Introduction
In fluid mechanics, the study of fluid flow is important to understand the behavior and characteristics of fluids. One way to analyze fluid flow is by studying the paths followed by fluid particles. These paths can be classified into different types, such as pathlines, streamlines, streaklines, and streamtubes. Each type represents a different aspect of fluid flow.

Pathline
A pathline represents the actual path followed by a specific fluid particle over a period of time. It is a continuous curve that traces the movement of a particle as it flows through a fluid. Pathlines can be visualized as the trajectory or history of a fluid particle. They can be observed by tracking a specific fluid particle using dye or other tracers. Pathlines provide information about the motion and displacement of individual fluid particles.

Streamline
A streamline is a line that is instantaneously tangent to the velocity vector of a fluid flow at every point. It represents the direction and magnitude of the flow velocity at a specific instant in time. Streamlines are used to visualize the flow pattern and direction of a fluid. They provide information about the flow velocity and the paths followed by fluid particles at a given moment.

Streakline
A streakline represents the locus of fluid particles that have passed through a specific point in space at different times. It is formed by connecting all the particles that have passed through the point at different instants. Streaklines provide information about the history and past motion of fluid particles. They are often observed by injecting a continuous stream of dye or smoke into the flow and capturing the resulting pattern.

Streamtube
A streamtube is a collection of streamlines that encloses a certain volume of fluid. It represents a three-dimensional flow channel that guides the movement of fluid particles. Streamtubes can be visualized as tubes or pipes through which the fluid flows. They provide a comprehensive view of the flow behavior within a specific region.

Conclusion
In the context of the smoke coming out of a chimney in concentric circles, the path followed by the smoke particles represents a streakline. This is because the smoke particles are released at different times and form a pattern that traces their past motion. It is important to understand the different types of fluid flow paths in order to analyze and predict the behavior of fluid flows in various engineering applications.

Picture of a fluid flow