Monday 18 June 2012

Science: Toroidal Vortex

Toroidal Vortices are a phenomenon.  Volcanoes make then, dolphins make them but how and why they are formed in this way we are not yet sure of.  If you would like to try to make one yourself here is how it's done...


Fill a tall glass to the brim with water.  Wait a minute because even though the water looks still it will keep swirling for ages, so the longer you wait the better this will work.


Squeeze gently and using a pipette so a drop of food colouring is just dangling on the tip of the dropper is hanging there then let it touch the water.
You will instantly notice that the drop of food colouring shoots down into the water and turns into a tiny little ring!  If you look very closely you'll see that the ring is swirling in on itself as it goes down the glass.  While it moves quickly it will keep it's shape.
When it's about half way down the glass it slows down.  The ring suddenly becomes unstable and begins to break up (this is called a vortex breakdown).  Depending on how still the beginning you may find it form a wonderful inverted crown.  The tips of the crown are even smaller vortex rings.

What's going on?

A vortex like this is formed when a fluid swirls around a central point. Whirlpools and tornadoes are two types of funnel shaped vortices you probably know well.

Why a toroidal vortex? Well, the mathematical name for a ring shape is a toroid. Hence why we say we’ve just created a miniature toroidal vortex!  Isn’t that super cool science?

Toroidal vortices form due to a complicated combination of friction and pressure. It’s the friction between the drop of food colour and the water that is what slows its descent.  However, the friction is not equal on all parts of the drop. The outer part of the ring (the toroid) has a sideways frictional force but the bottom of the drop meets the water head on and more friction it put on that part of the drop.  All at the same time, the little drop leaves a little bit of lower pressure behind it. The difference in pressure and friction is what changes the form of the drop into the swirling ring shape.

The clear water inside the coloured ring is called the vortex core. The vortex rings are quite stable while it is dropping with some speed but then it slows down.  At the point when the ring slow this is when the core enlarges very suddenly and this is what causes the ring to break apart.  I read up about this a little and found out it is actually a phenomenon know as a vortex breakdown.
This is what I read: ‘Vortex breakdowns are not yet fully understood but are of particular interest to aeronautical engineers and combustion scientists. A sudden change or breakdown in the structure of vortex rings above a wing can have detrimental effects on the aircraft’s performance. On the other hand, vortex breakdowns inside combustion chambers are highly desirable because they mix air and fuel more thoroughly.
Facts about toroidal vortices
Dolphins can blow perfect air bubble vortex rings too. They watch and chase them and even use their flippers to stop them rising in what appear to be games similar to those we humans play with soap bubbles.

The biggest vortex rings on Earth are made by volcanoes!  Sometimes volcanoes emit huge ring shaped clouds of steam and gas up to 200 metres in diameter. How they are formed is another mystery but they can fly up to 1000 metres high and last more than 10 minutes.

For other experiments go to my Physics page, Chemistry page or Biology page.

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