Short Answer:

A fluid is a substance that can flow and does not have a fixed shape. It takes the shape of the container in which it is kept. Fluids include both liquids and gases. The main characteristic of a fluid is that it continuously deforms when any external force or shear stress is applied to it, no matter how small the force is.

In simple words, fluids are materials that can move or flow easily. Examples of fluids are water, air, oil, and steam. Unlike solids, fluids cannot resist deformation, which means they keep changing shape as long as the force is applied.

Detailed Explanation :

Fluid

A fluid is a substance that can flow and does not have a definite shape. It can easily change its shape according to the container that holds it. Fluids include both liquids and gases because both of them have the ability to move and flow freely under the influence of external forces. The most important property of a fluid is that it cannot resist shear stress. Even a very small shear stress will cause continuous deformation in a fluid.

In mechanical engineering, the study of fluids and their behavior is very important because fluids are used in various applications such as hydraulics, lubrication, energy transfer, and aerodynamics. The science that deals with the behavior of fluids at rest and in motion is called fluid mechanics.

Characteristics of a Fluid

1. Ability to Flow:
   Fluids can flow or move from one place to another. This is why both liquids and gases are called fluids. Liquids flow under gravity, and gases can flow freely in all directions.
2. No Fixed Shape:
   A fluid does not have a fixed shape. It takes the shape of the container in which it is kept. For example, water in a glass takes the shape of the glass, and air fills the entire volume of a balloon.
3. Has Volume:
   Liquids have a definite volume but not a definite shape, whereas gases have neither a definite shape nor a definite volume. Gases expand to occupy the available space.
4. Deformation Under Shear Stress:
   When a shear stress (a force applied tangentially) acts on a solid, it deforms only up to a certain limit. But when the same stress acts on a fluid, it continues to deform as long as the force is applied. This continuous deformation is called flow.
5. Pressure at a Point:
   In a fluid at rest, the pressure at a point acts equally in all directions. This is one of the important properties of fluids that is used in hydraulic systems and fluid mechanics principles.
6. Density and Viscosity:
   Fluids are characterized by properties such as density (mass per unit volume) and viscosity (resistance to flow). These properties help determine how a fluid will behave under different conditions.

Types of Fluids

Fluids can be classified based on their behavior under applied forces or shear stress. Some common types are:

1. Ideal Fluid:
   An ideal fluid is an imaginary fluid that is incompressible and has no viscosity. It offers no resistance to shear stress. It is used only for theoretical analysis because no real fluid behaves ideally.
2. Real Fluid:
   A real fluid has viscosity and can be compressed slightly. All practical fluids like water, oil, and air are real fluids.
3. Newtonian Fluid:
   A Newtonian fluid follows Newton’s law of viscosity, which means that the shear stress is directly proportional to the rate of shear strain. Examples include water, air, and kerosene.
4. Non-Newtonian Fluid:
   These fluids do not follow Newton’s law of viscosity. The relationship between shear stress and shear strain rate is not linear. Examples include toothpaste, blood, and polymer solutions.
5. Compressible and Incompressible Fluids:
   If the density of a fluid changes significantly with pressure, it is called a compressible fluid (like gases). If the density does not change much with pressure, it is called an incompressible fluid (like liquids).

Behavior of Fluids Under Force

When a fluid is subjected to an external force, it behaves differently from a solid. In a solid, the applied force causes a finite deformation, and when the force is removed, the solid returns to its original shape. However, in a fluid, even a small tangential force causes continuous deformation, meaning the fluid keeps flowing as long as the force acts on it.

This behavior of fluids can be explained using shear stress and shear strain. Shear stress is the tangential force per unit area, and shear strain is the angular deformation caused by this force. In fluids, the rate of shear strain depends on the viscosity of the fluid.

Examples of Fluids

Fluids are present everywhere in our surroundings. Some common examples include:

• Water: Used in hydraulic systems and cooling systems.
• Air: Used in pneumatic systems, aerodynamics, and ventilation.
• Oil: Used for lubrication and energy transmission in hydraulic machines.
• Steam: Used in turbines and heating systems.
• Fuel: Like petrol or diesel, which flow through pipelines and injectors in engines.

Importance of Fluids in Engineering

Fluids are an essential part of mechanical and civil engineering systems. Engineers use the properties of fluids to design pumps, turbines, compressors, pipelines, and hydraulic machines. The knowledge of fluid properties helps in predicting pressure losses, flow rates, and energy conversions in different systems.

For example:

• In hydraulic systems, oil is used as the working fluid to transfer energy.
• In aerospace engineering, air is studied to understand the forces on aircraft wings.
• In automobile engines, fuel and air mixtures are analyzed to improve combustion and efficiency.

Thus, the study and understanding of fluids form the foundation for many engineering designs and operations.

Conclusion

In conclusion, a fluid is any substance that can flow and cannot resist shear stress. It includes both liquids and gases. Fluids have no definite shape and can easily change form according to their container. The behavior of fluids under various conditions helps engineers design machines and systems that work efficiently with liquids and gases. Understanding fluids is essential for studying mechanical systems, hydraulic machinery, and natural phenomena involving motion and pressure of fluids.