What is Fluid? – Phases, Types, & Fluid Flow

What is Fluid?

A fluid is a substance that is continuously (deformed) under constant shear stress, no matter how small. A fluid is a subset of the phases of matter and consists of liquids, gases, plasmas, and, to some extent, plastic solids.  In common usage, “fluid ” is often used as a synonym for “liquid”, which has no implication that gas may also be present. In this article, we discuss Types of Fluid And Various Types of Fluid Flow.

Matter around us exists in three phases (excluding plasma)

  • Solid
  • Liquid
  • Gas
Three Phases of Fluid
Three Phases of Fluid


Atoms of solids are in close contact, forces between them that allow atoms to vibrate but not to change positions with neighboring atoms. (These forces can think of as springs that can stretch or compress, but not easily broken.) Thus, a solid resist all types of stress. A solid cannot be easily deformed because the atoms that make up the solid are not capable of moving freely. Solids also resist compression, because their atoms are part of a lattice structure in which the atoms relatively fix distance. Under compression, the atoms will force into each other. Most of the examples we have studied so far include solid objects that are very few when stressed.


In contradiction, liquids easily deform when apply and do not return to their original shape after the force removed because atoms are free to slide and change about their neighbors, i.e. they flow (hence Are a type of fluid). Molecules held together by their mutual attraction. When a liquid placed in a container with no lid, it remains in the container (provided there is no hole under the surface of the liquid in the container!). Because atoms closely fill, liquids, such as solids, resist compression.


Atoms in gases separated by distances that are larger than the size of atoms. The forces between the gas atoms are therefore very weak, except when the atoms collide with each other. Thus gases not only flow (and are therefore consider fluid) but they are relatively easy to compress because there are too much space and little force between the atoms. When placed in an open container gas, unlike liquids, will escape. The major difference is that gases easily compress, while liquids are not. We will usually refer to both gases and liquids only as liquids, and distinguish between them only when they behave differently.

Types of Fluid

The fluids are classified in to the following types:

  • Ideal fluid
  • Real fluid
  • Newtonian fluid
  • Non-Newtonian fluid
  • Ideal plastic fluid
  • In-compressible fluid
  • Compressible fluid

On the basis of shear and velocity gradient relationship, fluids can broadly be classified into the following types of fluid:

types of Fluid
Types of Fluid

Ideal fluid: A fluid, which is incompressible and is having no viscosity, knows as an ideal fluid. Ideal fluid is only an imaginary fluid as all the fluid, which exists, has some viscosity.

Real fluid: A fluid, which possesses viscosity, knows as real fluid. All the fluids in actual practice are real fluids.

Newtonian fluid: A real fluid, in which the shear stress is direct, proportional to the rate of shear strain (or velocity gradient), knows as a Newtonian fluid.

Non-Newtonian fluid: A real fluid, in which the shear stress is not proportional to the rate of shear strain (or velocity gradient), knows as Non-Newtonian fluid. In the case of such a fluid, the relationship between the shear stress and the rate of strain is an arbitrary functional relation, either implicit or explicit.

Ideal plastic fluid: A real fluid, in which the shear stress is more than the yield value and the shear stress is proportional to the rate of shear strain (or velocity gradient), knows as ideal plastic fluid.

In-compressible fluid

When the density of the fluid doesn’t change with the application of external force, it knows as an incompressible fluid.

compressible fluid

When the density of the fluid changes with the application of external force, it knows as compressible fluid. Density and Viscosity is different according to the types of fluid as shown below.

The Table Below represents the Density and Viscosity of Different Types of Fluids

Table o property

Different Properties of Fluids

Though each fluid is different from others in terms of composition and specific qualities, there are some properties that every fluid shares.

These properties can broadly categorized under:

  • Kinematic properties: These properties help in understanding the fluid motion. Velocity and acceleration are the kinematic properties of the fluids.
  • Thermodynamic properties: These properties help in understanding the thermodynamic state of the fluid. Temperature, density, pressure, and specific enthalpy are the thermodynamic properties of the fluids.
  • Physical properties: These properties help in understanding the physical state of the fluid such as color and odor.


The density of a fluid defined as the ratio of the mass of the fluid to its volume. And the density of gases is dependent on pressure and temperature, while the density of liquid remains constant. The density of water is 1000 kg.m-3 while the density of air is 1.225 kg.m-3. There is some basic properties associate with fluids that help us to understand them in a better way.

ρ = mass of fluid / volume of fluid

ρ = Density

kg.m-3 = SI unit of density


  • Viscosity is the fluid property that determines the amount of resistance of a fluid to overcome stress. It is the property of a fluid that causes the fluid to provide resistance to the flow of one layer of fluid over another adjacent layer.
  • In a liquid, viscosity decreases with an increase in temperature. In a gas, viscosity increases with an increase in temperature.


  • It the term used in fluids which is analogs to the term stress used in solids. Both are the ratio of apply force and area of the surface (on which the force apply) Pressure of a fluid is the force per unit area of the fluid. In other words, it is the ratio of the force on a fluid to the area of the fluid held perpendicular to the direction of the force.
  • Pressure denoted by the letter ‘P’. Its unit is N/m2.


This is the property that determines the level of warmth or coolness or intensity of heat of a fluid. Temperature measured using temperature scales. 3 are the common use temperature scales.

  • Celsius (or centigrade) scale
  • Fahrenheit scale
  • Kelvin scale (or absolute temperature scale)

Kelvin scale is widely used in engineering. This is because, this scale is independent of properties of a substance.

Specific gravity

  • Specific gravity is the ratio of the specific weight of the given fluid to the specific weight of the standard fluid. It is denoted by the letter ‘S’. It has no unit.
  • Also, specific gravity may also be defined as the ratio between the density of the given fluid to the density of the standard fluid.

Specific Weight

  • A specific weight is a weight possessed by the unit volume of a fluid. It is denoted by ‘w’. Its unit is N/m3.
  • Specific weight varies from place to place due to the change of acceleration due to gravity (g).

Specific Weight

  • A specific weight is a weight possessed by the unit volume of a fluid. It is denoted by ‘w’. Its unit is N/m3.
  • And specific weight varies from place to place due to the change of acceleration due to gravity (g).

Different Types of Fluid Flow

Fluid flow has all kinds of aspects – static or unstable, compressed or inconsistent, viscous or non-viscous and rotational or irrational, to name a few. Some of these characteristics reflect the properties of the liquid, and others focus on how the fluid is growing.

Steady or Unsteady Flow

Fluid flow can be steady or unsteady, depending on the fluid’s velocity:

Steady: In steady fluid flow, the velocity of the fluid is constant at any point.

Unsteady: When the flow is unsteady, the fluid’s velocity can differ between any two points.

Compressible and In-compressible flows

Incompressible flow is a flow in which the variation of density within the flow is assumed to be constant. In general, all liquids are treated as incompressible fluids. On the contrary, flows that are characterized by a varying density are said to be compressible.

Gases are normally used as the compressible fluids. However, all fluids, in reality, are compressible because any change in temperature or pressure results in changes in density. In many situations, though, the changes in temperature and pressure are so small that the resulting changes in density are negligible.

Laminar and Turbulent Flows

Laminar And Turbulent flow
Laminar And Turbulent flow

A Laminar flow (Streamlines) is one in which each fluid particle has a well-defined path. In such flow, the paths of fluid particulars do not intersect with each other. In turbulent flow, the paths of fluid particles may intersect with each other.

Consider water flowing through a pipe at low speeds, there is a nice smooth condition, which is called laminar flow. The mixing of warm and cold air in the atmosphere by wind, which causes clear-air turbulence experienced during an airplane flight, as well as poor astronomical seeing.

Examples of fluids

  • Water
  • Air
  • Blood
  • Mercury
  • Honey
  • Gasoline
  • Any other gas or liquid
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