Kind: captions
Language: en
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 u.s.
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