Difference Between Stream Line, Streak Line, and Path Line

Contents

Stream Line	Streak Line	Path Line
It is an imaginary line showing the positions of various fluid particles.	It is a real line showing instantaneous positions of various particles.	It is a real line showing the successive position of one particle.
Particles may change streamline depending on the type of flow	May change from instant to instant.	Particles may cross its path line.
Streamlines cannot intersect with each other, they are always parallel.	Streak line changes with time. Two streak lines may intersect each other.	Two path lines for two particles may intersect each other.
No flow across streamline.	Flow across the streak line is possible.	Flow across a path line is possible by other particles.

A stream tube is an imaginary tube consists of streamlines, forming its boundary surface, which does not permit the fluid across it.
Stream tube may be of regular or irregular shape.
As stream tube is bounded by streamlines, therefore no fluid can enter or leave the stream tube from its boundary. Hence stream tube behaves as a solid surface tube
The general continuity equation can be applied to stream tubes though it has no solid boundaries.
In steady flow with uniform velocity, all streamlines are straight and parallel.

It is an instantaneous picture of the positions of all fluid particles in the flow which have passed or emerged from a given point.

Examples
- The line of smoke from a chimney is a streak line.
- A dye when injected into the flowing fluid and a resultant colored lines after some time gives the streak lines.

Path Line

It is the actual path traveled by an individual fluid particle over a period of time.

The locus of the same fluid particle over a time period 𝑡1 to 𝑡2 is called a path line
The path line may intersect at different times.
A path line is a Langrangian concept in that we simply follow the path of an individual fluid particle as it moves around in the flow field.