What Is Fluid? | Types of Fluids | Compressible Fluid | Type of Fluid Flow

Types of Fluids

What Is Fluid?

What Is Fluid

Fluid flows are part of fluid mechanics & are related to fluid dynamics. It involves the motion of a fluid subject to unbalanced forces. These motions continue as long as an unbalanced force is applied. For example, if you are pourings water from mugs, the water velocity is very high above the lip, moderately high near the lip, and very low at the bottom of the mug.

The unbalanced force is gravity, & the flow continues as long as the water is available and the mug is tilted. Liquids generally include liquids, gases, and plasma. To some extent, plastic solids are also considered to be liquids. Before discussing liquids, let us first discuss the phases of matter.

Matter around us exists in three phases except for plasma. Plasma is a very hot substance – so hot that electrons are pulled away from the atoms making up the ionized gas. There are three states of matter

  • Solid
  • Liquid
  • Gas

From the above three phases, liquid and gas are jointly known as liquids. The main difference between liquids and solids lies in their ability to resist shear stresses. Let us understand liquids with their definitions.

When a constant shear force is applied, a solid eventually stop deforming, whereas a fluid never stops deforming and reaches a constant rate of strain.

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Types of Fluids:

Types of Fluids

Based on the shear stress & velocity gradient relationship, liquids can be broadly classified into the following five types:

#1. Ideal Fluid

An ideal fluid is a fluid that is incompressible in nature and has no viscosity. In practice, no liquid is an ideal liquid because all liquids have some viscosity. Hence it is also called imaginary fluid. The ideal fluid is incompressible and has no viscosity. It is an imaginary fluid and does not really exist.

#2. Real Fluid

Real Fluids are liquids that have some viscosity and are compressible in nature. In fact, all liquids are real liquids.

#3. Newtonian Fluid

Newtonian fluids are liquids that obey Newton’s law of viscosity. In other words, a reals fluid whose shear stress is proportional to the rate of shear deformation is called a Newtonian fluid. For a Newtonian fluid, viscosity depends entirely on the temperature and pressure of the liquid.

Examples: Water, Air, Emulsion, Hydrogen Main Features: Newton’s Liquids have no elastic properties. They are incompressible, isotropic, and unreactive. Viscosity is temperature-dependent; viscosity further depends on the different pressures at which it is found. At a certain temperature, their viscosity remains constant with an increase in temperatures of the fluid, the viscosity decreases. The viscosity of this type of liquid is inversely proportional to the increase in its temperature.

Newtonian fluid was named after Sir Isaac Newton, who defined it as a viscous flow. They obey Newton’s law of viscosity. A real fluid that obeys Newton’s law of viscosity is known as a Newtonian fluid.

Note: According to Newton’s law of viscosity, the shear stress is directly proportional to the strain rate.

#4. Non-Newtonian Fluids

Non-Newtonian fluid is liquid that does not obey Newton’s law of viscosity. In other words, a reals fluid in which the shear stress is not directly proportional to the rate of shear deformation is known as a non-Newtonian fluid. A real fluid that does not obey Newton’s law of viscosity is known as a non-Newtonian fluid.

#5. Ideal Plastic Fluid

Fluids in which the shear stress is greater than the yield value and the shear stress is directly proportional to the rate of shear deformation are known as ideal plastic fluid. Properties of liquids There are some basic properties associated with liquids that help us understand them better.


It is the ratio of the applied shear stress to the shear strain rate achieved in liquids. Viscosity provides resistance to the movement of fluid layers.


It is a term used in liquids that is analogous to the term stress used in solids. The ratio of both the applied force and the area of ​​the surface on which the force is applied.

Specific Gravity:-

The specific gravity of fluids is defined as the ratio of the density of a particular fluid to the density of a standard fluid, usually water.

Examples of fluids:

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

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Incompressible Fluid:

When the density of the liquid does not change by the use of external force, it is known as an incompressible fluid.

Compressible Fluid:

When the density of a liquid changes by the use of external force, it is called compressible fluid. The table below shows the density and viscosity of different types of liquids.

Types of fluid Density  Viscosity 
Ideal fluid Constant Zero
Real fluid Variable Non-Zero
Newtonian fluid Constant/ Variable T=u(dudy)
Non-Newtonian fluid Constant/ Variable T≠u(dudy)
Incompressible fluid Constant Non-Zero/ Zero
Compressible fluid Variable Non-Zero/ Zero

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Type of Fluid Flow:

Fluid flows have all kinds of aspects – stationary or unstable, compressive or incompressible, viscous or non-viscous, and rotational or erratic, to name a few. Some of these characteristics reflects the property of the fluid themselves, & others focus on how the fluid is moving.

#1. Steady or Unsteady Flow

The flow of a fluid may be stationary or unstable depending on the velocity of the fluid:

  • Constant: In a steady fluid flow, the velocity of the fluid remains constant at any point.
  • Unsteady: When the flow is unstable, the velocity of the fluid can vary between any two points.

#2. Viscous or Non-Viscous Flow

The fluid flow can be vicious or non-viscous. Viscosity is the measure of the thickness of a fluid, & very viscous liquids such as motor oil or shampoo are called viscous fluids.

#3. Fluid Flow Equation

Mass flow rates are the rate of movement of a mass of fluid through a unit area. In simple words, it is the speed of mass per unit of time. The formula for the mass flow rate is given as follows:

  • Massflowrate=ρAV

From the equations, we can see that the rate of mass flow depends on the density, velocity, and area of ​​the cross-section of the fluid.

#4. Solved Example

A fluid moves through tubes of 15 m/s, and the transverse area of ​​the tube is 0.4 m2. If the densities of liquids are = 1.5 g/m, then what is the amount of mass flowing through the tube?

To calculate the total mass of the fluid flowing through the tube, we use.

  • Massflowrate=ρAV

Substituting the values in the above equations, we get.

  • Massflowrate=1.5×15×0.4=9g/s

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Frequently Asked Questions (FAQ)

Incompressible Fluid

An incompressible fluid is a fluid the density of which remains constant during flow. Liquids are normally treated as being incompressible, as a gas can be when only slight pressure variation occurs.

Types of Fluids

  • Ideal Fluid
  • Real Fluid
  • Newtonian Fluid
  • Non-Newtonian Fluids

Compressible Fluid

Compressible flow (or gas dynamics) is the branch of fluid mechanics that deals with flows having significant changes in fluid density.

Type of Fluid Flow

  • Steady or Unsteady Flow
  • Viscous or Non-Viscous Flow
  • Fluid Flow Equation
  • Solved Example

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