Lecture - 1 ( basics of fluid )

Fluids

The fluid is a substance that is having the ability to flow or deform continuously under the action of shear force, no matter how much small the force is. All types of liquids and gases are known as fluids. Examples are water, Mercury, steam, air. If fluid is static or stationary then it means no shear force acting on it.

In the above diagram initially when fluid is static we consider some molecules of fluids which are in straight line. After that when shear force is applied, for the time interval 't1' those molecules change their position with angle 'θ1' except one molecule which is at boundary because we are considering no slip condition. In upcoming lectures we study about no slip conditions. Similarly after time interval 't2' molecules change it's position by 'θ2'. 'Patm ' is the atmospheric pressure acting on surface of fluid.

Difference between solids and fluids

  • In case of solids the deformation is constant with respect to time whereas in case of fluids the deformation is continuously varying with respect to time and hence in solids Deformation is important, in fluids rate of change of defamation 'dθ/dt' is more important.
  • In case of solids after removing the load the solids will try to regain their original position whereas Fluids will never try to regain their original position.
  • In case of solids all types of deformations are possible like torsion, bending, longitudinal and volumetric deformations whereas in fluids only angular deformation is possible.
  • In solids, it will show resistance to all types of loads whereas Fluids will show resistance to only compressive loads. 

Cohesion and adhesion 

  • Cohesion is the intermolecular forces of attraction between molecules of same nature. Example - when water is in contact with plastic will show Cohesion more, when mercury is in contact with glass it will show Cohesion more.
  • Adhesion is the intermolecular forces of attraction between molecules of different nature. Example - water in contact with glass will show adhesion more. 
(From above example we can conclude that Cohesion and adhesion are dependent on nature of surfaces in contact)

Properties of fluids 

  • Density (ρ)
  • Specific weight (w)
  • Specific gravity (S)
  • Compressibility (β)
  • Viscosity
  • Surface tension (σ)
  • Capilarity
We will study detailed explanation of each property in our next lectures

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