# Science: It's Always a Fluid Situation

## Article excerpt

EVERY TWO weeks a 500-page journal covering the field of fluid mechanics lands on Keith Moffatt's desk. He has, for the past five years, been the director of the Isaac Newton Institute for Mathematical Sciences in Cambridge. Here, mathematical discussions are apparently ceaseless and must be seen to be happening, as there are blackboards everywhere, not only in the lift but in the toilets of both sexes. Mathematical problems, Moffatt assured me, will go on forever, new ones arising as the old ones are solved.

But why on earth, I wondered, should a mathematician such as he is care about something that sounds as old-fashioned as fluid mechanics? How wrong I was. The movement of fluids - which unlike solids are infinitely deformable - plays a central role in all our lives and presents some very difficult mathematical and physical problems.

While Leonardo da Vinci made some wonderful drawings of water showing eddies and turbulence, these were observations without theory. It was in the 18th century that Leonhard Euler, with undoubted genius, first derived the equations of fluid motion that predicted, for instance, how fast water should flow from a vessel of a given depth. But there was something seriously wrong with his idealised equations - they ignored viscosity, or internal friction. Then in 1824, Claude Navier, a Frenchman, and Sir George Stokes, an Irishman, put viscosity into their equations and thus laid the cornerstone of all modern fluid dynamics.

The flow of fluids dominates our lives. The Earth's atmosphere is a huge gaseous fluid whose motion is driven by temperature differences and the rotation of the Earth. Glaciers, and even the Earth's mantle, are fluid.

Fluid mechanics is also fundamental to the design of aeroplanes. As long ago as 1738 Daniel Bernoulli, who was born into a family of mathematicians, understood that air flowing over a curved surface, such as the wing of an airplane would be, caused lift. It is also crucial in the design of ships, submarines, harbours and waves, and in the mixing of chemicals. All these are governed by the same laws. Also, since human beings are about 90 per cent liquid, so too are the flows through our vessels and cells.

But all these systems and the calculations related to them can be upset by the devil of fluid dynamics - turbulence. …