In this section, the concepts of a streamline, streakline and pathline will be introduced.

    Streamline, Streakline and Pathline



In the study of fluid mechanics, streamlines are often drawn to visualize the flow field. At every point in the flow field, a streamline is tangent to the velocity vector. That is, for 2D flow in Cartesian coordinates,


The above equation can be integrated to give the equation of a streamline once the velocity field (i.e., u and v) is known. For every streamline, there is a stream function (Ψ) associated with it. The definition of the stream function will be covered in differential Conservation of Mass section.



A streakline consists of all fluid particles that have previously passed through a specified point. This is essentially the same as injecting dye or smoke continuosly at a given location and observing how the dye or smoke moves along with the fluid motion. This is a technique that is often used in experiments to visualize the flow field.

On the other hand, the actual path that a single fluid particle takes is referred to as the pathline, i.e., it is the trajectory of a particular fluid particle. This is referred to as the Lagrangian viewpoint of the flow field. Experimentally, it can be achieved by tagging a fluid particle and tracing its motion throughout the flow field.

In general, a streamline, streakline and pathline are not the same; however, they coincide when the flow is steady.

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