Let us first define what a flow is: a flow is the continuous movement of a fluid, i.e. either a liquid or a gas, from one place to another. Basically there exist two types of flows, namely laminar flows and turbulent flows. Roughly speaking we can say that a laminar flow is a 'simple' flow while a turbulent flow is a 'complicated' flow. We will illustrate what we mean by 'simple' and 'complicated' using the following, simple experiment. Go to your kitchen sink and open the faucet. The stream of water that emerges from your faucet is very smooth and very regular. The flow of water is smooth because all the water molecules move, at more or less the same speed, in the same direction. This is called a laminar flow. Furthermore, if you did not open the faucet too much, the water will also flow down the drain in a laminar flow. Now place a cup under the stream of water emerging from the faucet. Although the stream is still laminar, the flow pattern of the water in the sink has become very complicated. This is due to the fact that now the water molecules tend to move in different directions at different speeds. Such a flow is called turbulent.

Another example of turbulence you can easily observe at home is the flow of water in a boiling pot of water. Put water in a pot and heat it up on your electric cooker. If you wait for a short while, the water will start to move in a laminar fashion, i.e. in a very regular way. If you wait a bit longer bubbles will start to rise from the bottom to the surface and the motions of the water become very complicated or turbulent. In this particular case the turbulence is due to convection. In order to see how complicated the turbulent motions are look at the picture of the boiling kettle.

Laminar Flow	Turbulent Flow	Boiling kettle>

Now we are able to define the terms laminar and turbulent in a more precise fashion. In a laminar flow all the molecules in the fluid move more or less smoothly in the same direction and at the same speed. In a turbulent flow, however, the molecules in a fluid move in many different directions and at many different speeds.

Turbulent motions are very common in Nature. Turbulence occurs nearly everywhere: in the oceans, in the atmosphere, in rivers, even in stars and galaxies. In fact it is easier to find a turbulent flow than a flow that is really laminar. Here are a few examples of turbulent flows:

1. The wake of a ship or submarine is turbulent.
2. The swirls and eddies in a fast flowing river are turbulent.
3. The air currents in the atmosphere are turbulent.
4. Turbulence was observed in the ash plume of the Mt. Spurr volcano as it erupts
5. The outer layer of the Sun, i.e. the convection zone, is highly turbulent.

Wake of submarine	Rapids	Air currents in the atmosphere	Eruption of the Mt.Spurr near Anchorage in 1992	The Sun.

How easily a fluid becomes turbulent depends to a large extend on its viscosity. Simply speaking, viscosity is the resistance of a fluid (i.e. either a liquid or a gas) to movement. The more viscous a fluid is the less likely it is to become turbulent. Thus, water or air which have a low viscosity can become turbulent relative easily, while honey or syrup, which are very viscous, tend not to become turbulent.

1. Heating: If you heat a fluid at the bottom and cool it at the top the fluid becomes turbulent due to convection.. This is what happens in a boiling pot of water.
2. Pressure: The water stream that emerges from the faucet in the picture above is laminar. This is because the faucet is not fully open and the pressure in the pipe is fairly low. If you open the faucet to its full extend the water will shoot out in a very wild manner. When the faucet is fully open the pressure in the pipe is very large.

1. Turbulence occurs if a heavy fluid, let us say water, is lying on top of a lighter fluid, like oil. This gives rise to Rayleigh-Taylor instabilities which lead to turbulence. For a computer generated movie of turbulence generated by Rayleigh-Taylor type instabilities click here. In this movie blue represents the heavy fluid, while red represents the light fluid.
2. Turbulence through Kelvin-Helmholtz instabilities occurs at the interface between two fluids flowing in opposite directions. This happens for example in the atmosphere, when different air masses flow over each other at different speeds. They often result in very beautiful cloud formations. Another example is the wind flowing over the oceans. For a computer generated movie of a Kelvin-Helmholtz instability click here.

Computer generated picture of a Rayleigh-Taylor instability	Computer generated picture of a Kelvin-Helmholtz instability	Formation of clouds generated by Kelvin-Helmholtz instability.

Note, that there exist many more ways in which a liquid can become turbulent.

Turbulence is a very complex physical phenomena and even today we do not fully understand how turbulence works. Richard Feynman, a Nobel Prize-winning physicist, once said that turbulence is the most important unsolved problem of classical physics.

The first person that described and studied turbulence was probably Leonardo da Vinci (1452-1519). His sketches of turbulence can still be seen today. Since then many famous scientists have worked on the problem of turbulence like for example Leonhard Euler (1707-1783), Hermann Ludwig Ferdinand von Helmholtz (1821-1894), William Thompson (Lord Kelvin) (1824-1907), Lord Rayleigh (1842-1919) and Andrey Nikolayevich Kolmogorov (1903-1987) to name but a few.

Today, many problems related to turbulence remain unsolved and research in turbulence is very active. Outstanding problems related to turbulence are:

1. Turbulent diffusion/Turbulent Magnetic Diffusion
2. Turbulent transport of energy and angular momentum
3. MHD Turbulence. Researchers investigating MHD (magneto-hydrodynamic) turbulence investigate what happens to the turbulence in the presence of magnetic fields