Fluid mechanics is a branch of physics that deals with the study of fluids, which are substances that can flow and change shape when subjected to external forces. It encompasses both liquids and gases and explores how these substances behave under various conditions and interactions.

Fluid mechanics involves the analysis of fluid properties such as velocity, pressure, density, and temperature, and how they change in response to external forces like gravity, friction, and applied mechanical forces. It also examines the forces that fluids exert on solid surfaces when they come into contact with each other.

The field of fluid mechanics is essential in various engineering disciplines, including aerospace, civil, mechanical, and chemical engineering. It plays a crucial role in understanding and designing systems such as pumps, turbines, pipelines, and aircraft, as well as in applications like weather prediction, oceanography, and environmental studies.

### Fluid Mechanics Fundamentals

Fluid mechanics encompasses the study of both liquids and gases, but there are fundamental differences between the two. While many laws of fluid mechanics apply to both liquids and gases, liquids are generally considered to be nearly incompressible compared to gases.

In liquids, the volume remains relatively constant when pressure is applied, making them nearly incompressible. This means that a given mass of liquid will maintain its volume even when subjected to external pressure. On the other hand, gases are much more compressible compared to liquids. When pressure is applied to a gas, its volume can change significantly, leading to changes in density and other properties.

The focus of interest in fluid mechanics is often centered around gases, particularly in contexts like atmospheric flight where air is the primary fluid of concern. Air, which is composed mostly of nitrogen (about 78%) and oxygen (about 21%), is a compressible fluid with certain characteristics that vary depending on conditions such as pressure, temperature, and velocity.

While air is generally considered to be compressible, under certain conditions, such as low flight velocities, it can be treated as incompressible for simplification purposes. This simplification allows for the application of certain principles and equations that are commonly used in the analysis and design of aerospace systems and other engineering applications involving fluid flow.

#### Gas:

A gas is composed of countless molecules in constant, random motion. Each molecule possesses its own unique velocity, position, and energy, which change as a result of collisions with other molecules. When these molecules collide with a surface, they exert a force per unit area, which we perceive as pressure.

As long as the distance between molecules is significant enough that the forces of attraction between them are negligible, the gas behaves as a continuous substance with properties determined by statistical averages of the individual particle effects. This type of gas is often referred to as a perfect gas.

In a perfect gas, the behavior of the gas is described by certain idealized relationships, such as the ideal gas law, which relates pressure, volume, temperature, and the number of molecules present. These relationships provide a useful framework for understanding and predicting the behavior of gases under various conditions, making them valuable tools in fields like fluid mechanics, thermodynamics, and aerospace engineering.

#### Stream line:

In fluid dynamics, particularly in the context of airflow, the concept of streamlines plays a crucial role in understanding the behavior of fluids. Streamlines represent the path followed by individual particles within the fluid as it flows under certain patterns and within a defined space, such as the Earth’s atmosphere.

At any given instant in time, streamlines are visualized as curves that are tangent to the velocity vector of the fluid flow at each point. In simpler terms, if you were to place a small object in the fluid, the path it would follow would trace out a streamline. These streamlines provide a visual representation of the flow patterns and directionality of the fluid at different points within its domain.

One fundamental characteristic of streamlines is that they cannot intersect or cross each other, except at points where the velocity of the fluid is zero. If streamlines were to cross, it would imply that a single point in the fluid has two different velocities simultaneously, which is physically impossible within the constraints of fluid dynamics.

#### Stream tube:

In fluid dynamics, a stream tube is a conceptual construct that helps visualize the flow of a fluid, particularly in situations where streamlines are involved. It is defined as the collection of all streamlines that pass through a closed curve at a specific moment in time.

To better understand the concept, imagine enclosing a region of fluid flow with a closed curve. The stream tube represents the space within this enclosed region where the fluid particles are moving along the streamlines that intersect the curve. In other words, if you were to trace the paths of individual fluid particles passing through this closed curve, their trajectories would define the boundaries of the stream tube.

To understand about What Fluid Mechanics is?, you need to read following articles one by one.

- What is Continuity Equation – Derivation, Examples, Formula.
- What is Momentum Equation? Derivation, Formula.
- What is Euler Equation? Derivation, Formula.
- What is Bernoulli effect? Bernoulli Equation, Formula & Derivation.
- What is Viscosity? Viscosity Stress Calculation.
- What is Boundary layer in Aerodynamics?
- What is Reynolds number? How to Calculate it?
- What is the Speed of Sound? Formula, Factors, Basic Understanding, Use Casses.
- What is Mach number? Basic Understanding.