General features of fluid mechanics

Abstract

In this chapter we introduce some basic concepts of fluid mechanics. After observing in Sect. 3.1 that viscous effects are necessary to justify the existence of friction and drag forces, in Sect. 3.2 we define the Reynolds number, Re, as the ratio between convective and diffusive momentum fluxes, deriving a scaling of the drag force in the limits of small and large Re. Then, in Sect. 3.3, we observe that within these scaling laws there are some inconsistencies, which can be resolved only by introducing the concept of boundary layer. Finally, after a brief discussion on the boundary conditions, in Sect. 3.5 we present a brief qualitative overview of turbulence. A more complete description of this very complex subject can be found in Chap. 18.