Subsonic tunnel
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) (Barlow, Rae, Pope; 1999). They may be of open-return type (also known as the
Eiffel type, ), or closed-return flow (also known as the
Prandtl type, ) with air moved by a propulsion system usually consisting of large axial fans that increase the dynamic pressure to overcome the
viscous losses.