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# Properties of Fluid | QnA | Fluid Mechanics | By Ashutosh Nautiyal

What is Density or Mass Density? Density or Mass Density of a fluid is defined as the ratio of the mass of fluid to its volume. It is denoted by ρ.

## Properties of Fluid Properties of Fluid

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## Describe Density or Mass Density.

• Density or Mass Density of a fluid is defined as the ratio of the mass of a fluid to its volume.

• It is denoted by ρ (rho).

• Its SI unit is kg/m3.

Mass of fluid

•   ρ  =  ______________

Volume of fluid

• It is also known as Specific Mass of Fluid.

• It increases with increase in pressure.

• For water, it decreases with increase in temperature and it is maximum at 4 °C.

• The Mass Density of water is taken as 1000 kg/m3 at 4 °C.

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## Describe Specific Weight or Weight Density.

• Specific Weight or Weight Density of fluid is defined as the ratio of the weight of a fluid to its volume.

• It is denoted by w.

• Its SI unit is N/m3.

Weight of fluid                Mass of fluid * Acceleration due to gravity

• w =  ________________   =   _______________________________________

Volume of fluid                                 Volume of fluid

• w = Mass density * Acceleration due to gravity

• Hence, w =  ρ * g

• It depends upon mass density and acceleration due to gravity.

• Since gravitational acceleration varies from place to place, therefore, specific weight also varies from place to place.

• It increases with increase in pressure.

• For water, it decreases with increase in temperature and it is maximum at 4 °C.

• The Specific Weight of water is taken as 9810 N/m3 at 4 °C.

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## Describe Specific Volume.

• Specific Volume of a fluid is defined as the ratio of the volume of fluid to the mass of fluid.

• It is the reciprocal of Mass Density.

• It is denoted by v.

• Its SI unit is m3/kg.

Volume of fluid                   1                         1

• v = _______________  =  ______________   =  ___

Mass of fluid              Mass Density             ρ

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## Describe Specific Gravity.

• Specific Gravity is defined as the ratio of the weight density (or density) of fluid to the weight density (or density) of a standard fluid.

• For liquids, the standard fluid is taken as water.

• For gases, the standard fluid is taken as air.

• It is denoted by S.

• It is a dimensionless quantity.

Weight density (or density) of liquid

• S (for liquids) =  _________________________________

Weight density (or density) of water

Weight density (or density) of gas

• S (for gases) =  ________________________________

Weight density (or density) of air

• It is also known as Relative Density.

• The specific gravity of water may be taken as unity for all practical purposes.

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## Describe Vapour Pressure.

• Vapour Pressure of a liquid is the partial pressure exerted on its surface by the vapours of the same liquid which gathers into the container containing the liquid, above the surface of the liquid due to evaporation.

• It increases with increase in temperature as an increase in temperature will evaporate more vapours which will exert more vapour pressure.

• It will go on increasing until the boiling point of the liquid is reached when it will be equal to the external pressure on the liquid surface.

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## Describe Cohesion.

• Cohesion is the property of liquid by the virtue of which it can withstand slight tension.

• This property is due to the intermolecular attraction between the molecules of the liquid.

• The property of surface tension is also due to cohesion.

• Due to the property of cohesion, the droplet of water hanging down the tap keeps its entity together.

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• Adhesion is the property of liquid by the virtue of which it sticks or adheres to the solid body with which it is in contact.

• This property is due to forces of attraction between the molecules of the liquid and the molecules of the solid body.

• The property of adhesion is exhibited by a droplet of water before falling from the tip of the finger.

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## Describe Surface Tension.

• Surface tension is the property of liquid by virtue of which the free surface of the liquid acts as a stretched elastic membrane capable of bearing a slight amount of tension.

• This property is due to the cohesive forces between the molecules of the liquid.

• Mathematically, surface tension is the ratio of the surface force F to length L along which the force acts.

• It is denoted by symbol σ.

• Its SI unit is N/m.

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## Describe Capillarity.

• Capillarity is the phenomenon by which a liquid surface rises up or falls down in a thin glass tube in comparison to the general liquid level in the vessel when the glass tube is dipped into the mass of liquid.

• The rise of the liquid surface is known as capillary rise.

• The fall of the liquid surface is known as capillary depression.

• It is generally expressed in terms of mm or cm of liquid.

• Its value depends upon the specific weight of the liquid, the diameter of the tube and surface tension of the liquid.

• This phenomenon is due to the combined effect of cohesion and adhesion of liquid particles.

• If the cohesion between the liquid particles is less than the adhesion with the glass tube, then there will be a capillary rise and the convexity of the surface of the liquid in the tube will be downward.

• If the cohesion between the liquid particles is more than the adhesion with the glass tube, then there will be a capillary depression and the convexity of the surface of the liquid in the tube will be upward.

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## Describe Compressibility.

• Compressibility is the property of a fluid by virtue of which the fluid undergoes a change in volume under the action of external pressure.

• It is expressed as the reciprocal of Bulk Modulus of Elasticity (K).

• Compressibility = 1/K

• The SI unit of Compressibility is m2/N.

• For practical or numerical purposes, water is considered as an incompressible fluid as the variation in the volume of water with the variation of pressure is so small.

*Self Typed
*Source- Books, Internet, Self-Analysis
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