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
