The wind tunnel

10.1 UNDERSTANDING THE WIND TUNNEL

The basic idea of a wind tunnel is easy to grasp. The forces on a wing in flight may be exactly imitated if the wing is held fixed and an airstream blows over it at an equivalent speed. To make a very simple wind tunnel is easy and has been undertaken as a project in schools. A fan draws the air through a duct A section of the duct is fitted with removable panels for access to allow models of wings or other components to be mounted safely in the flow. Simple spring balances can be used to measure forces, and probes connected to pressure manometers can be moved by hand to investigate flow speeds, etc. Much can be learned from the simplest such tunnels but to make accurate measurements is difficult For work at model aircraft speeds and sizes, it is particularly vital to keep the flow in the test section of the tunnel as free as possible from turbulence. This requires not only flow straighteners in the tunnel but diffusers and fine mesh grids, or even screens of fabric through which the air is drawn. These reduce turbulence to such fine dimensions that natural damping tends to reduce the small disturbances in the flow very quickly. In addition, the flow after passing through the screens enters a carefully designed contraction in the tunnel before the test section. The contraction has a venturi effect (see Figure 2.7), speeding the flow up while at the same time narrowing the stream. This further reduces turbulence, since any remaining small lateral oscillations in the flow become stretched out longitudinally and restricted laterally.

After the test section has been passed, further flow straighteners are usually fitted and, since the fan rotates, the shape of the tunnel in cross section has to be changed to circular from rectangular or square. This change has to be fairly gradual since it is easy for disturbances in the flow downstream to make themselves felt in the test section.

Because of the effects of sound on the boundary layer (see 8.12), the noise of the fan blades and the fan motor itself must be suppressed and vibrations must be prevented from disturbing the measuring instruments.

Tunnels of the open return type, in which the air after passing the fan is allowed to escape into the laboratory building or even to open atmosphere, with new air constantly drawn in through the screens at the other end, are often affected by external weather, especially wind which can cause fluctuation in the flow speed through the tunnel. Such tunnels may be sited in sheltered places, such as wood or forest lands, to shield them. The closed return type of tunnel is less subject to weather but because the same air is re­circulated to the intakes after passing the fan, additional precautions are needed to prevent vortices from the fan blades persisting all the way through the tunnel. Figure 10.1 shows in schematic fashion the layout of a very good modem wind tunnel.