1. What is Fluid ?
Fluid is a substance that continuously deforms (flows) when subjected to shear stress, no matter how small the force is. Fluids include liquids and gases.
Examples: Water, air, oil, steam.
Fluid properties help engineers analyze fluid flow, pressure, heat transfer, and hydraulic systems.
Table of Contents
2. What is Density (Mass Density) ?
Density is described as the mass per unit volume of a fluid.Unit
SI Unit = kg/m³
Importance
- Determines buoyancy.
- Used in flow and pressure calculations.
- Important in hydraulic machines.
Example
Density of water ≈ 1000 kg/m³
3. Specific Weight (Weight Density)
Definition
Specific weight (γ) is defined as the weight per unit volume of a substance (fluid). Also can be represented as Specific weight (γ) = Density X Acceleration due to gravity.
Where
ρ = density
g = acceleration due to gravity
Unit
N/m³
Application
- Used in pressure and head calculations.
- Important in pumps and turbines.
4. Specific Gravity (Relative Density)
Definition
Specific gravity is defined as the ratio of the density of a substance to the density of a reference substance (usually water at 4°C for liquids).Characteristics
- Dimensionless.
- Used in petroleum and chemical industries.
Example: Oil SG ≈ 0.8
5. Viscosity
Definition
Viscosity is the property of a fluid that offers resistance to flow or internal resistance between adjacent layers of fluid. In simple word, Viscosity is the internal friction of a fluid.
Newton’s Law of Viscosity
Where:
- τ = Shear stress
- μ = Dynamic viscosity
- du/dy = Velocity gradient
Shear stress is directly proportional to velocity gradient. Two Types of Viscosity (a) Dynamic Viscosity (b) Kinematic Viscosity.
Types of viscosity
(a) Dynamic Viscosity
Where
μ = dynamic viscosity.
Unit : Pascal-second (Pa·s).
(b) Kinematic Viscosity
Unit = m²/s
Importance
- Lubrication systems.
- Pipe flow.
- Oil and fuel design.
Example
Honey → high viscosity
Water → low viscosity
6. Compressibility
Definition
Ability of fluid to change volume under pressure. Characteristics
- Liquids → low compressibility.
- Gases → high compressibility.
Application
- Gas dynamics.
- High-pressure systems.
7. Surface Tension
Definition
Surface tension is a physical property of liquids that allows their surface to resist an external force, behaving as if it were covered by a stretched elastic membrane.
It is essentially the result of cohesion, where liquid molecules are more attracted to each other than they are to the air or gases surrounding them.
In classical mechanics, surface tension (γ) is the force (F) acting parallel to the surface and perpendicular to a line of unit length (L).
Unit : Newton/Meter (N/m)
Cause
Due to cohesive forces between liquid molecules.
Effects
- Soap bubble formation.
- Formation of droplets.
- Capillary action.
8. What is Capillarity ?
Capillarity (or capillary action) is the phenomenon by which a liquid rises or falls in a narrow tube (called a capillary tube) due to the combined effect of surface tension, adhesion, and cohesion forces.
Factors
- Surface tension.
- Tube diameter.
- Contact angle.
Example
Water rises in glass tube.
How Capillarity Works
Capillary action happens because of two main forces:
1. Adhesion
It is the attraction between liquid molecules and solid surface molecules.
Example: Water sticking to the glass wall.
2. Cohesion
It is the attraction between liquid molecules themselves.
Example: Water molecules attracting each other.
Capillary Rise/Depress Formula
The height of rise is given by:Where:
- h = height of liquid rise or depress
- σ (sigma) = surface tension
- θ = contact angle
- ρ = density of liquid
- g = acceleration due to gravity
- r = radius of tube
Types of Capillary Effect
1. Capillary Rise
When the liquid rises in a narrow tube.
Example:
- Water in a thin glass tube
- Ink rising in a pen
- Water in plant roots
Reason: Adhesion force is stronger than cohesion.
2. Capillary Depression
When the liquid level in a narrow tube is lower than the outside liquid level.
Example:
- Mercury in a glass tube
Reason: Cohesion force is stronger than adhesion.
9. Vapour Pressure
Definition
Pressure exerted by vapour in equilibrium with liquid.
Importance
- Cavitation in pumps.
- Boiling point of fluids.
Example
Water vapour pressure increases with temperature.
10. Compressibility vs Elasticity
Elasticity in fluids means ability to return to original volume after pressure removal.
Liquids → nearly incompressible
Gases → highly elastic.
11. Newtonian and Non-Newtonian Fluids
Newtonian Fluid
Viscosity remains constant.
Example: Water, air.
Non-Newtonian Fluid
Viscosity changes with shear rate.
Examples: Blood, toothpaste.
12. Temperature Effect on Fluid Properties
- Liquid viscosity decreases with temperature.
- Gas viscosity increases with temperature.
- Surface tension decreases with temperature.
13. Importance in Mechanical Engineering
Fluid properties are used in:
- Hydraulic machines.
- Refrigeration and air conditioning.
- Power plants.
- Automotive cooling systems.
- Solar refrigeration (like your project idea).
- Heat exchangers and boilers.
14. Summary Table
| Property | Unit | Importance |
|---|---|---|
| Density | kg/m³ | Flow and buoyancy |
| Viscosity | Pa·s | Resistance to flow |
| Surface tension | N/m | Droplets and capillarity |
| Specific gravity | – | Comparison of fluids |
| Vapour pressure | Pa | Cavitation |