The Knudsen Number,, is the dimensionless characteristic of the rarefied gas medium. The molecular flow of gases is considered typical when the mean tree path length λ of molecules exceeds the characteristic linear dimension of a streamlined body d. In many cases, it is the correct choice of a simple model that guarantees progress in the solution of a posed problem.įor gases, two models of fluid flow are used: molecular and viscous for liquids, only a viscous model is used. As a rule, only the most essential properties of the medium are considered because the wider the context of the problem, the more difficult it is to close the mathematical model, to carry out physical analysis, and in the long run, to correlate theory with experiment. To describe the motion of the fluid medium, an appropriate mathematical model for this phenomenon must be chosen. Inside the fluid medium, sharp boundaries of the flow can arise (for instance, cavities in cavitation or shock waves in retarding supersonic flows). When a fluid medium flows around bodies immersed in a fluid, the zone of influence of the body on the flow can extend over a considerable distance both up and down the flow. To a large extent, this difficulty is associated with defining boundary conditions.

At present, this problem has been solved only for some particular cases. In the general case, the motion of a fluid medium in a certain volume can be specified if, at any moment of time t, the fluid velocity field of the medium at any point of the volume can be determined (or calculated with the required accuracy). External flows may be addressed by the methods of fluid mechanics. External flows are defined as those motions of a fluid medium which occur when the medium flows around a body immersed in it.

Eddy viscosity ratio external flow