Subsonic and transonic wind 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, 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.

The working principle is based on the continuity and Bernoulli's equation:

The continuity equation is given by:

${\displaystyle AV=constant\Rightarrow {\frac {dA}{A}}=-{\frac {dV}{V}}}$

The Bernoulli equation states:

${\displaystyle P_{total}=P_{static}+P_{dynamic}=P_{s}+{\frac {1}{2}}\rho V^{2}}$

...
Wikipedia