A line is drawn and marked following fluid particles show the streak line. It concentrates on every single particle to highlight the streak line of each and every particle as shown in Fig. 4.3

Let us consider a fixed point A at time t_{1} is in the fluid flow. Three particles P_{1}, P_{2} and P_{3} follows unsteady flow and traced new paths after crossing A to reach B, C and D respectively at time t_{2}. Now, draw line join AB, AC and AD to make streak lines.

From Lagrangian method, if the fluid particle passes through any fixed point is given by,

= S (_{0} , t)

So, the equation of streak line can be written as-

At time t,

= *f* (_{0} , t)

Or,

=*f*[F(_{1} , t), t]

It shows the equation for streak line w.r.t fixed point.