In physics, a handy way of visualizing the flow of a fluid is through streamlines. You draw a fluid’s streamline so that a tangent to the streamline at any point is parallel to the fluid’s velocity at that point. In other words, a streamline follows the fluid flow.
You can see an example in the figure, where the streamline is the darker line in the middle of the fluid flow.
If you plot streamlines for any flow, you get an immediate picture of how that fluid is flowing. You can have as many or as few streamlines as you need to get an accurate picture of fluid flow.
When fluid flow is turbulent, streamlines can become all mixed up. That’s why dealing with turbulent flows in a precise, mathematical way is very hard.
You can have a number of streamlines that form a tube of flow. That is, the streamlines form the walls of a tube. The interesting thing about tubes of flow is that fluid does not pass through the walls of such a tube — it’s always transported inside the tube.