Properties of Fluid | What is Fluid, Density, Specific Volume, Specific Gravity, Viscosity

Fluids are liquids or gases and the study of liquid at rest as well as in motion is called fluid mechanics. Fluid tends to take the shape of the container in which it is kept. Fluid has the property to flow. If we do the study of fluid at rest then it is called fluid statics and if we study its motion and pressure is not considered then it is called fluid kinematics, and if the pressure forces are also considered for the fluids which are in motion then this branch of fluid is called fluid dynamics.

Properties of Fluid

To understand the fluid, we need to understand the basic terminology used for fluid and the properties of the fluid. The properties of the fluid are described below:

Density or Mass Density of Fluid

The Density or Mass density of a fluid is defined as the ratio of the mass of the fluid to its volume. The units of mass density in the SI system is kg per meter cube i.e kg/m3. The density of the fluid is denoted by the symbol rho (ρ).

Mathematically, the density or mass density of the fluid is written as:

rho (ρ) = Mass of Fluid/ Volume of Fluid

The density of the fluid is considered contact while the density of the gases tends to change with the temperature and pressure.

The density of water is 1gm/cm3 or 1000kg/m3.

Specific Volume

Specific Volume of fluid is defined as the volume of a fluid per unit mass of fluid. Mathematically, it can be written as:

Specific Volume= Volume of fluid/mass of fluid = 1/(Mass of fluid/volume) = 1/ρ

Hence, we can also say that the Specific volume of a fluid is the reciprocal of the mass density of the fluid. The units in the SI system are m3/kg.

Weight Density or Specific Weight

Weight density or Specific weight of a fluid is defined as the ratio of the weight of the fluid to the volume of fluid and it is denoted by ‘w’.

Mathematically, it can be written as:

w= Weight of fluid/Volume of fluid = Acceleration due to gravity * mass of fluid/ Volume of fluid

w= (mass of fluid x g)/ Volume of fluid = ρ x g

Specific Gravity

Specific Gravity is defined as the ratio of weight density of the liquid to weight density of water in the case of liquids and weight density of the gases to weight density of air in the case of gases. It is also called Relative density and is denoted by ‘S’.

Mathematically, it can be written as:

S (for liquids) = Weight density of liquid/Weight density of water

S (for Gases) = Weight density of Gases/Weight density of Air

Viscosity

Viscosity is the property of the fluid which offers the resistance of one layer of fluid to the adjacent layer of the fluid. Viscosity is denoted by ‘μ‘.

Units of viscosity

Units of viscosity can be calculated as below:

We know, that Viscosity = Shear Stress/Shear strain

= Shear Stress / (Change of Velocity/ change of distance)

By solving this equation for units, we will get (Force x Time) / (Length)2

We can get the units in MKS and in CGS.

In MKS, force is represented by kg-f and time is represented in seconds and mass is represented in m.

So, μ = kgf-sec/m2

In CGS, force is represented by dyne and time is represented in seconds and mass is represented in cm.

So, μ = dyne-sec/cm2

Effect of Temperature on viscosity

The temperature on viscosity has a direct effect. The viscosity of gases increases with an increase in temperature while the viscosity of liquid decreases with an increase in temperature. Similarly, the viscosity of gases decreases with a decrease in temperature while the viscosity of liquid increases with a decrease in temperature.

Kinematic viscosity

Kinematic viscosity is defined as the ratio of dynamic viscosity and density of the fluid. It is denoted by ‘ν’ and called ‘nu’. Mathematically, it can be written as:

‘ν’= Viscosity/Density

Newton’s Law of viscosity

Newton’s Law of viscosity is defined as shear stress on a layer of the fluid element that is directly proportional to the rate of shear strain. Mathematically, it can be written as:

τ (Tau) = μ(du/dy)

What is the Density of water?

The density of water is 1gm/cm3 or 1000kg/m3.