In fluid dynamics, what does Bernoulli’s principle refer to?

Bernoulli’s principle refers to the concept that within a flowing fluid, an increase in the fluid's speed occurs simultaneously with a decrease in pressure or potential energy. This is succinctly captured in the first option, which highlights the relationship between pressure and velocity in fluid flow. As a fluid accelerates through a constriction, for example, its velocity increases while the pressure decreases. This principle is fundamental in explaining various phenomena in fluid dynamics, such as the lift generated by airplane wings and the behavior of fluid in pipes.

The other concepts mentioned in the incorrect choices address different aspects of fluid or gas behavior: the second choice deals with the relationship between temperature and pressure specifically in the context of gases, the third discusses the role of viscosity in affecting fluid flow patterns—which is separate from Bernoulli’s considerations—and the fourth focuses on temperature's effect on fluid density. Each of these topics is important in fluid mechanics, but they do not describe Bernoulli's principle, which is specifically about the interplay between pressure and velocity in a flowing fluid.