Turbulence: All Mixed Up
Turbulence is disordered motion of a fluid. What does disordered mean? Well, as some scientists are fond of saying, "it's hard to define but you'll know it when you see it" (the same can be said for alot of other things too, including life itself).
You can get a good idea of what turbulence is by thinking about what all turbulent flows have in common:
A spinning column of fluid is called a vortex (plural vortices) and since turbulence is loaded with vortices we say it possesses vorticity. This is especially true if the object itself is spinning like a planet or star in which case vorticity is generated by the Coriolis effect. Examples of vortices include tornadoes and hurricanes on Earth, dust devils on Mars, and the Great Red Spot on Jupiter. Even spiral galaxies are vorticies.
- A wide range of
Scales here means different sizes and time intervals. For example, Jupiter's atmosphere has vortices more than 100,000 km (60,000 miles) wide which encircle the entire planet, vortices less that 1 km (0.6 miles) wide which are no bigger than a tornado, and everything in between, including the Great Red Spot, which is about 15,000 km (10,000 miles) across.
- Unpredictability (chaos)
Turbulence is unpredictable in the sense that two turbulent flows which look almost exactly the same at one instant may look completely different a short time later. This is sometimes called the butterfly effect and it's the reason why we have to use statistics to describe turbulence. For example, scientists can predict the probable path of a hurricane but they can never be 100% certain.
- Efficient mixing and dissipation
Turbulent fluids are good at stirring; if you put milk in your tea turbulence will mix it up far more rapidly than if you put milk in a cup of molasses. Turbulence also is good at dissipating energy - this is why cars and airplanes are designed to be as streamlined as possible. Streamlined shapes reduce turbulence and therefore reduce drag.
In astronomical objects such as planets and stars turbulence is usually caused by buoyancy (warm fluid rises and cool fluid sinks due to gravity) or shear (winds or currents going in different directions).