Streamlines, streaklines and pathlines

Summary

Streamlines, streaklines and pathlines are three families of curves that picture a flow. A streamline is tangent to the velocity at one instant. A pathline is the track of one fluid parcel. A streakline joins all parcels that passed through one fixed point. In a steady flow the three families coincide; in an unsteady flow they differ. They are basic tools of flow visualization and of the kinematics of fluids, linking the Lagrangian and Eulerian descriptions of a flow.