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A portion of this video was sponsored by loda
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This is like a scientist trap.
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It certainly is; case in point, that is Space Station commander Chris Hadfield
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What this isn't is turbulent. Nope, this is largely laminar flow.
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“Did somebody say peculiar flow! ?”
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no i dont
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If you didn't know, Destin from smarter every day loves laminar flow,
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where all the particles of the fluid move parallel to each other in organized layers or laminae
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look at that it made a bubble!!!
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Where I live people will roll down the window in their car when they see me in the street
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and they will scream “turbulent flow” to me
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That happens, that happens in Huntsville. Yeah.
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Here- Here's my argument to you, Destin
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nashe
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Okay.
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but turbulent flow if you make that effort is actually more awesome.
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Um...
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no.
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Turbulent flow is not better than laminar. It is awesome
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But it is not better than laminar flow.
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Can I just say I get it
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I get where Destin is coming from
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I mean laminar flow is pretty and it's well behaved,
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Meanwhile turbulent flow is a mess in more ways than one.
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I mean, there isn't even a universally agreed-upon definition of turbulent flow.
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You know it when you see it
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[Laughing]
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Hahaha, So that's the deal with turbulence, you know it when you see it?
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Pretty much. Yeah.
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So instead of a formal definition, in this video
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we are going to build a checklist of characteristics of turbulent flow,
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so that you know it when you see it
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and the first characteristic of turbulent flow is that it is unpredictable
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That's right. Turbulent flow is messy it's unpredictable.
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It is literally definitionally chaotic
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meaning it is sensitively dependent on initial conditions.
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So if you were to change something somewhere in the fluid
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well, it would completely change the final state
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and that means you can't make predictions with turbulent flow
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All you can do is speak about it statistically.
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I mean, there are the Navier-Stokes equations which are meant to govern all fluid flow
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Including turbulence, but they are notoriously difficult to solve.
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In fact, there is a million-dollar prize
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for anyone who can even make progress towards getting insight into these equations that would explain turbulence,
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so yeah, I get it, turbulence is a mess,
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laminar flow is easy to love
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It's like the bell of a ball
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whereas turbulent flow is kind of an ugly duckling
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But in this video I want to transform that ugly duckling into a beautiful swan
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I want you to see that if you make the effort
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The love you can have for turbulent flow is so much deeper and richer than that superficial fling you have with laminar flow
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You are looking at the motion of air in a room, which is generally turbulent
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the physics girl and friends imaged a cross-section of air using a fog machine and a laser sheet
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one of the defining characteristics of turbulent flow is that
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it consists of many interacting swirls of fluid also called Eddies or Vortices
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These eddies span a huge range of sizes
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In the case of air in a room, from the micrometer scale all the way up to meters in diameter
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Can you think of another physical phenomenon that exhibits structures over such a range of sizes?
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But turbulence can be much larger.
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The surface of the Sun is turbulent as hot plasma rises to the surface in huge convection currents.
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The cell like structures here are roughly the size of Texas
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Larger still are the turbulent swirls on Jupiter.
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The Great Red Spot is a vortex bigger than the Earth
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The rest of the planet is covered in Eddie's of all sizes
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down to the limits of our ability to measure them from orbiting spacecraft
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Even the dust between the stars is in turbulent motion
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It makes radio sources twinkle the same way the turbulence in our atmosphere makes stars twinkle
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a stunning example of this turbulent dust is the Orion Nebula: twenty four light years across
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Turbulence is cosmic. In contrast, laminar flow has to be small.
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This was shown experimentally in 1883.
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Osborne Reynolds passed water through a glass pipe at different flow rates
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and to visualize the flow, he introduced a stream of dye in the middle of the pipe
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He found at low flow rates the dye remained in a steady stream: laminar flow
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but as the flow rate increased the dye began to oscillate back and forth
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and beyond a certain critical point, the dye became completely diffused throughout the pipe.
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This was turbulent flow.
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Reynolds had observed another essential characteristic of turbulence,
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It is diffusive, meaning it mixes things together
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Turbulent flows caused things to spread out
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not only dye, but also heat or momentum
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They all become distributed throughout the fluid
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Reynolds found the transition to turbulence was not only dependent on the flow rate
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turbulence occurred more readily in wider pipes
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But less readily with more viscous fluids, things like honey
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He calculated a dimensionless quantity now called the Reynolds number
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Equal to the velocity of the fluid times the characteristic length, say the diameter of the pipe
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Divided by the kinematic viscosity of the fluid
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which you can think of as a measure of its internal friction
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high Reynolds numbers result in turbulent flow
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Have a look at the smoke rising from a candle flame
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At first, it's laminar. But the hot gases accelerate as they rise,
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and once the Reynolds number gets too big the smoke transitions to turbulence
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so laminar flow only occurs at low Reynolds numbers
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Which means it is limited to low speeds small sizes or viscous fluids
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This is why in our everyday lives most fluid flow is turbulent
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Turbulent flow is the rule.
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Laminar flow is the exception.
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The air flowing in and out of your lungs is turbulent,
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the blood pumping through your aorta is turbulent
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the Atmosphere near the surface of the earth is turbulent
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as is the air flow in and around cumulus and cumulonimbus clouds
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In fact modeling shows that turbulent flow plays an essential role in the formation of rain drops
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so turbulence literally makes it rain.
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[Thunder crashes, Rain sounds]
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I'm going to create turbulence in this rheoscopic fluid
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Rheoscopic just means that it shows the currents
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and it does that by having these tiny particles suspended in the water
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But what you notice if you look at this turbulent flow is that it gradually dies away
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And that's because another characteristic of turbulence is that it's dissipative
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That is it takes in energy at the largest scales at these big eddies,
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and then that energy gets transferred down to smaller and smaller eddies
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until on the smallest scales that energy gets dissipated to the fluid as heat
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And so in order to maintain turbulence
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You need a constant source of energy, something to keep generating those large eddies,
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which is why we often think about turbulence around objects that move through a fluid
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things like planes cars or boats.
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So I want to think about the interface between an object and the fluid.
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So picture fluid flowing over a flat surface
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far away from the surface, the fluid isn't affected. It keeps moving with what will call its free stream velocity
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But right at the surface,due to friction and adhesion
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The molecules of the fluid are effectively stuck to the surface. Their velocity is zero.
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The fluid next to it can flow only slowly due to friction with this stationary layer
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with increasing distance from the surface,the fluids velocity increases from zero until it reaches the free stream velocity
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and this region of velocity adjustment is known as a boundary layer.
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In this case, it's a laminar boundary layer
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To form this boundary layer, the surface is applying a force to the fluid
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That means the fluid is applying an equal and opposite force on the surface
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and this is known as skin friction
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Now if the fluid velocity is particularly fast or if the surface is long
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the boundary layer will grow and eventually transition to turbulence
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in a turbulent boundary layer,the fluid swirls and mixes bringing faster flowing fluid closer to the surface
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and this increases the skin friction
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so turbulent boundary layers result in significantly more drag than laminar ones
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and the boundary layers around planes and large ships are mostly turbulent
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and skin friction accounts for the majority of the drag they experience
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to make matters worse laminar boundary layers can be tripped into becoming turbulent by small obstacles or rough surfaces
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in practice this means clean smooth surfaces can significantly reduce drag saving on fuel costs
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If your car is really dirty, it likely gets worse gas mileage than if it were clean
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This is what the Mythbusters found when they tested it.
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It also explains why planes are frequently washed
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So when you think about airplanes, I imagine that they would be built as smooth as possible
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I think of the scene in The Aviator
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where Leo says he wants all of the rivets shaved down flush
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and you can see that with this plane all of these screws are are set in to the wing
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and really to make the smoothest surface possible,
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but then you look over here and there are these ridges that stick up out of the plane,
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which seem to make no sense
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I mean, why would you add roughness to the surface of the wing?
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the answer is actually to induce turbulence in the flow of air over the wing
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when cruising in level flight, air smoothly follows the curve of the wing
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but at low speeds or higher angles of attack the airflow can separate
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you can think of it as not having enough energy to follow the curve of the wing
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This leads to a condition known as stall which dramatically decreases lift
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Here you can see the airflow of via strings taped onto the wing
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and as the plane slows the flow separates
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and the strings go wild
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This plane has stalled.
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The way to delay flow separation and stall is by adding small fins on the wing called vortex generators
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What these vortex generators do, is they actually cause turbulence
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which mixes the faster flowing higher up air down closer to the surface
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so you're energizing that fluid flow as it passes over the wing
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and because that flow has greater energy it is able to follow the surface of the wing for longer
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That means the air flow remains attached
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and if you have attached airflow over the wing then you can maintain lift
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so in the case of airplanes
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You actually need turbulence and you induce more turbulence on the wing
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in order to fly efficiently and effectively and be able to climb at higher angles of attack.
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A similar principle is at work with golf balls.
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The Scott found out about turbulence the hard way
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because they started playing with a very smooth golfball
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and it wouldn't fly as far as it would once it got sort of dimple nicked and dirty
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you can see why by observing the airflow in a wind tunnel
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with a smooth ball the air forms a laminar boundary layer over its surface
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this leads to low skin friction, which is a good thing
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But it also means the air flow separates easily
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leaving a large wake of low pressure turbulent air behind the ball
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and that leads to a different form of drag.
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Is that a pressure difference drag?
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That's right, that's a pressure drag.
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So the boundary layer itself has a skin friction drag and then if it separates there's a pressure drag
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And if you force that boundary layer to become turbulent
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So you have mud or roughness or mix on the golf ball
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then a turbulent boundary like this can get further around the golf ball before it separates
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And so it reduces that wake and reduces that pressure drag.
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So by reducing the pressure drag to more than your increase in this kind of drag, golf ball travels further
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Yep!
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Golfers started carving grooves into their golf balls
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before the aerodynamics of this was fully understood
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And since then dimples have found to work the best for creating a turbulent boundary layer
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Dimples are very shallow compared to the diameter of the golf ball, but they have a pretty massive effect
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What sort of effect are we talking?
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Looking at the drag, and we call it drag coefficient
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You see a really big drop almost a factor of two when the boundary layer becomes turbulent.
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So having a turbulent boundary layer reduces the size of the turbulent wake
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but turbulent wakes themselves are interesting and
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scientists are looking for ways to harness the energy they contain
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I came to Caltech to see this experiment
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where the water flows around a cylinder and transitions to turbulence in its wake
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The flow is visualized here using a fluorescent dye
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You can see how under the right conditions
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Vortices are shed by one side of the cylinder, and then the other,
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alternating back and forth in a regular pattern
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This is known as periodic vortex shedding
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and the pattern it creates downstream is called a von Karman Vortex Street
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These patterns appear all over the place, most spectacularly in images taken from space
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At this scale, an Island acts as the obstacle that creates the periodic vortex shedding
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and the vortex street is made visible by patterns in the clouds
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These patterns can even be seen from ground level
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Obviously this phenomenon is not strictly turbulent because it follows a predictable pattern
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but it is part of the transition to turbulence
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and these scientists are looking for ways to harness the energy in these vortex structures
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One experiment showed that if you put a dead fish in the wake of an object
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it will actually swim upstream
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This suggests fish can take advantage of turbulent water to swim more efficiently
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It's just one way that animals have adapted to live in a turbulent world
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So to sum up, turbulence is everywhere,
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it's inside you around you from the smallest scales up to the largest structures in the universe
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and it's useful for flying airplanes, forming raindrops,
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making golf balls fly further, and helping fish, dead or alive, swim upstream
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In contrast, laminar flow is small, superficial, it's a toy
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That's why it's most notable use is in decorative fountains
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It appeals to your desire for order, but the world like turbulence is messy
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That's why I personally prefer the richness, the unpredictability of turbulent flow
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No, but but turbulent flow has its places too.
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I'm actually like studying turbulent flow for like my my schooling,
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Like I'm studying turbulent flow in rocket nozzles. That's a thing.
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So cheating on laminar flow, is what are you doing.
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Um, no, yes. Yes, maybe, I don't know
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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
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So I will concede and I agree with you turbulent flow is awesome. I will agree
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All right. All right.
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Well it's just not as awesome as laminar flow. Let's be honest
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Hey, I just wanted to let you know that this video was filmed
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before the COVID outbreak and before the shelter-in-place guidance was put into effect
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Now this portion of the video was sponsored by Cottonelle flushable wipes
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and since the outbreak they have been working around the clock to get their products back on shelves
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And back when I filmed this video I actually did a little experiment with these wipes to find out how flushable they really are
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So let's check that out
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So here I have a baby wipe, a paper towel, and a Cottonelle flushable wipe
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and I'm gonna submerge all three of these in the fish tank for 30 minutes
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and then test how strong they are
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Flushable wipes actually became really important to me a couple years ago
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When the main sewer for my building backed up into my condo and flooded the entire downstairs
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And the reason was my neighbor was flushing baby wipes down the toilet and that blocked up the whole system
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So it was pretty awful.
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But in fact, this is a thing people do a lot
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There was this study from 2016 that found in the US
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60 million baby wipes are purchased every year
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and seven million of them end up being flushed down the toilet
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In fact when they looked in the New York City sewer system
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They found that 38 percent of the stuff you find in there is actually these baby wipes
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Meanwhile, 14 million flushable wipes are purchased every year and flushed down toilets
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But they make up only 2% of what you find in the sewer system
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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.
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Okay, 30 minutes have elapsed and it is time to test the strength of these three wipes
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So I'm gonna test their strength with a roll of pennies.
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Here we go on the baby wipe
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It can still support that weight.
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What about the paper towel?
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Still supports that weight
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What about the Cottonelle flushable wipe?
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Ah!
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It fell through
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so this is what makes the Cottonelle flushable wipe flushable
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it immediately starts to break down after flushing
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So you should purchase some cotton nail flushable wipes and try them out for yourself
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I want to thank Cottonelle for sponsoring this video,
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And I want to thank you for watching
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