MODELLING: AERODYNAMICS
Turbulent times
According to the legendary physicist Richard Feynman, turbulence is the last great unsolved problem of classical physics. Even so, computers allow us to study it in great detail, but you must apply such tools with great care. Paul Schreier tries to bring some order to the world of software intended for these studies of chaos
When you study aerodynamics or indeed any fluid flow, in most cases you’re actually studying turbulence. Laminar (‘smooth’) flow is not terribly interesting in the real world, whereas turbulent flow is very common in nature and occurs nearly everywhere: in rivers, in the oceans, around our cars and airplanes, even in stars and galaxies. We’re all familiar with the difference between laminar and turbulent flow in our
daily lives. When water flows slowly out of a faucet, the stream is smooth and regular, because the water molecules move at more or less the same speed in the same direction. This is laminar flow. When you increase the flow’s speed, the stream becomes rough and irregular; the water molecules tend to move in different directions – this flow is turbulent. Similarly, when you drive your car faster, turbulence builds up behind it and
drag develops. How easily a fluid becomes turbulent depends to a large extent on its viscosity; the more viscous a fluid, the less likely it is to become turbulent. Water and air, which have a low viscosity,
can become turbulent relative easily; in contrast, oil and molten glass or metal tend not to become turbulent.
A key parameter
Another way of specifying this behaviour is with the Reynold’s number, which is a measure of the way in which a moving fluid encounters an obstacle. It is proportional to the fluid’s density and speed as well as the size of the obstacle; it is inversely proportional to the fluid’s viscosity. A small Reynolds number defines a flow in which the fluid moves slowly or has a large viscosity to keep it organised. In such a situation, the fluid can move around an obstacle smoothly with laminar flow. On the other hand, a large Reynolds number indicates a flow in which the fluid can’t get
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SCIENTIFIC COMPUTING WORLD JUNE/JULY 2010
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