•    It is the measure of elasticity in fluid.
•    Fluids are compressed under pressure due to change in their mass density
•    More mass can be accommodated in the unit volume and when pressure is removed the fluids regains to its original volume.
•    As the pressure is increased, the volume is decreased.

dp ∝- dv / V

dp = K x -dv / V

K = dp / (-dV / V)

Where K is the constant of proportionality is known as bulk modulus.




•    If K is more, dv/v is less indicating less volume change i.e. less compressibility. That is why compressibility expressed as inverse of bulk modulus.

•    Higher the bulk modulus less is the compressibility of fluid

•    Liquids are generally considered to be incompressible. For instance, a pressure of 113078 kPa will cause a given volume of water to decrease by only 5% from its volume at atmospheric pressure.

•    Gases on the other hand, are very compressible. The volume of a gas can be readily changed by exerting an external pressure on the gas.

•    Water hammer and cavitations are the examples of the importance of compressibility effects in liquid flows.

•    Compressibility effects are very important in the design of modern high-speed aircrafy, missile power plants, fans and compressors

•    It is the ratio of compressive stress to volumetric strain. It si denoted by ‘K’.

K = Increased in pressure / Volumetric strain

K = dp / -(dV / V)

V = Volume of gas enclosed in cylinder
dv = Change in volume
dp = Change in pressure

•    Negative sign indicates decrease in volume with an increase in pressure.
•    Value of K; Water is 2.05 x 10⁹ N/m² , Air is 1.02 x 10⁵ N/m²

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