Incompressible Aerodynamics
An incompressible flow is characterized by a constant density despite flowing over surfaces or inside ducts. While all real fluids are compressible, a flow problem is often considered incompressible if the density changes in the problem have a small effect on the outputs of interest. This is more likely to be true when the flow speeds are significantly lower than the speed of sound. For higher speeds, the flow will compress more significantly as it comes into contact with surfaces and slows. The Mach number is used to evaluate whether the incompressibility can be assumed or the flow must be solved as compressible.
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