A streakline is the locus of points of the all fluid particles that have passed continuously though a particular spatial point in the past.
A streakline is a Pathline that moves more than a single point through the flow. In a streakline, the entire line is moved through the flow. The result is a more realistic visualization showing how air (or a fluid) moves around the object, without the effects of drag or mass.
Engineers often use dyes in water or smoke in air to see streaklines, and then use the patterns to guide their design modifications, aiming to reduce the drag. Most drag is caused by eddies in the fluid behind the moving object, and the objective should be to allow the fluid to slow down after passing around the object and regain pressure, without forming eddies.
Streaklines are a good way to find discontinuities in a flow field. Points that suddenly become separated by an unusually large distance can easily be seen and analyzed further to find vortices, eddies, or changes in flow.
- Flow Description, Streamline, Pathline, Streakline and Timeline from "Aerodynamics for Students"
- Streamlines, Streaklines, & Pathlines on Wikipedia