Subsonic wind tunnels are used for operations at low Mach numbers, 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) or closed-return flow (also known as the Prandtl type). These tunnels use large axial fans to move air and increase dynamic pressure, overcoming viscous losses. The design principles of subsonic wind tunnels are based on the continuity equation and Bernoulli's principle, which allow for the calculation of important parameters such as the tunnel's contraction ratio.
Transonic wind tunnels (0.75 < M < 1.2) are designed on similar principles as subsonic tunnels but present additional challenges, primarily due to the reflection of shock waves from the walls of the test section. To mitigate this, perforated or slotted walls are used to reduce shock reflection. In transonic testing, both Mach number and Reynolds number are critical and must be properly simulated. This often necessitates the use of large-scale facilities and/or pressurized or cryogenic wind tunnels. These tunnels are crucial for studying aerodynamic properties of objects at speeds approaching and surpassing the speed of sound, such as high-speed aircraft and spacecraft during critical phases of flight.