What does the term 'kinematic viscosity' refer to in fluid mechanics?

Kinematic viscosity is a fundamental property in fluid mechanics that describes how easily a fluid flows under the influence of gravity. It is defined as the ratio of dynamic viscosity to the fluid's density, allowing for a better understanding of a fluid's behavior in motion. This property essentially measures the internal resistance of a fluid to shear or flow when subjected to external forces.

When discussing kinematic viscosity, it is important to note that it is not simply about how thick or thin a fluid appears (which relates more to dynamic viscosity), but rather about how that fluid behaves in motion and how it interacts with its surroundings at a certain density. It plays a critical role in various engineering applications, such as predicting flow patterns in pipes or understanding the movement of fluids in different conditions.

The other options refer to different physical properties: the speed at which a fluid moves through a pipe deals with velocity and flow rate, the boiling temperature relates to thermal properties, and the density under pressure pertains to compressibility and specific gravity. Each of these terms addresses distinct concepts in fluid mechanics, highlighting the unique aspect of kinematic viscosity as a measure of resistance to flow.