Properties of Fluid [Notes with PDF]

A fluid is a substance that has the tendency to flow. The properties of fluid include Density or Mass Density, Weight Density or Specific Weight, Specific Volume, Specific Gravity, and Viscosity.

Definition of Fluid:

A fluid is a substance that deforms continuously (change in shape due to relative motion) under the action of shear force. Example: liquid and gases. Fluid is also called as zero-memory substance.

Properties of Fluids:

There are five properties of fluids and those are:

1. Density or Mass Density
2. Weight Density or Specific Weight
3. Specific Volume
4. Specific Gravity
5. Viscosity

1. Density or Mass Density:

The mass density or density of a fluid is defined as the ratio of the mass of a fluid to the the volume of the fluid. Density is also called a Mass per unit volume of a fluid. Density is denoted by symbol ρ (rho) and the unit of mass density is (kg/m³).  The density of liquid may be constant but the density of gases changes with the variation of temperature and pressure. The Density of water is 1000 kg/m³ or we can say 1 g/cm³).

Mathematically,

Following are the densities of some common fluids :

• At 4⁰ density of H₂O is 1000 kg/m³ which is maximum.
• As T increases the molecule will move away and hence the density increases.

2. Weight Density or Specific Weight:

Weight density or the specific density of a fluid is defined as the ratio of the weight of the fluid to the volume of the fluid. Weight density is also called as the Weight per unit volume of a fluid. This is denoted by the symbol ‘w’ and the unit of mass density is (N/m³). 

Mathematically,

w = (Weight of Fluid) / (Volume of fluid) = (Mass of fluid) * (Acceleration due to gravity) / (Volume of fluid)

As we know from the previous formula of Density, we get : w = ρ * g

• w depends on ρ and g while ρ is dependent on T and P, So
• w=f(T, P, location(g))

The value of weight density or specific weight  for water is 9.81*1000 N/m³

3. Specific Volume:

The specific volume of a fluid is defined as the ratio of the volume of fluid to the mass of fluid. The volume of a fluid occupied by a unit mass or volume per unit mass of a fluid is also called Specific volume. The unit of Specific volume is m³/kg. This quantity is commonly applied to Gases.

Mathematically,

Specific Volume = (Volume of fluid) / (Mass of fluid)

When we divide by volume of fluid to the Numerator and Denominator the fraction we get is : = (1) / (ρ)

Hence we can see that specific volume is the reciprocal of Mass Density.

4. Specific Gravity:

Specific Gravity is defined as the ratio of the Density or weight density of a fluid to the density or weight density of a standard fluid. We know that for liquid water is a standard fluid and. For gases, the Standard fluid is taken as Air. The specific gravity is also called Relative Density. This is denoted by the symbol ‘S‘ and this is a dimensionless quantity.

Mathematically,

S (liquid) = ( Weight Density of liquid)/(Weight Density of water)

S (Gases) = ( Weight Density of Gases / (Weight Density of Air)

Now,

Weight density of liquid  = S * Weight Density of water =  S * 9.81 * 1000 N/m³

Density of liquid = S * Density of water = S * 1000 kg/m³

5. Viscosity:

Viscosity is defined as the resistance to flow deformation. It is an internal resistance between two particles. The reciprocal of the viscosity is called fluidity.

Mathematically, viscosity is the measure of internal resistance between two adjacent fluid particles in motion. Unit is 1/s

dθ is angular deformation.

dθ/dt is the rate of angular deformation to the rate of shear strain or strain rate.

du/dy is the velocity gradient.

FAQ’s:

Conclusion:

In conclusion, Fluids are type of substance that can take the space of the container they are in. They are of various types and understanding each one is important for various industries such as Engineering, Physics, Biology etc.

References [External Links]:

• Fluid | Definition of Fluid by Merriam-Webster
• What are Fluids? – Edinformatics