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Science, Maths & Technology

Blowing in the wind

Updated Wednesday 8th July 2009

There's more to wind tunnels than wind, as Ian Johnston explains with a visit to Thurleigh testing centre

Thurleigh testing centre - exterior view Copyrighted image Icon Copyright: BBC The exterior One of the most important tools for aerodynamics research is the wind tunnel - in fact, the first really good wind tunnel was built by the Wright brothers and was crucial to their success. The idea is quite simple - instead of moving a wing, fuselage, plane or other object through the air, you use a fan to blow the air past it.

Well, simple in theory. In practice, it all gets rather complicated, and needs some pretty impressive engineering. The main wind tunnel test section at Thurleigh was eight feet in diameter and could run up to Mach 2.2. That's 2.2 times the speed of sound: a whopping 1700mph. To get the air moving that fast requires a colossal amount of power: 80,000hp, to be precise. That's twelve times the traction power of a Pendolino train.

More problems start as soon as the air has left the fan. Getting air to move requires a pressure - that's what fans do - and compressing air makes it hot. Very hot. That's bad news for the model under test, and potentially bad science too, if you're trying to study normal atmospheric conditions. So, after the fans came a huge heat exchanger, pulling a fair proportion of those 80,0000hp out of the air again and losing them in a cooling tower.

After the heat exchanger came a series of baffles and guides to remove turbulence from the air and direct it as smoothly as possible through the test Inside the Bang windtunnel Copyrighted image Icon Copyright: BBC The building interior section and over the model being tested. As it leaves this area it's still going very fast, so, to avoid wasting all that nice kinetic energy, the Thurleigh wind tunnel, like most other large scale ones, sent the same air round again.

Side view of fan Copyrighted image Icon Copyright: BBC Side view of fan

That's the point of the circular hole in the side wall. The fans filled the whole of the end space, where the set is now, and the circular hole in the end wall led on towards the heat exchangers.

The support infrastructure for all this was huge and complicated. The main drive motor was a synchronous one - that means that it always went round at exactly one speed, set by the supply frequency. To allow operation at different speeds, Thurleigh had two 25MW variable frequency generating sets - a small power station of its own. Setting the output of them set the speed of the drive motor and hence the air speed in the wind tunnel. For operation at full blast, the on-site generators could be synchronized to the national grid, allowing the motor to run on a mixture of home-made and mains electricity.

Just starting the synchronous motor was a major undertaking as well - that took another two 14,000hp DC motors and a large motor generator set to convert AC mains into DC for those.

Dials and switches Copyrighted image Icon Copyright: BBC Banks of switches

The electrical control room for all of this is still in place and more or less intact. Nowadays much or most of it would be done electronically and controlled from a PC, but in the 1960s electromechanical engineering ruled supreme and the control room has a wall  of switches, dials and beautifully complicated, Swiss-made automatic controllers for the generating sets.

There is another control room too, dedicated to controlling the oil supply to the system. An 80,000hp electric motor is a heavy lump of metal, and with two 14,000hp motors to start it and all the fans at the other end, the shaft bearings were huge. The supply of oil to these was absolutely crucial, so another wall of instruments allowed operators to monitor and control the oil temperature and pressure in all the bearings. That would be done on a PC too, nowadays, but sixty feet of bakelite and gauges is somehow more satisfying: there is little of the romance of engineering in a digital display.

Dials showing oil supply instruments. Copyrighted image Icon Copyright: BBC Oil supply instrument controls

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