The study of fluids in motion, or flow, is one of the most complex branches of engineering. This complexity is reflected in such familiar examples as the flow of a river during a flood or a swirling cloud of smoke from a process plant smokestack. Each drop of water or each smoke particle is governed by Newton's laws, but the equations for the entire flow are very complicated. Fortunately, idealized models that are simple enough to permit detailed analysis can represent most situations in process control.