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DIANNA COWERN: I made a video
recently on one of the simplest
but most mesmerizing phenomena
I've ever discovered.
You drag a plate
through the water,
creating two dips
on the surface.
And sunlight shining
through the dips
will create two
crisp black spots
on the bottom of the
pool that can persist
across the length of the pool.
But the dips in the water are
just the tip of the iceberg,
as they say.
Underneath the surface is
an entire half-ring vortex,
a.k.a., a toroidal vortex.
I had never seen
these before, so there
is so much more to explore.
And I know not everyone
else has a pool,
so I took some of your
awesome suggestions
and I'm going to try them out.
For this one I have
to get in the pool.
Ooh!
I know I look ridiculous,
but this is not warm.
You can see here,
as the food coloring
has dropped into the dips.
The best part, I think,
are the reflections
you get off of the surface.
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I couldn't see
what I was filming,
but this just looked way better
than I could have imagined.
So we tried two in sequence.
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The red one cut the
green one in half.
The red one didn't die,
but the green one kind of
disintegrated.
It was cool.
And, of course, it runs
into my camera and dies.
The vortex hit on this
end of the camera,
and then you could see the
death of the vortex just moving
along the rest of the vortex.
It almost looked like
it was unraveling, which
I'm hoping won't happen here.
We're going to put
the pole in the pool
and let the vortex pass
by so that the pole cuts
it right in the middle.
And I'm hoping that it'll
meet back on the other side
or it'll split
into two vortices.
Aw, it just kills it.
I think there's
too much turbulence
when there's an asymmetric
collision of the vortex
with an object in the water.
Unlike when you hit a
wall with the vortex,
it's a lot more symmetric and it
can keep that nice laminar flow
in the spin of the vortex.
I've been told that yellow is
not the best color for a pool.
So we're going to try blue and
red to see how the fluid mixes.
Nice.
That looks awesome.
It almost looks like the
red is engulfing the blue.
It's surprising that the
colors don't mix together.
It's almost like they stay in
their own layers of the vortex.
We tried a smaller
plate just for fun.
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Check that out.
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I was a bit surprised
by this one.
I thought the ping
pong ball would
disturb the flow of the vortex.
But it dropped right
in and shows us
how fast it's spinning, at
least close to the center.
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The motion of these vortices
is really unpredictable.
Like, sometimes they just
seem to die for no reason.
Sometimes they turn.
Sometimes they hit the wall.
Sometimes they spread out.
Sometimes they go faster.
When it's spinning too fast,
there's a lot more turbulence,
and that breaks up the vortex.
And if there are
disturbances on the surface,
that actually causes
sort of that unraveling
death of the vortex
to move across it.
But I guess the
unpredictability is
what makes fluid dynamics
so fun and beautiful.
Thanks for all of the great
ideas, and thanks for watching.
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If you've enjoyed these videos
and would like to see them
continue, head over to
www.physicsgirl.org/support.
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