22. Fundamental Concepts2.1. The Continuum assumption• Fluids are composed of many finite-sizemolecules with finite distance betweenthem. These molecules are in constantrandom motion and collisions.• This motion is described by statisticalmechanics (Kinetic Gas Theory).• This approach is not acceptable, for the timebeing, in almost all practical flows.• There is a need to look at properties of fluidparticles that are large on a micro-scopicscale but small on a macroscopic scale.• The microscopic properties are the focalpoint of the Rarefied Gas Dynamics.Within the continuum assumption there are nomolecules. The fluid iscontinuous.Fluid properties as density, velocity etc. arecontinuos and differentiable in space & time.A fluid particle is a volume large enough tocontain a sufficient number of molecules of thefluid to give an average value for any propertythat is continuous in space, independent of thenumber of molecules.
3Characteristic scales for standard atmosphere:*atomic diameter≈10-7mm*