Continuity Assumption
The foundation of aerodynamic prediction is the continuity assumption. In reality, gases are composed of molecules which collide with one another and solid objects. To derive the equations of aerodynamics, fluid properties such as density and velocity are assumed to be well-defined at infinitely small points, and to vary continuously from one point to another. That is, the discrete molecular nature of a gas is ignored.
The continuity assumption becomes less valid as a gas becomes more rarefied. In these cases, statistical mechanics is a more valid method of solving the problem than continuous aerodynamics. The Knudsen number can be used to guide the choice between statistical mechanics and the continuous formulation of aerodynamics.
Read more about this topic: Aerodynamics
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