ABSTRACT
Shear forces, however small they may be, cause fluids to deform. This, or some version of it, defines substances called fluids—they may be either liquids or gases, or mixtures of phases. When the motion of a solid is studied, it is often sufficient to consider the solid as a rigid particle on which all attention is focused. Fluid motions may also be analyzed by the particle approach (the method is sometimes called the Lagrangian approach), but most fluid motions consist of an enormous number of particles, and this treatment of each of them is often impractical. In such cases the more usual control volume or Eulerian point of view is adopted in which the changes within a region of fluid flow, or a flow field, are considered.
