Newtonian fluid is the fluid whose viscosity does not change with the rate of deformation. Let's understand this concept in detail.

Definition of Newtonian Fluid:
A Newtonian fluid is a type of fluid where the shear stress is directly proportional to the rate of shear strain. In other words, the viscosity of a Newtonian fluid remains constant regardless of the rate of deformation or shear rate.

Explanation:
Viscosity is a measure of a fluid's resistance to flow. In most fluids, the viscosity changes with the rate of deformation. However, in the case of Newtonian fluids, the viscosity remains constant at all shear rates. This means that the fluid's behavior does not change whether it is sheared quickly or slowly.

The behavior of Newtonian fluids can be described by Newton's law of viscosity, which states that the shear stress (τ) is directly proportional to the velocity gradient (du/dy) in the direction perpendicular to the flow.

τ = μ (du/dy)

Where:
- τ is the shear stress
- μ is the dynamic viscosity
- (du/dy) is the velocity gradient

Key Characteristics of Newtonian Fluids:
1. Constant viscosity: The most important characteristic of Newtonian fluids is that their viscosity remains constant regardless of the shear rate or rate of deformation.

2. Linear relationship between shear stress and shear rate: Newtonian fluids exhibit a linear relationship between shear stress and shear rate, as described by Newton's law of viscosity.

3. Examples of Newtonian fluids: Some common examples of Newtonian fluids include water, air, gasoline, and most oils.

Importance in Engineering:
The concept of Newtonian fluids is widely used in engineering applications. It helps engineers predict and analyze the flow behavior of fluids in various systems, such as pipes, channels, and pumps. Understanding the constant viscosity of Newtonian fluids is crucial for designing efficient systems and ensuring proper fluid flow.

Conclusion:
In conclusion, a Newtonian fluid is the type of fluid whose viscosity remains constant regardless of the rate of deformation or shear rate. This characteristic makes it distinct from other types of fluids, such as non-Newtonian fluids, which exhibit variable viscosity with changing shear rates.