Turbulent Flow is MORE Awesome Than Laminar Flow

published on August 2, 2020

A portion of this video was sponsored by Cottonelle this is like a scientist trap it certainly is case in point that Space Station commander Chris Hatfield what this isn't is turbulent nope this is largely laminar flow did somebody say laminar flow if you didn't

Know Destin from smarter every day loves laminar flow where all the particles of the fluid moved parallel to each other in organized layers or laminae look at 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 is better to me that happens that happens in Huntsville yeah here's my argument to you Destin that laminar flow is easier to love 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 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 belle of the 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 Eddie's or vortices these Eddie's 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 die 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 its 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 I'm going to create turbulence in this Rio scopic fluid Rio scopic 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 Eddie's and then that energy gets transferred down to smaller and smaller Eddie's 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 Eddie's 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 service 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 your 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 hires angles of attack a similar principle is at work with golf balls the skull found out about turbulence the hard way because they started playing

With a very smooth gospel and it wouldn't fly as far as it would once it got sort of dingo 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 I can actually 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 more than your increase in this kind rag that's right 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 right what we call a 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 flows what are you doing um no yes yes maybe I don't know but I what 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

Kovat 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 cotton L

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 the

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 a toilet 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 wait what about the paper towel can still support that wait what about the cut now 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 want to thank cotton al for

Sponsoring this video and I want to thank you for watching

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