Fluid as a continuum or continuum based approach

• Fluid is made of molecules. However, for most of the engineering applications, when we speak of fluid's properties such as density, or conditions such as pressure and temperature, we do not imply such properties or conditions of individual molecules, but those of “fluid” as a whole.
• In other words, we refer to the average or macroscopic aggregate effects of the fluid- molecules, reflected in pressure, temperature, density, etc.
• Such an approach to treating a fluid is called continuum based approach. In other words, fluid is treated as continuum.
• However, there is a restriction. The continuum approach can be applied only when the mean free path of the fluid (largely, gas) is smaller (actually much smaller!!) than the physical characteristic length of the system under consideration, say, the diameter of the tube in which the gas flows, or size of a container in which gas is stored.