Wind tunnels are usually designed for a specific purpose and speed range. There are special tunnels for propulsion, icing research, subsonic, supersonic, and hypersonic. A wind tunnel may be open and draw air from outside the tunnel into the test section and then exhaust back to the outside, or the tunnel may be closed with the air recirculating inside the tunnel. The tunnel in the figure is a closed tunnel which we are viewing from above. The air inside the tunnel is made to move by the fan on the far side of the tunnel. In this figure, air continuously moves counter-clockwise around the circuit, passing over the model that is mounted in the test section.