In Nature, most practical applications are associated with turbulent flows. These include river flows, wind flows around land forms (e.g. hills, mountains), oceanwaves. In engineering applications, many fluid flows are also turbulent: e.g., spillway flows, wind flows between buildings in a city. The fluid involved may be gas or liquid. In external flows, some relative motion occurs between an object and a large mass of fluid: e.g., the wind flow around an airship (Fig. 1.1). Internal flows include pipeline systems, and irrigation water channel. The flow may be steady or unsteady: e.g., a plane at take off. Whilemany applications involve turbulent flows, laminar flows occur in medical engineering (e.g. blood flows), slow-moving landslides and lubrication.