Low-speed wind tunnels are used for operations at very low Mach number, with speeds in the test section up to 480 km/h (~ 134 m/s, M = 0.4). They may be of open-return type (also known as the Eiffel type, see figure), or closed-return flow (also known as the Prandtl type, see figure) with air moved by a propulsion system usually consisting of large axial fans that increase the dynamic pressure to overcome the viscous losses.
