's why I personally prefer the richness, the unpredictability of turbulent flow

No, but but turbulent flow has its places too.

I'm actually like studying turbulent flow for like my my schooling,

Like I'm studying turbulent flow in rocket nozzles. That's a thing.

So cheating on laminar flow, is what are you doing.

Um, no, yes. Yes, maybe, I don't know

But I wonder you will not get me to say turbulent flow is not awesome and not beautiful, you will not get me to say that

So I will concede and I agree with you turbulent flow is awesome. I will agree

All right. All right.

Well it's just not as awesome as laminar flow. Let's be honest

Hey, I just wanted to let you know that this video was filmed

before the COVID outbreak and before the shelter-in-place guidance was put into effect

Now this portion of the video was sponsored by Cottonelle flushable wipes

and since the outbreak they have been working around the clock to get their products back on shelves

And back when I filmed this video I actually did a little experiment with these wipes to find out how flushable they really are

So let's check that out

So here I have a baby wipe, a paper towel, and a Cottonelle flushable wipe

and I'm gonna submerge all three of these in the fish tank for 30 minutes

and then test how strong they are

Flushable wipes actually became really important to me a couple years ago

When the main sewer for my building backed up into my condo and flooded the entire downstairs

And the reason was my neighbor was flushing baby wipes down the toilet and that blocked up the whole system

So it was pretty awful.

But in fact, this is a thing people do a lot

There was this study from 2016 that found in the US

60 million baby wipes are purchased every year

and seven million of them end up being flushed down the toilet

In fact when they looked in the New York City sewer system

They found that 38 percent of the stuff you find in there is actually these baby wipes

Meanwhile, 14 million flushable wipes are purchased every year and flushed down toilets

But they make up only 2% of what you find in the sewer system

So I think it's so important that whatever you throw in the toilet has to be able to break apart so it doesn't clog everything up.

Okay, 30 minutes have elapsed and it is time to test the strength of these three wipes

So I'm gonna test their strength with a roll of pennies.

Here we go on the baby wipe

It can still support that weight.

What about the paper towel?

Still supports that weight

What about the Cottonelle flushable wipe?

Ah!

It fell through

so this is what makes the Cottonelle flushable wipe flushable

it immediately starts to break down after flushing

So you should purchase some cotton nail flushable wipes and try them out for yourself

I want to thank Cottonelle for sponsoring this video,

And I want to thank you for watching