Bernoulli's equation only yields perfect predictions if the following conditions exist:
i- The liquid is inviscid; that is, it has no viscosity, and offers no frictional resistance to movement through it. (In other words, not like honey. More like alcohol, although even alcohol has some viscosity.)
ii-Steady flow; that is, no turbulence. The smoke rising from a candle is non-turbulent for a few inches; that is, it rises in a straight, thin column. After that, the flow becomes turbulent, breaking into swirls and eddies. (The transition from non-turbulent to turbulent flow is governed by a physical constant known as the Reynolds Number. In this day of spaceflight and supercomputers, the physical bases of the Reynolds Number are still unknown.)
iii- Incompressibility; that is, the fluid is like water, and not like air.
iv-No heat is gained or lost by the fluid during the process being analyzed.
v- The process involves a negligible change in height, implying constant potential energy due to gravitation.
How realistic are these conditions, under various circumstances? Give examples. When, where, and why would anybody pay any attention to Bernoulli's Law?