A wind tunnel can be an open- or closed-loop system. In an open-loop wind tunnel, the intake and exhaust ends of the tunnel are not connected, but this isn't very economical. Today, most wind tunnels are closed-loop.

A schematic of a closed-loop tunnel.

It doesn't take a rocket scientist to see that fast-moving air coming out of the tunnel can be recirculated into the intake end of the tunnel to help boost total airspeed. This is how a closed-loop tunnel works. Special vanes are used to turn the airflow around the corners of the tunnel while minimizing turbulence and power loss. Less loss means more speed for your dollar!