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Viscosity Terms and Definitions

By Joe Honeywell



Why do some fluids such as honey flow very slowly from a cup while fluids such as water flow rapidly? Will fluids flow the same if they are heated to a higher temperature? The answer to these questions relates to a fluid’s physical property called viscosity and its behavior under various conditions. The interaction of molecules causes them to move relative to each other at different rates. Since each fluid has a different molecular structure that interacts with itself differently at a given temperature, the fluids flow behavior will act differently when it flows from a cup.

The science of a fluid flow behavior is called rheology and is important to almost every field of engineering. For example, the field of hydraulics is influenced by the fluid’s viscosity. At given operating conditions the viscosity will have an effect on the flow rate, pressure drop and required power. Viscosity also plays a critical part in the design of chemical processes, sizing and selection of equipment such as heat exchangers and rotating machinery, performance of mechanical components such as bearing and seals, manufacture of food, cosmetics and medicines, and many other engineering fields. As a matter of fact, viscosity touches the design of almost every science that deals with moving fluids. Understanding the basic science of rheology is the key to understanding the behavior of flowing fluids. It is a complex topic but a good understanding of terms and definitions related to fluid viscosity will go a long way to better system designs, selection of equipment and performance of operating systems.

This paper will discuss topics related to the basic understanding of fluid viscosity. It will start by defining the basic terms that relate to all fluids. It will discuss the relationships of viscosity to three important parameters; temperature, velocity and pressure. The flow behavior and classifications of fluids are discussed along with representative curves of each fluid category. It will also review viscosity measurement methods by discussing two of the most common viscometers. Finally, this paper will give conversion factors and typical viscosity values for common fluids used by many engineering disciplines.



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