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Wind Speed

High-speed tunnels are those having a Mach number approaching or exceeding one—the sonic velocity. The flow in such tunnels is considered “compressible,” which means the density of air changes significantly from place to place in the tunnel or around test objects. In low-speed tunnels (Mach number less than one-third), the density of air is essentially constant everywhere, and the flow can be considered “incompressible.”

Wind tunnels are commonly referred to as low-speed tunnels if the Mach number is less than one-third which under atmospheric condition corresponds to a speed of 110 m/s (250 mph) approximately. They are referred to as high-speed tunnels if the Mach number is greater than one-third, as supersonic tunnels if the Mach number is greater than 1.0, and as hypersonic tunnels if the Mach number is greater than 3.0.


  

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