Stress, Strain & Quicksand: Crash Course Engineering #12

No matter where we are, we’re almost always
affected by our environment. Because, we’re affected by the medium that
we’re in. Usually that medium is a fluid, like air. And, to understand how these fluids work, and
to be able to optimize our designs, we’re going to
need to learn about fluid mechanics. Fluid mechanics explains how the air moves
around your car, how food coloring moves through
water, making all those pretty patterns. And it explains what makes quicksand act like,
well, quicksand. All of which seems worth knowing, don’t
you think? [Theme Music] So far, we’ve talked systems versus their
surroundings. We’ve learned that mass and energy
cannot be created or destroyed, but that
they can be converted. What this means for us as engineers is that,
in the real world, we can’t simply focus on
what’s going on inside our system. We need to think about the outside as well. That’s because machines, buildings, even our
own bodies, are influenced by their surroundings. To see what I mean, let’s say we’re designing
a new car. We’ve already learned a good deal about
engines, so you probably have a decent idea
of what should go inside the car. But what about the outside of the car? While a car is moving, it’s going to interact
with the fluid that it’s moving through which
will be air in most cases. Unless you’re that Tesla that got sent into space. As the car interacts with the air, the two
can affect one another, and this can lead
to what we call transfer. You know what transfer is, right? When something moves, or is moved, from one
place to another? There can be a transfer of momentum, or a
transfer of heat. Maybe even mass. But if we’re looking at moving fluids, then we’ll often
have a transfer of momentum, which can be better
understood with the help of fluid mechanics. Fluid mechanics studies how fluids respond
to the forces exerted on them. So how exactly do fluids move? And how does a particle, or anything for that
matter, move within a fluid? To answer these questions, we need to know
about stress, strain, and viscosity. Now, if you’ve been a student of physics for any amount
of time, I’m sure you’re familiar with stress and strain in
the colloquial sense – especially around exam time. But here, of course, they mean something completely
different. Suppose we have a fluid between two flat plates. If we were to move the bottom plate, what
would happen to the fluid? How would it move? Well, at the top and bottom, where the fluid is in contact
with the surface, the individual particles of the fluid will
go through something called the no-slip condition. In the no-slip condition, a fluid in motion will come
to a complete stop at a solid surface and assume
a zero velocity relative to the surface. Basically, the particles of the fluid that
are touching the solid will stick to its surface,
meaning that they won’t slip. Because of this, the fluid particles in contact with the
bottom plate will move with it, while the fluid particles
at the top will stay in place with the stationary plate. This is all happening due to stress, the
force that’s applied to a cross-sectional area
of an object or substance. If the force is normal, or perpendicular
to the surface of the object, then we have
normal stress. If it’s parallel, then we have shear stress. You thought I was gonna say “parallel stress,”
didn’t you? We can find stress by taking the applied force
and dividing it by our cross-sectional area. Now, once a fluid is stressed, the degree
to which it stretches is called strain. Simply put, strain is the deformation that
stress causes on a system. If the deformation causes something in a
system to become either shorter or longer, then we can find its strain by taking the change
in length and dividing it by the initial length. And this is called normal strain But if the deformation is a change in angle between
two segments that had been perpendicular to each
other, then we have shear strain. And we can find that by subtracting the change in
angle from the original angle, which will either be pi
over 2 or 90 degrees, depending on your units. So all of this is what would happen if our
bottom plate was moving. But what if neither plate moved, and we had a
pump driving the flow of the fluid between them? Well, the same no-slip condition would apply, so
while the fluid moved, its particles at the surface
of the two plates would stay stationary. But another thing that we’d need to take
into account is viscosity. Viscosity is essentially a measure of a fluid’s
resistance to flow, and it’s often referred to
as the thickness of a fluid. For example, water has a low viscosity, since
it flows pretty easily, while honey and other thick,
sticky fluids have a much higher viscosity. Some fluids will also thicken when exposed
to stress, like when you whisk water mixed
with cornstarch. But others will thin out when exposed to stress, like
quicksand, which is just water mixed with lots of sand. The great Sir Isaac Newton gave us a
way of describing how fluids move with his
law of viscosity. Simply put, this law describes Newtonian fluids
as fluids with a viscosity that’s independent of stress. No matter how much stress you put on the fluid,
its viscosity never changes. While no real fluid is perfectly Newtonian,
many fluids, like air and water, are close enough
that we can think of them that way. Non-Newtonian fluids, on the other hand, don’t
follow the law of viscosity. Their viscosity can change under stress, and
therefore their strain can, too. A non-Newtonian fluid will actually thicken or thin
out if you apply a force to it, which is exactly what
happens when you step into quicksand. The stress of you stepping onto it makes it
become less viscous. And then you’re in trouble. Now, there’s still the matter of how one
fluid moves within another fluid. And to understand that, we should turn to
the work of British engineer Osborne Reynolds. In 1868, just a year after graduating college,
Reynolds became the first professor of engineering
at Owens College in Manchester. He spent much of his career studying fluid
mechanics, and one of his greatest contributions
was his work on fluid flow patterns. In 1883, Reynolds conducted an experiment
that revealed there are two main types of flow
in a pipe: laminar and turbulent. This experiment was so influential that the
device that he used to conduct it was still used
well into the 21st century. In his experiment, Reynolds used a colored
fluid with the same density of water, and injected a very thin stream of it into a large
transparent tube with water flowing through it. When he injected the dye into slow-moving
water, the flow of the dye maintained its place
and pattern in the center of the water. We call this laminar flow. But when Reynolds injected the dye into water that
was moving fast, the dye spread out and diffused,
mixing with the water and coloring it. This is called turbulent flow. Those were the two main types of flow that
Reynolds found, but we also have something
called transitional flow. Transitional flow is a mixture of laminar
and turbulent flow. It often has turbulence in the center of the
pipe, and laminar flow near the edges. Reynolds’ experiment allowed him to
determine when the transition would occur
from laminar to turbulent, giving us the quantity we now know as
the Reynolds number. We can find the Reynolds number for the
flow of a fluid in a pipe by taking the diameter of the pipe and multiplying it
by the velocity of the fluid and the density of the fluid,
then dividing all of that by the viscosity of the fluid. Now, the value we get for our Reynolds number will be
dimensionless, meaning there are no units attached to it,
but it can tell us a lot about the movement of a fluid. It lets us know how predictable, or chaotic,
our fluid flow will be. That’s because we can look at the Reynolds
number as a ratio of inertial forces to viscous forces. Inertial forces represent the driving kinetic movement
of the fluid, which result in chaotic flow movement, like
the swirling motion of eddies and vortices. Viscous forces represent resistance to flow and
are more likely to provide slow, steady motion. So the more powerful your viscous forces are,
the slower and more controlled the fluid’s motion
will be. The higher your inertial forces are, the more
chaotic your flow will be. Therefore, a low Reynolds number
represents laminar flow, while a high Reynolds
number represents turbulent flow. In a pipe, laminar flows will usually have a
Reynolds number lower than 2100, while turbulent
flows will usually be higher than 4000. And a Reynolds number between these two values
typically represents transitional flow. Understanding fluid mechanics allows us
to use and apply equipment for fluid flow, like
pumps and pipes. And we can also apply this knowledge so
we can better understand how things move
inside a fluid. Like that new car we’re designing. If we’re testing our new car in a wind tunnel, we’ll
want to see how the air around the car is moving
and design our vehicle to account for the flow. The more resistance our cars have from the
air, the harder it is to go faster and the more fuel
they’ll need to use to travel at a given speed. This is why understanding fluid flow is so
important. So remember, it’s not just the system that
matters, but also the surroundings. Today was all about fluid mechanics. We talked about the different scales that
we work with as engineers as well as mass
and energy transfers. We learned about the no-slip condition and
the different types of stress and strain that
we’ll encounter. Newton’s law of viscosity gives us a way to
describe fluid movement, and Reynold’s number helps in determining the
difference between laminar and turbulent flow. I’ll see you next time, when we’ll learn
even more about fluid flow and some of the
equipment behind momentum transfer. Crash Course Engineering is produced in association
with PBS Digital Studios. You can head over to their channel to check out a
playlist of their amazing shows, like The Art Assignment,
The Origin of Everything, and Physics Girl. Crash Course is a Complexly production and this
episode was filmed in the Doctor Cheryl C. Kinney
Studio with the help of these wonderful people. And our amazing graphics team is Thought Cafe.

About the author


  1. Me, an almost 7th grader: Mmhmmm yep. I actually understand some of this. This is actually pretty interesting.

  2. A good transport phenomena analysis course covers all these topic in differential terms. Interesting stuff. William m deen wrote a very book on that…

  3. meaning that as the bottom plate is moving minuscule friction is transferred through the Air from the statistics nary plate to the moving one

  4. I'm starting my fall semester in 10 days and I'm taking fluid mechanics too. thank you so much I'm actually excited to learn about fluids now lol

  5. I'd like to traslate these amazing series into Spanish since It's my mother tongue and I am really sure that some people around me or just that don't speak English would find them so helpful and interesting. Would it be possible drop the link here for that porpouse?

  6. I am taking Fluid Dynamics next Semester for my Engineering Studies. Thanks for the first insight, Shini & CC-Team 🙂

  7. Ansys Fluent CFD is an interesting and helpful program for fluid mechanics/dynamics. Kind of wish they would talk about problems like cavitation or water hammer/hydraulic shock, unless that is in the next part…

  8. My first time watching— thank you, crash course, for giving a platform to a woman of color in STEM. You’re being the change we need in the world.

  9. CrashCourse, Are you able to make your content available on podcasts? I would love to listen to your videos while I work

  10. The quicksand graphic is false due to buoyancy. Please do more vetting of the graphics, and have them work with physics people more.

  11. In microfluidics, the flow is always laminar because the diameter is tiny (less than a millimeter), so when you pump in two different fluids, they don't mix easily. If you look up "microfluidic chip", you'll see that most of them have a zig-zag shape. That increases the length of the channel so the fluids can diffuse into each other and mix evenly along the way (although there are other reasons why you would want to make the channel longer).

  12. thank you for help.. i made it through the examination theory part.. all with imagination and your examples😁😁 no puns?😂😂

  13. Very nice lesson! Has anyone noticed how amazing and (scuse me) beautiful Shini is? 10/10 would take out for coffee and discuss research projects

  14. Best CrashCourse ever… EVER!!!!
    I love it…
    I think it is easier to understand stress and strain in the context of beam theory. And then defined a fluid as a solid not able to carry stress in tension. But this works too…
    I am looking forward to the FE and CFD videos it is going to be amassing.

  15. Even though quicksand becomes less viscous when you touch it, your body is less dense than quicksand. While you can't stand on it, nor will you sink. You will float, kind of like a boat on water. I don't believe there are any recorded fatalities due to quicksand.

  16. In the UK we don't graduate from college, we graduate from university. College is like an alternative for school, for A-Levels and other qualifications before you'd go to uni 😛 Must have felt weird for Dr. Shini to say that 😛

  17. Crashcourse could vastly improve their view count if they stopped this identity politics garbage and let someone host the show who is actually charismatic and entertaining to watch rather than someone who lets them feel good about themselves because they're fulfilling their PC quota.

  18. Great video, thank you. Just a small point I think might need correcting: In the diagram accompanying the normal strain equation, I think the subscript of the total length (on the top) should be T for total. This would differentiate it from the shear strain LF.

  19. I'm taking biomedical transport phenomenon and it is basically fluid dynamics.. I can assure you all it is not fun. Thanks crash course for breaking this down for others.

  20. You can either watch these videos for overview skim which actually makes your sch/uni/coll lesson easier or you have already learnt these at sch/uni/coll then come and watch for better understanding. Both actually worked. I have already tried with chemistry, physics, biology, anatomy and physiology.

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