Crazy pool vortex


[SQUEAKING] [POP] Check this out. I’m just gonna drag this plate
through the water in the pool. That’s it. But look what forms–
perfect black circles on the bottom of the pool. And it’s so weird. They don’t die. They just keep going
right next to each other. I sped this footage
up because the circles took 3 minutes to cross
the pool and didn’t seem to be dying out. These got sharper as they went
and made it all the way across. So cool. And look closer. You can see what’s causing them. There are two almost
unnoticeable dips in the surface of the water. Why don’t they die out? And how do these
dark spots form? And why do the
dips move together? So many questions. Let’s first look
at what they are. When I dragged the
plate through the water, you could see it
forming little vortices, like you get when you move
a paddle through the water. A vortex is a gas
or a liquid moving in circles with what we
call a vortex line going up through the circles. They are everywhere–
tornadoes, water flowing down the drain, hurricanes,
or the movement of air over an airplane wing. Vortices often form when there’s
a difference in fluid speed, like fast wind moving
over slow wind. As you move the plate
forward, the water right next to the boundary
moves with the plate because of friction. The water further
out is not moving. It’s that difference in velocity
that causes a shear force and starts the water spinning. This is accentuated by the water
in front of the plate being forced around the
sides and the water behind the plate
being sucked forward as the plate moves along. In this coherent
vortex, our dips can travel much further
across the pool than a ripple without dissipating. Why? The vortex wants to keep
its angular momentum. So it continues spinning as
it moves through the water and only loses some
energy through friction to the surrounding
stationary water. So what we appear to
have here is two vortices moving across the pool. But it seems weird that
they stay right together. Well, when we
create this vortex, we’re forcing water to move
around the whole plate, not just on the
surface of the water. Our vortex line is curved. We’re creating a
half vortex ring. You know, like a smoke ring,
bubble ring, or even the plume rising up from an
explosion, but half of that. These two dips on
the water’s surface are just the ends
of the half ring. They’re connected. And that’s why
they stay together. We can see this if we put
a little food coloring into the ends. The food coloring travels
from one end to the other. And you can see the
entire half ring. So cool. We just made a bubble
ring without the bubble. This also tells you
something interesting. The half ring is not a wave. A wave is a disturbance
moving through a medium that transfers energy but
doesn’t transfer matter. When we added food
coloring, we saw clearly that the vortex is
transferring matter, just like a smoke ring does. So sadly, it’s not a wave. It’s cooler than a wave. Now back to this
half-ring vortex. Isn’t it weird? You’ve probably never seen
half of a smoke ring before. Why? Well, according to
Helmholtz’s law, a vortex line cannot
start or end in a fluid. So a vortex line either
wants to close into a loop or end at a boundary. If you tried to cut
a smoke ring in half, it would either break
up or, even cooler, it would break up into
two smaller smoke rings. I don’t know if that happens. It would be awesome. Since the line can’t end in the
fluid, it ends at the surface. You almost never see
these half vortices though because
they have to occur on the boundary of two media. If you can think of another
way to make the entire half ring visible, like a smoke ring
or a bubble ring, let me know. So now, why does our vortex
create those cool dark spots? Well, the balance between
gravity, centripetal forces, and surface tension
creates a funnel shape which acts like a lens. Lenses often work by
focusing light to a point, like this Fresnel lens, which
can melt pennies in the sun. But I digress. Our vortex lenses focus the
sunlight away from the center, like so. And you have a bright ring
surrounding a dark circle. The ring highlights the
dark circle further. And you get these incredibly
clear black spots. So cool. Now I highly recommend
trying this at home, especially if it’s too
cold to swim in the pool. Here are some tips. The water has to be very still. Even a small breeze makes
it harder to create these. The sun has to be
out, or it won’t work. And you can try some
other really cool things. Send your vortices at a wall. You’ll see the dark
spots spread out. Or try colliding two of them. Happy physics-ing. [THEME MUSIC]

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