Here at the Ri recently we've been having a lot of fun with smoke rings.
I built this giant cannon for this year's CHRISTMAS LECTURES
and I thought, why stop at smoke?
Smoke rings, fire rings, bubble rings, they are all vortex rings.
Vortex rings are formed when a fluid, which can be a liquid or a gas,
is forced through a hole.
This forces the fluid to curl back in on itself, which creates a distinctive
ring shape called a torus.
Fluid dynamics can help us understand why the vortex rings form,
which is why I've put together this plastic bag DIY cannon.
When you fire a smoke ring, you force the smoke from inside the cannon
through the relatively still air of the room.
To start with, the smoke is basically a fast-moving ball.
As it emerges through the opening, the smoke on the outside of the ball
is being slowed down because of the friction between it and the edges of the hole.
Once it leaves the cannon, there is friction at the interface
between the smoke and the air in the room.
This means that you get smoke in the center of the ball
moving faster than the smoke around the edges,
and so the smoke on the edges starts to curl around
and form a mushroom cloud.
As the smoke reaches the back of the cloud,
it's drawn into the faster moving current of air in the centre,
it's this flow pattern that eventually causes the ring to form.
The amazing thing about smoke rings
is how far they can travel without dispersing,
they hold their shape for a really long time.
The spinning flow pattern actually reduces the impact of friction between
the smoke and the air in the room.
This is similar to how it's easier to roll a ball across the floor
than it is to slide one.
This means that even though vortex rings can travel a really long way,
they don't lose much kinetic energy as they go.
So I can use an air cannon like this one to knock over unsuspecting plastic cups.
You can also blow out candles
And if you add butane, you can do this...
The same basic physics can be shown with bubbles underwater.
So here, we have our underwater bubble ring set up,
in place of our air cannon we have the water bottle you can see in here.
Underneath we've got those jars just for structural support.
To help us visualize the rings we have these bubbles
and to force the water out of the opening we have this handy bat
which we like to keep in the prep room for emergencies.
You might have seen videos of dolphins and whales making bubble rings underwater.
They somehow move their fins to create the vortex,
and exhale to create the bubbles.
A vortex ring also forms when a small mass of liquid
moves through a larger body of still liquid.
The friction at the interface between the two fluids
causes the vortex ring to form.
Think about raindrops falling on a lake.
Under each one a vortex ring is actually forming,
it's just normally we can't see it
Thank you for watching!
If you like this video don't forget to subscribe,
and now we'll leave you with some slow-mo footage
of even more vortex rings.