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.


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