Difference between Streak Line and Path Line
Fig 7.4 Description of a Streak line
Above diagram can be described by the following points:
Describing a Path Line:
a) Assume P be a fixed point in space through which particles of different identities pass at different times.
b) In an unsteady flow, the velocity vector at P will change with time and hence the particles arriving at P at different times will traverse
different paths like PAQ, PBR and PCS which represent the path lines of the particle.
Describing a Streak Line:
a) Let at any instant these particles arrive at points Q, R and S.
b) Q, R and S represent the end points of the trajectories of these three particles at the instant.
c) The curve joining the points S, R, Q and the fixed point P will define the streak line at that instant.
d) The fixed point P will also lie on the line, since at any instant, there will be always a particle of some identity at that point.
Above points show the differences.
a) For a steady flow, the velocity vector at any point is invariant with time
b) The path lines of the particles with different identities passing through P at different times will not differ
c) The path line would coincide with one another in a single curve which will indicate the streak line too.
Conclusion: Therefore, in a steady flow, the path lines, streak lines and streamlines are identical.