Wind tunnels provide controlled air streams to study aerodynamic and fluid flow phenomena. They are frequently used to replicate the actions of an object flying or moving in an air stream, such as airplanes or vehicles. Table 7.1 lists several types of wind tunnels. Most wind tunnels are designed to have a uniform velocity profile in the test section at a low turbulence level. The testing object is placed in the test section for flow visualization and quantitative measurements such as lift and drag forces. To create an air flow, electric fans are usually used to blow air in or out of the tunnel. In large wind tunnels for testing real-size airplanes, rockets, or cars, powerful fans are required to generate a wind speed comparable with real conditions.