Fluid | Definition, Types with Examples and Properties


Fluid: A fluid is defined as a material which will continue to deform with the application of shear force. however small the force may be. Thus fluids are the matters which easily flow.

(i) Fluid statics: The study of fluids at absolute and the relative rest position is called fluid statics. The study of incompressible fluid (e.g. water liquid) under static condition is called Hydrostatics and that dealing with the compressible fluid (e.g. air, etc.) is termed as Aerostatic.

(ii) Fluid kinematics: The study of fluid when the fluid is in motion without considering the force and energy causing such motion is called fluid kinematics. It deals with velocity and acceleration or fluid and its effects on flow pattern of fluid elements.

(iii) Fluid Dynamics: It deals with the relation between velocity and acceleration or fluid and the forces which are exerted by or upon the moving fluid.

fluid velocity diagram


A fluid maybe classified as;
(a) Ideal fluid and real fluid
(b) Newtonian fluid and non-Newtonian fluid
(c) Compressible fluid and incompressible fluid.

(A) Ideal fluid: A fluid which is incompressible and having no viscosity is called an ideal fluid. There is no internal resistance in the ideal fluid. Ideal fluid is only an imaginary fluid. No fluids in actual practice are ideal fluids.

(B) Real fluid: A fluid which possesses viscosity is called real fluid. All the fluids in actual practice are real fluid. e.g. water, oil, air, nitrogen.

(C) Newtonian fluid: A real fluid in which the shear stress is directly proportional to the rate of shear strain (or rate of deformation or velocity gradient) is known as newtonian fluid. e.g. water, kerosene, air. Newton's law of viscosity state that the shear stress (τ) on a fluid element, the layer is directly proportional to the rate of shear deformation.

(D) Non-Newtonian fluid:
A real fluid in which the shear stress is not proportional to the rate of shear strain (or velocity gradient) is called non - Newtonian fluid.

(E) Compressible fluid: A fluid in which there is a variation in density with the change in temperature and pressure is known as compressible fluid. e.g. all gases at STP.

(F) Incompressible fluid: A fluid in which the density remains constant with variation in temperature and pressure is called incompressible fluid. e.g. all liquid at NTP (Normal temperature and pressure)

Also Read: Welding Defects


(1) Density or mass density: It is defined as the ratio of the mass of a fluid to its volume. Thus it is the mass per unit volume.

                                                Mass of fluid 
                        Mass density = Volume of fluid 

The unit of density in SI unit is kg/m^3. The density of liquid may be considered as constant. While that of gas changes with variation of pressure and temperature. The mass density of water is taken as 1000 kg/m^3  at STP. 

(2) Specific weight or weight density (ω): It is defined as the ratio of the weight of a fluid to its volume, Thus it is the weight per unit volume. It is denoted by symbol ω. unit of weight (force) is newton (N). 1 kgf = 9.81 N


     Specific weight = Weight of fluid
                               Volume of fluid

(3) Specific volume (v): Specific volume is defined as volume per unit mass. Thus it is a ratio of the volume of a fluid to its mass. It is denoted by symbol v. 

                                  Total volume 
     Specific volume =     Total mass

Its unit is m3/kg. 

"Specific volume is inverse of the mass density" 

(4) Specific gravity: Specific gravity is defined as the ratio of the specific weight (or density) of fluid to the specific weight (or density) of a standard fluid. For liquid, the standard fluid is water at 4*C, and for gases, the standard fluid is taken either air at O*C or hydrogen at the same temperature. specific gravity is dimensionless and has no units. It is denoted by symbol S.

(5) Viscosity or Dynamic viscosity (µ): It is the property of the fluid which offer resistance to the movement of one layer of fluid over another adjacent layer of fluid.

(6) Kinematic viscosity (ϑ): It is defined as the ratio of the dynamic viscosity (or viscosity) to the density of the fluid. It is denoted by symbol (ϑ) (new greek letter) 

(7) Surface tension (σ): It is defined as the tensile force acting on a surface of a liquid in contact with gas or on the surface between two immiscible liquids such that contact surface act as a very thin layer under tension. It is denoted by symbol σ. Surface tension is expressed in force per unit length. So its unit is N/m in Sl system. 

Hence surface tension is also defined as "Force required to maintain the unit length of film in equilibrium". Surface tension mainly depends upon intramolecular cohesive force. Surface tension decreases with rise in temperature. 

(8) Cohesion: It is the property of the fluid by which particles of the same fluid are attracted. Surface tension is mainly due to the cohesion property of the fluid. 

(9) Adhesion: It is the property of the fluid by which particles of different fluids or solid and fluid are attracted. Capillarity is mainly due to the adhesion property of a fluid. 

(10) Compressibility (β): It may be defined as a change in volume of fluid under the action of external pressure. 

(11) Bulk modulus of elasticity (K): It may be defined as a change in pressure of fluid needed to cause the rate of change of volume (ΔV/v, volumetric strain). In Sl system, bulk modulus of elasticity is expressed in N/m2. 

(12) Vapour Pressure: Liquids vaporize at any temperature. Such vaporization occurs due to continuous escaping of molecules at a liquid free surface. When the liquid is confined in a vessel, such vaporized molecules get accumulated above the free surface and exerts pressure. Which is called vapour pressure. 


The fluid flow may be classified as follows. 

  1. Steady and unsteady flow; 
  2. Uniform and non-uniform flow; 
  3. One, two and three-dimensional flow; 
  4. Rotational and irrotational flow; 
  5. Laminar and turbulent flow; 
  6. Compressible and incompressible flow; 
  7. Ideal and real flow; and 
  8. Pressure and pressure less flow; 

(1) Steady and Unsteady flows:

(i) Steady flow: The fluid flow, in which the fluid characteristics like velocity, pressure, density etc. at any point remain constant with respect to time is called steady flow. 


(l) Flow-through a constant cross-sectional area pipe at a constant rate.
(2) The flow of water in the discharge pipeline when centrifugal is running at 
a uniform rotational speed. 

(ii) Unsteady flow: The fluid flow, in which the fluid characteristics like velocity, pressure density etc. at a point vary with respect to time is called unsteady flow. 


(l) Flow-through a pipe whose valve is being opened or closed gradually. 
(2) The flow of water in the suction and delivery pipes of a reciprocating pump. 

(2) Uniform and non-uniform flows: 

(i) Uniform flow: The fluid flow, in which the velocity at any given time does not vary with respect to space is called uniform flow. 


(l) Flow-through a straight pipe having a constant cross-sectional area. 
(2) The flow between parallel plates. 

(ii) Non-uniform flow: The fluid flow. in which the velocity at any given time vary with respect to space, Thus for a non-uniform flow, 

Example: Flow through a pipe of a constant cross-sectional area at an increasing or decreasing rate. 

(3) One, Two and Three Dimensional flows: 

(i) One dimensional flow: The type of flow, in which the flow parameter such as velocity variation occurs only along the flow direction. The variation of velocities in other two mutually perpendicular directions is insignificant or assumed to be negligible is called one-dimensional flow. The velocity is a function of time and one space coordinate. 

(ii) Two-dimensional flow: The type of flow, in which the flow parameter such as velocity varies along with two directions. That is velocity is a function of time and two rectangular space coordinate, say x and y. 

The velocity variation in the third direction is negligible. 


(i) Flow over notch and weir 
(ii) The viscous flow between two parallel plates. 

(iii) Three-dimensional flow: The type of flow, in which flow parameter such as velocity vary in all the three directions, that is velocity is a function of time and three mutually perpendicular directions. 


(i) Flow at the inlet of the nozzle 
(ii) Flow in river 
(iii) Flow within fluid machines like the centrifugal pump, turbine. 

(4) Rotational and Irrotational flows: 

(i) Rotational flow: The type of flow in which the fluid particles rotates about their own mass centres while moving along a streamline.  

(ii) Irrotational flow: The type of flow in which the fluid particles do not rotate about their mass centres while moving along a streamline. As shown in fig. the same fluid particle V1V2 does not rotate about its own axis as it moves along the circular streamline. 


(l) A vortex or whirlpool develops above a drain hole of the washbasin. 
(2) Flow in a casing of the centrifugal pump. 
(3) The flow above a drain hole in a stationary tank. 

(5)Laminar and Turbulent flows:

Laminar and Turbulent flow

(i) Laminar flow: The type of flow, In which the fluid particles move in a well-defined path or streamline and they retain the same relative position at successive cross-sections of the flow passage. Fluid particles do not cross one another. 

Thus the fluid particles move in layers gliding smoothly over the adjacent layer. It Is also called a streamline or viscous flow. 

Laminar flow occurs in a smooth pipe due to the following factors. 
(l) When fluid flow velocity in pipe is low. 
(2) The fluid has a high viscosity. 
(3) The diameter of the smooth pipe is very small (capillary tube) 
(4) Reynold number is less than 2000 

(ii) Turbulent flow: A turbulent flow is that type of flow in which the fluid particles move in erratic and unpredictable paths. Fluid particles are individually subjected to fluctuating transverse velocities. So that eddies formation takes place. Which are responsible for high energy losses. 


(1) Flood In river 
(2) The discharge of smoke into atmosphere from a chimney. 
(3) High-velocity flow in large size of conduit. 

Reynolds number is defined as a ratio of inertia force to viscous force. 

(i) Reynolds number, Re < 2000 - Laminar flow occurs pipe 

(ii) Reynolds number, Re > 4000 -Turbulent flow occurs in pipe 
(iii) Reynolds number, Re between 2000 to 4000 laminar or turbulent 

Flow may occur in a pipe (It is also known as transition flow) 

(6) Compressible and incompressible flows: 

(i) Compressible flow: A compressible flow is that type of flow in which the density of fluid changes from point to point due to change in pressure and temperature. So that the density of fluid does not remain constant for this flow. 


(l) Gas flow in turbines. 
(2) The flow of gas through nozzle and orifice. 

(ii) Incompressible flow: In this type of fluid flow density of fluid does not changes or the density changes due to pressure and temperature variation is negligible in the flow field. For all practical purpose, liquids are generally considered flowing incompressible. 

Example: Flow of water in nozzle, turbine and pipe.

(7) Ideal and real flows: 

(i) Ideal flow: It is ideal frictionless flow. In this flow, no shear stress is assumed to exist between adjacent fluid layers and between the fluid layers and the boundary. So that the fluid is considered non-viscous (µ = O) 

(ii) Real flow: In this type of flow, due to viscosity of the fluid, shear stress comes into existence. This shear stress opposes the sliding of one layer over another and there is a frictional resistance to fluid motion. 

(8) Pressure flow and pressure less flow: 

(i) Pressure flow: It is that type of flow in which fluid motion is bounded by the solid boundary on all the sides, Therefore the free surface of the liquid does not exist. This type of flow exist due to the pressure gradient along the pipe length. 

Example: Flow-through pipe. 

(ii) Pressureless flow: It is that type of flow in which the fluid motion is bounded by three sides and the fourth side is exposed to the atmosphere. It is also called as gravity flow. This type of flow occurs due to the own weight of the fluid. 

(l) Flow in an open channel and river 
(2) Flow in partially filled pipelines. 

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Fluid | Definition, Types with Examples and Properties Fluid | Definition, Types with Examples and Properties Reviewed by Harish on June 27, 2020 Rating: 5

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