Fluid Mechanics- Introduction


Fluid Mechanics is a science which deals all aspects of  fluids whether in motion or static. Fluid is a common name to liquids, vapors and gases.

Three parts of fluid mechanics

(a) Fluid statics deals with forces applied by fluids at rest. This fluid force is pressure. Pressure is a normal force exerted by a fluid per unit area. It is called the static pressure of the fluid. Some laws are used in fluid statics. These are

Pascal’s law (the pressure applied to any part of the enclosed liquid will be transmitted equally in all directions through the liquid)

Newton’s law (momentum principle) applied to a static fluid. ΣF = ma = 0 for a static fluid. i.e., ΣFx = ΣFy = ΣFz = 0. It deals with equilibrium of forces on a fluid

Buoyancy Law (Archimedes’ principle) states that a body completely or partially submerged in a fluid (liquid or gas) AT REST is acted upon by an upward buoyant force, the magnitude of which is equal to the weight of the fluid displaced by the body.

(b)Fluid Kinematics deals with fluids in motion without considering forces or energy acting. In this we study displacement, velocity and acceleration. The other name of kinematics is geometry of motion. In this we study ideal flow, real flow, in-compressible flow and compressible flow depending on the properties of the fluid. In this we also study various types of flow like laminar flow, turbulent flow, steady flow, unsteady flow, uniform flow, non-uniform flow, rotational flow, ir-rotational flow, one, two and three dimensional flows. Out of Lagrangian and Eulerian methods, Eulerian method is used as it is relatively easy to apply. It concerns velocity field and  continuity equation.

© Fluid Dynamics deals with fluids in motion with forces/energy acting on the moving fluids.

Three types of fluids

  • Liquids are in-compressible. There is no effect of pressure and temperature. Examples of liquid are water, milk, kerosene oil, petrol, fatty oils etc.
  • Vapor is compressible. Vapors have effect of pressure and temperature. It is studied only with the help of tabular data and charts.  Gas laws are not applicable to vapors. Examples of vapors are steam and refrigerants.
  • Gases are compressible. These are effected by pressure and temperature. It is studied with gas laws and universal equation. Examples of gases are air, oxygen, nitrogen, hydrogen etc.


  • Water flowing in pipe lines at home or factory
  • Refrigerant flowing in a refrigerator or air conditioner
  • Water flowing in water turbines and pumps
  • Steam flowing in pipes to steam turbines
  • Gas flowing to gas turbines
  • Blood flowing in the human body



NOTE: Why we consider the motion of a fluid particle in a fluid motion?

There is a basic difference between the motion of a solid and the motion of a fluid. A solid body is compact and moves as one element. There is no relative motion between the particles of a solid body. Thus we study the motion of the entire body and there is no necessity to study the motion of any particle of a solid body. But in fluids we consider the motion of individual particles as there is relative motion between various fluid particles.

It is done with the help of prototypes (these are small size working objects for pumps, turbines, submarines, aero-planes and dams). These prototypes have geometric similarity, dimensional similarity, kinematic and dynamic similarities. This study is carried out with empirical equations using dimensionless numbers.