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Basic Fluid properties that every one should know

Various fluids can be differentiated by their fluid properties, some of the basic fluid properties are density, specific weight, specific gravity, viscosity & specific volume. Let's take a look at the properties of fluids one by one in detail. Fundamental knowledge of fluid properties is very essential to learn Fluid Mechanics (FM) subject.


Properties of fluid

  1. Density or Mass Density
  2. Specific Weight or Weight Density
  3. Specific Volume
  4. Specific Gravity
  5. Viscosity

1. Density or Mass Density

  • The density of the fluid can be defined as a ratio of the mass of fluid to the volume of fluid. Hence it can be call mass per unit volume.
  • It is represented by a symbol ⍴ (rho) and the SI unit of density is kg/m³.
  • Mathematically,  
  • The density of water is taken as 1000 kg/m³ or 1 g/cm³.
  • The density of a liquid is considered as constant with the variation of pressure and temperature.

2. Specific Weight or Weight Density

  • The specific weight of fluid can be defined as the ratio of the weight of the fluid to the volume of fluid.
  • It is represented by w  and also called weight density and the SI unit is N/m³.
  • Mathematically, 
     
  • The value of specific weight or weight density of water is 9.81* 1000 N/m³.

3. Specific Volume

  • The specific volume of fluid can be defined as the volume of the fluid occupied by a unit mass.
  • Hence, it is reciprocal of density and it's SI unit is m³/kg.
  • Mathematically, 


4. Specific Gravity

  • Specific gravity is defined as the ratio of the weight density of the fluid (or density) to the weight density (or density) of standard fluid.
  • For liquid, water is taken as standard fluid.
  • For gas, the air is taken as standard fluid.
  • Specific gravity is also called relative density. as specific gravity is a ratio of two density, it is a dimensionless quantity and it is represented by S. 
  • Mathematically,

  • Example: Density of Mercury is 13600  kg/m³, hence sp. gravity of mercury = 13.6 

5. Viscosity

  • Viscosity (or dynamic viscosity) is defined as the property of the fluid that offer resistance to the movement of one layer of fluid over another adjacent layer of fluid.
  • Viscosity is represented by μ and its SI unit is N.s/m².
  • For liquid, the viscosity of a fluid is due to inter-molecular attraction force.
  • For gas, the viscosity of a fluid is due to the random motion of molecules.
  • As per Newton's law of viscosity, shear stress between two fluid layers is proportional to the rate of angular deformation. Hence, 
Fig.-Velocity distribution near a solid boundary
  • Mathematically, 
        where, τ is shear stress between two layers of fluid, and μ is constant of proportionality and called viscosity.
  • From the above equation, viscosity also defined as the shear stress required to produce a unit rate of shear strain.
  • Another unit of viscosity is poise. 10 poise = 1 N.s/m².




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