Fluid Properties : Basic definitions used to define fluids

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Today we will discuss about fluids properties. Properties of any material or fluid are used to define its state, condition, behavior and distinguish it from others. There are various types of fluids which behave different in different conditions. For example when we take petrol in a open container, it will evaporate suddenly but the kerosene in same container doesn’t. These all behavior of fluid can be described by its’ properties which knowledge is essential to study about fluid mechanics. 
There are various properties of fluids which are used to define a fluid and its behavior in various fluid applications. These are
Fluid Properties : Basic definitions used to define fluids

Fluid Properties:


  • Density of a fluid is defined as the mass of the fluid per unit volume.
  • Mathematically it is defined as the ratio of the mass to the volume of the fluid. 
ρ=  Mass/Volume
  • It is depends on the mass and size of the atom of the fluid. Fluids have same volume and different mass have different density. 
  • The SI unit of density is Kg/m3. It is generally denoted by ρ

Fluid Properties : Basic definitions used to define fluids

Specific gravity:

  • It is density of a fluid compared to the density of water which is 1000 Kg/m3. It shows the substance is how much heavy compare to water. 
  • Mathematically it is defined as the ratio of the density of a fluid to the density of water.
S=  (Density of Fluid)/(Density of Water)
  • If the value of specific gravity is greater than one it means the fluid is heavy compared to water and if that fluid mix with water the fluid remain downside of the mixture. If specific gravity is less than one means the fluid is light and can flow over the water in a mixture. 
  • It is a unit less quantity and shown by the S.


  • The property of fluid due to which, a fluid layer which is flowing with a velocity U, exerts a resistance force on the other layer known as viscosity. 
  • It is a property which offers the resistance force in the flow. A fluid has more viscosity has less flow velocity compare to a fluid has less viscosity. For example oil has more viscosity compared to water.
  • Viscosity of a liquid increases with decreasing in temperature and viscosity of gas increases with increasing in temperature. 
  • According to newton’s law of viscosity, the shear stress is directly proportional to the velocity gradient. The constant of the proportionality is known as viscosity. Mathematically 
τ  = µ du/dy
Where µ is known as dynamic viscosity. The unit of dynamic viscosity is Pa-s or N-s/m2.
  • There are two types of viscosity used in fluid mechanics one is known as dynamic viscosity (µ) and other on is known as kinematic viscosity (ν). The kinematic viscosity is ratio of the dynamic viscosity to the density of the fluid. 
ν  = µ/ρ
  • The SI unit of kinematic viscosity is m2/s. 

Specific Weight:

  • It is defined as weight of fluid per unit volume. 
  • Mathematically, it is defined as the ratio of the weight to the volume of the fluid.  

w=  (Weight of the fluid)/(Volume of the fluid)

It can also be expressed as 
w= ρ*g
  • The SI unit of the specific weight is N/m3.

Specific volume:

  • It is the reciprocal of the density or we can say that it is the volume of the fluid per unit mass. 

Specific Volume=  (Volume of the fluid)/(Mass of the fluid)
  • The SI unit of specific volume is m3/Kg.

Vapor Pressure:

  • The pressure exerted by its vapor in phase equilibrium with its liquid at a given temperature is known as vapor pressure.  
  • The vapor pressure of the fluid is increased by increasing in temperature. 
  • It the liquid pressure drops below its vapor pressure at a given temperature, the liquid starts to evaporate. Petrol have more vapor pressure with respect atmospheric pressure at atmospheric temperature, so it starts to evaporate while water doesn’t. 
Today we have discussed about fluids properties. If you have any doubt about these properties, ask by commenting. 

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