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Subsonic and transonic wind tunnel
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<p>Transonic wind tunnels, between Mach 0.75 and Mach 1.2 (920 and 1,500 km/h; 570 and 910 mph; 260 and 410 m/s), are designed on similar principles as subsonic tunnels but present additional challenges, primarily due to the reflection of <a href="/facts/Shock_wave/CqrG1Xm5">shock waves</a> from the walls of the test section. To mitigate this, perforated or slotted walls are used to reduce shock reflection. In transonic testing, both <a href="/facts/Mach_number/Ee01MIgw">Mach number</a> and <a href="/facts/Reynolds_number/4sdungsf">Reynolds number</a> are critical and must be properly simulated. This often necessitates the use of large-scale facilities and pressurized or cryogenic wind tunnels. These tunnels are crucial for studying aerodynamic properties of objects at speeds approaching and surpassing the <a href="/facts/Speed_of_sound/dYtbt1nf">speed of sound</a>, such as high-speed aircraft and spacecraft during critical phases of flight. </p>