ABSTRACT
Initiation of relative fluid motion, and thus development of velocity gradients, occurs under the action of external force gradients, such as those due to pressure, elevation, shear stresses, density, electromagnetic forces, etc. For example, rain falls to earth due to elevation differences (i.e., gravity differential), and butter spreads thin on toast due to the shearing action of a knife. Additionally, industrial liquids are transferred by means of piping systems after being pushed by pumps or pulled by vacuum, both of which generate pressure differentials. Meteorological phenomena are primarily due to air circulation, as a result of density differences induced by nonisothermal conditions. Finally, conducting liquids flow in nonuniform magnetic fields.
