Introduction to Fluid Mechanics | |||||
Fluid as a continuum |
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In the definition of the fluid the molecular structure of the fluid was not mentioned. As we know the fluids are composed of molecules in constant motions. For a liquid, molecules are closely spaced compared with that of a gas. In most engineering applications the average or macroscopic effects of a large number of molecules is considered. We thus do not concern about the behavior of individual molecules. The fluid is treated as an infinitely divisible substance, a continuum at which the properties of the fluid are considered as a continuous (smooth) function of the space variables and time. To illustrate the concept of fluid as a continuum consider fluid density as a fluid property at a small region.(Fig.L1.2(a)). Density is defined as mass of the fluid molecules per unit volume. Thus the mean density within the small region C could be equal to mass of fluid molecules per unit volume. When the small region C occupies space which is larger than the cube of molecular spacing, the number of the molecules will remain constant. This is the limiting volume |
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