Introduction

Physical Properties of Fluids

Introduction

The study of fluid motion began with idealized, non-viscous fluids, and many mathematical ideas were developed during the later part of the nineteenth century. That branch of study came to be known as hydrodynamics. However, equations built for ideal fluids could not be applied directly to real fluids because actual liquids and gases behave differently in practical situations.

As experimental work on water flow increased, engineers developed empirical methods that could be used in design and analysis. This practical field became known as hydraulics. Later, as industries expanded and engineering problems became more complex, there was a need for theories that could describe the behavior of all real fluids under different flow conditions.

To meet this need, researchers combined experimental observations with mathematical analysis. This broader field is now called fluid mechanics. It includes the study of fluids at rest as well as fluids in motion, making it one of the most important subjects in engineering science.

A proper understanding of fluid properties is essential for analyzing both static and flowing systems. Some of the most important properties include:

  • Density
  • Viscosity
  • Surface tension
  • Bulk modulus
  • Vapour pressure

Each of these properties affects fluid behavior in a different way. Viscosity represents internal resistance to flow. Surface tension is related to the force acting at the surface of a liquid and is responsible for capillary effects. Bulk modulus describes how a fluid responds when pressure changes and is important in wave propagation, such as sound. Vapour pressure can influence the formation of vapor bubbles and may cause flow disturbances when the local pressure becomes too low.

In this chapter, these properties are discussed in detail because they play a major role in engineering applications. The basic laws used in the study of fluids are:

  1. Newton’s laws of motion
  2. Conservation of mass and energy
  3. Laws of thermodynamics
  4. Newton’s law of viscosity

A fluid is a substance that continues to deform whenever a shear force is applied, no matter how small that force may be.

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