Types of Wind Tunnels

The first wind tunnels were open-ended tubes through which air was blown. These were soon replaced by wind tunnels that blew the same air around in an endless loop. This layout is called a closed-circuit wind tunnel. The temperature, humidity (amount of moisture), and speed of the air can be better controlled in a closed – circuit tunnel.

Open wind tunnels still are used for some tests. If a working engine was being tested in a wind tunnel, a closed circuit tunnel would recirculate the engine exhaust gases, and that would affect the engine’s performance and thrust. In such cases, an open wind tun­nel is used. Fresh air is taken in, acceler­ated through the test section once, and then expelled along with the engine exhaust gases.

Wind tunnels also can be classi­fied according to the airspeed in their test section. There are subsonic tunnels, with airspeeds less than the speed of sound, and transonic tunnels, in which the air travels at about the speed of sound. Supersonic tunnels have airspeeds greater than the speed of sound, while hypersonic tunnels are for tests at more than five times the speed of sound.