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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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