Fluid Properties

Fluid :

       Fluid is a substance which can flow. Technically the flow of any substance means a continuous relative motion between different particles of the substance. Now, how and why does a fluid flow? The answer to "how" is the continuous relative motion between the particles of the fluid when shear force is acting on it. And the answer to "why" is the particles of fluid move past each other when shear force is applied because they cannot resist the shear force, i.e., they are inelastic to shear stress. A fluid can deform under shear stress indefinitely without returning to its original position.

        The properties outlines below are general properties of fluids which are of interest in engineering. The symbol usually used to represent the property is specified together with some typical values in SI units for common fluids. Values under specific conditions (temperature, pressure etc.) can be readily found in many reference books. The dimensions of each unit is also give in the MLT system.

1. Density:

        Density is the mass per unit volume of a fluid. In other words, it is the ratio between mass (m) and volume (V) of a fluid.

Density is denoted by the symbol ‘ρ’. Its unit is kg/m3.

      In general, density of a fluid decreases with increase in temperature. It increases with increase in pressure.

The ideal gas equation is given by:

     The above equation is used to find the density of any fluid, if the pressure (P) and temperature (T) are known.

Note: The density of standard liquid (water) is 1000 kg/m3.

2. Viscosity

     Viscosity is the fluid property that determines the amount of resistance of the fluid to shear stress. It is the property of the fluid due to which the fluid offers resistance to flow of one layer of the fluid over another adjacent layer.

      In a liquid, viscosity decreases with increase in temperature. In a gas, viscosity increases with increase in temperature.

3. Temperature:

      It is the property that determines the degree of hotness or coldness or the level of heat intensity of a fluid. Temperature is measured by using temperature scales.There are 3 commonly used temperature scales. They are :

Celsius (or centigrade) scale

Fahrenheit scale

Kelvin scale (or absolute temperature scale)

      Kelvin scale is widely used in engineering. This is because, this scale is independent of properties of a substance.

4. Pressure:

      Pressure of a fluid is the force per unit area of the fluid. In other words, it is the ratio of force on a fluid to the area of the fluid held perpendicular to the direction of the force.

Pressure is denoted by the letter ‘P’. Its unit is N/m2.

5. Specific Volume:

      Specific volume is the volume of a fluid (V) occupied per unit mass (m). It is the reciprocal of density.

Specific volume is denoted by the symbol ‘v’. Its unit is m3/kg.

6. Specific Weight:

      Specific weight is the weight possessed by unit volume of a fluid. It is denoted by ‘w’. Its unit is N/m3.

      Specific weight varies from place to place due to the change of acceleration due to gravity (g).

7. Specific Gravity:

      Specific gravity is the ratio of specific weight of the given fluid to the specific weight of standard fluid. It is denoted by the letter ‘S’. It has no unit.

     Specific gravity may also be defined as the ratio between density of the given fluid to the density of standard fluid.