Basic Concepts of Fluid Flow
Chapter 2, with its description of gas layers moving over each other, sets the stage for developing a macroscopic picture of fluid motion. Instead of accounting for the motion of individual molecules, we simplify the problem by considering the average effects of molecules in a given volume, i.e., by considering a volume element or parcel of air whose dimensions are much larger than the intermolecular spacing. The fluid is then regarded as a continuum, i.e., as a hypothetically continuous substance. The determination of the properties of the fluid as a function of position and time is the objective of many fluid mechanics problems. Properties of the fluid include quantities such as velocity, pressure, density, and temperature.