What is a Fluid? Properties of Fluids. Difference between Liquids & Gases

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By saying “study of fluids” we mean a broad area of study including liquids and gases and also a generalized study including different types of liquids and gases. In hydraulics only liquids are considered, and for hydraulics in civil engineering, water is the prime subject of study. Civil engineers have to design the water supply system, drainage system, irrigation canals, and dams. To do all this design and analysis a civil engineer should have a sound understanding of the basic properties of fluids.

What is a Fluid?

Fluid is a substance which can flow. Technically the flow of any substance means a continuous relative motion between different particles of the substance. Now, how and why does a fluid flow? The answer to “how” is the continuous relative motion between the particles of the fluid when shear force is acting on it. And the answer to “why” is the particles of fluid move past each other when shear force is applied because they cannot resist the shear force, i.e., they are inelastic to shear stress. A fluid can deform under shear stress indefinitely without returning to its original position.

Properties of Fluids

The term fluid includes both liquid and gases. The main difference between a liquid and a gas is that the volume of a liquid remains definite because it takes the shape of the surface on or in which it comes into contact, whereas a gas occupies the complete space available in the container in which it is kept. In hydraulics in civil engineering, the fluid for consideration is liquid, so, we will examine some terms and properties of the liquids.

Mass Density: It is the mass of the fluid per unit volume. It unit is kg per cubic meter.

Specific Weight: It is the weight per unit volume of the fluid. This quantity depends on the gravitational force of the place where the fluid is kept. The units for it are newton per cubic meter.

Specific Volume: It is the volume occupied by the unit mass of the fluid. Its unit is cubic meter per kg.

Relative Density or Specific Gravity: It is defined as the ratio of mass density of the fluid concerned and the mass density of water at standard pressure and temperature, i.e., 4 degrees Celsius and atmospheric pressure.

Viscosity: Viscosity is the property of fluid which defines the interaction between the moving particles of the fluid. It is the measure of resistance to the flow of fluids. The viscous force is due to the intermolecular forces acting in the fluid. The flow or rate of deformation of fluids under shear stress is different for different fluids due to the difference in viscosity. Fluids with high viscosity deform slowly.

Compressibility: When pressure is applied on a fluid, its volume decreases. This property of a fluid is called compressibility.

Elasticity: When the force generating the pressure on the fluid, is released it returns to its original volume. This property of a fluid is called elasticity of the fluid.

Vapor Pressure: Molecules of a liquid escape from its surface to fill the space above the liquid surface and the container until such time when the pressure due to these molecules above the liquid surface reaches the vapor pressure of the liquid. This is how the vapor pressure of a liquid is defined.

Surface Tension: The molecules on the surface of a liquid, that is, the interface between the liquid and the air are bound together by a week force called surface tension. This force makes the liquid form a layer and is caused due to the cohesive force between the molecules of the liquid.

Capillarity: The molecules of a liquid have two types of forces acting on them. One is, cohesive force, the force among the molecules of the liquid only, and the other one is adhesive force, the force acting between the molecules of the liquid and some other substance. When the adhesion between the liquid and the container wall is more than the cohesion among the liquid molecules, the liquid sticks to the container walls and this results in capillary rise. The opposite of this behavior happens when the cohesion is more than the adhesion - the capillary level dips.

This post is part of the series: Basics of Hydraulics

Hydraulics deals with the study of fluids, their behavior, motion of fluids and the interaction of fluids with other bodies. In this article series on Hydraulics fluids will be analyzed when in motion and as well as the effect of interaction of fluids with different bodies when static.

  1. What is Hydraulics? Civil Engineering and Hydraulics
  2. Basic Properties of Fluids
  3. Hydrostatics or Fluid Statics: Pressure Variation
  4. Measurement of Pressure
  5. Pressure Measurement: Pressure Measurement Devices