ABSTRACT
So far we have considered mainly the theory of the inviscid fluid flow. In spite of its idealization, the inviscid theory gives satisfactory explanations for many phenomena of fluid flow. In the inviscid theory, we have to introduce, on many occasions, surfaces of discontinuity, such as shock waves, vortex sheets and flame fronts. These surfaces of discontinuity are allowable in the flow of inviscid fluid but not in a real fluid. In the real fluid, these surfaces of discontinuity should be replaced by transition regions. If the thickness of the transition region is very thin, to a first approximation, we may replace it by a surface of discontinuity. The flow pattern outside such a transition region will not be influenced greatly by such a replacement. This is why the theory of inviscid fluid flow gives satisfactory explanations for a great many phenomena of fluid flow outside such a transition region. If we are interested in the flow inside the transition region or in flow phenomena closely related to the transition region, we must consider the effect of viscosity. For instance, the theory of inviscid flow fails to explain such phenomena as skin friction, form drag of a body, no slippage on the surface of a solid body, etc. To explain these phenomena, we must take into consideration the effect of viscosity. In §2, we shall discuss the viscosity of a fluid and its influences on the fluid flow.
