Chp2_331 - 2 Fundamental Concepts 2.1 The Continuum assumption Fluids are composed of many finite-size molecules with finite distance between them These

Chp2_331 - 2 Fundamental Concepts 2.1 The Continuum...

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2 2. Fundamental Concepts 2.1. The Continuum assumption • Fluids are composed of many finite-size molecules with finite distance between them. These molecules are in constant random motion and collisions. • This motion is described by statistical mechanics (Kinetic Gas Theory). • This approach is not acceptable, for the time being, in almost all practical flows. • There is a need to look at properties of fluid particles that are large on a micro-scopic scale but small on a macroscopic scale. • The microscopic properties are the focal point of the Rarefied Gas Dynamics. Within the continuum assumption there are no molecules. The fluid is continuous . Fluid properties as density, velocity etc. are continuos and differentiable in space & time. A fluid particle is a volume large enough to contain a sufficient number of molecules of the fluid to give an average value for any property that is continuous in space, independent of the number of molecules.
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3 Characteristic scales for standard atmosphere: * atomic diameter 10 -7 mm *
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