Science Ouroboros: Vortex mania

Posted on November 30, 2010

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Important physics stuff going on here.

There’s some difficult science coming up, so pay attention.

You know how when you flush a toilet there’s that spinning, swirly part in the middle?

That’s a vortex.

Vortexes (or as they’re formally known: “vortices”) are a part of the fluid dynamics affecting liquids and gases. In a vortex, the mass at the top moves to bottom of a column in a tight central spiral, while the mass from the bottom moves to the top up the edges. This is why people are pulled into the centre of a whirlpool while motor homes are drawn up along the sides of tornadoes.

It’s also why we stir a hot cup of coffee — it mixes the cooler liquid from the top with the hotter liquid at the bottom.

A vortex is caused by forces moving in opposite directions along the sides.

One way to cause a vortex is to put liquid in a cylinder, then spin the cylinder. If you spin the cylinder slowly, nothing much happens. If you spin it faster, a vortex forms. And the faster you spin the cylinder, the wider and deeper the vortex.

The "Pet Tornado" by Tedco. Only $4.95 at Physlink.com!

There’s a toy you can get. The “Pet Tornado.”

It’s only $4.95 and is pretty safe. They recommend it for ages five and up. It’s filled with a viscous liquid and suspended particles. By turning the container quickly with your wrist you can make a miniature tornado inside.

(Trailer park not included.)

So why am I telling you this?

Because of a recent press release from the University of California — Santa Barbara.

Public release date: 29-Nov-2010
Contact: Gail Gallessich
gail.g@ia.ucsb.edu
805-893-7220
University of California – Santa Barbara

Tempest in a teapot: International team of scientists describes swirling natural phenomena

(Santa Barbara, Calif.) –– Scientists can use cylinders as small as teapots to study the mechanisms involved in powerful hurricanes and other swirling natural phenomena.

The earth’s atmosphere and its molten outer core have one thing in common: Both contain powerful, swirling vortices. While in the atmosphere these vortices include cyclones and hurricanes, in the outer core they are essential for the formation of the earth’s magnetic field. These phenomena in earth’s interior and its atmosphere are both governed by the same natural mechanisms, according to experimental physicists at UC Santa Barbara working with a computation team in the Netherlands.

Using laboratory cylinders from 4 to 40 inches high, the team studied these underlying physical processes. The results are published in the journal Physical Review Letters.

This team is being led by Guenter Ahlers, professor of physics at UCSB, and Stephan Weiss, a postdoctoral fellow at UCSB. What they’ve done, according to their press release, is “filled the laboratory cylinders with water, and heated the water from below and cooled it from above.’

Are you with me so far?

I know — it’s complicated. But in these modern times it’s important to keep up with the cutting edge of science.

And who knows? This information  may even save your motor home some day.

Now the hot water at the bottom travels towards the top, pushing the colder water down in what is called a “convection current.” However, when they spin the container, something incredible happens.

Ahlers and his team discovered a new unexpected phenomenon that was not known before for turbulent flows like this. When spinning the container slowly enough, no vortices occurred at first. But, at a certain critical rotation speed, the flow structure changed. Vortices then occurred inside the flow and the warm fluid was transported faster from the bottom to the top than at lower rotation rates. “It is remarkable that this point exists,” Ahlers said. “You must rotate at a certain speed to get to this critical point.”

You see? When you spin the cylinder slowly, no vortices form. When you spin it faster, you get vortices. This truly is a “new and unexpected phenomenon.”

But wait — there’s more.

Further, it was found that vortices do not exist very close to the sidewall of the cylinder. Instead they always stayed a certain distance away from it. That characteristic distance is called the “healing length.”

To translate this into layman’s terms, the vortex itself forms in the middle — not the side.

Got it?

Good. Because I really don’t think I can add anything more to this.

Except maybe these:

 

Tornado Maker by Slinky.
Make your own tornado in a bottle.
Make your own tornado in a ball.
Jackie — a party girl who makes her own tornadoes.

WARNING: Keep all these out of the reach of theoretical physicists. (Especially Jackie.)

Related post: CSI MIdwest

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