Tunnel Vision

In the latest in our racecar design 101 series we take a detailed look at the fundamentals of wind tunnel aero development By

- Ricardo Divila

It won’t come as a surprise to most to hear that wind tunnels were initially developed for developing aircraft. But when road car speeds increased, this naturally brought the use of these same tunnels into the automotive world. Either full-scale or model aeronautic facilities were then used for decades, but the proximity to the ground caused some problems peculiar to cars. We will discuss this later, but first let us examine the wind tunnel itself.

Wind tunnels fall into two fundamental types, open and closed. The open or Eiffel type – made by Gustave Eiffel, of Parisian tower fame – certainly have their advantages. For a start, they are cheaper to build, while the pollutants – smoke, flow viz paints or fumes if an engine is running – are easily purged. On the other hand, there are also disadvantages: for instance, the size of the tunnel must be compatible to the size of the room it is housed in, as the room is the return path for the air. They tend to be noisy, and more expensive to run than closed tunnels.

The closed, or Gottingen, type of tunnel is where the air runs in a closed loop. The advantages are that they are cheaper to run and the quality of the flow can be easily controlled, while they are less noisy than the open type. On the other hand, they are more expensive to build, while the continuous losses of energy in the tunnel heats up the air, so the air may need cooling, especially in summer. Also, as the air recirculates, pollutants are not easy to purge.

Wind scale

These basic types can be of varying dimensions, depending on the type of work required, but we can use a scaled-down model of whatever we are testing, again because of developments in the aeronautics world, though this will mean issues with scaling (see boxout). Incidentally, talking of scale, some tunnels, like the Nasa Langley facility, can house a Boeing 737.