Streak Lines
Definition: A streak line is the locus of the temporary locations of all particles that have passed though a fixed point in the flow field at any instant of time.
Features of a Streak Line:
- While a path line refers to the identity of a fluid particle, a streak line is specified by a fixed point in the flow field.
- It is of particular interest in experimental flow visualization.
- Example: If dye is injected into a liquid at a fixed point in the flow field, then at a later time t, the dye will indicate the end points of the path lines of particles which have passed through the injection point.
- The equation of a streak line at time t can be derived by the Lagrangian method.
If a fluid particle 
passes through a fixed point 
in course of time t, then the Lagrangian method of description gives the equation
passes through a fixed point in course of time t, then the Lagrangian method of description gives the equation | (7.5) |
Solving for 
,
, | (7.6) |
If the positions of the particles which have passed through the fixed point are determined, then a streak line can be drawn through these points.
of the particles which have passed through the fixed point are determined, then a streak line can be drawn through these points.
Equation: The equation of the streak line at a time t is given by
 | (7.7) |
Substituting Eq. (7.5) into Eq. (7.6) we get the final form of equation of the streak line,
 | (7.8) |
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