Explain the concept of a tube of flow.

$A$ tube of flow is defined as a bundle of streamlines that form a tubular region in a fluid flow.
Key characteristics of a tube of flow include:
$1$. Since the boundary of the tube of flow is formed by streamlines,no fluid can cross the boundary of the tube. The fluid enters at one end and leaves at the other.
$2$. The velocity of all fluid particles passing through any cross-section perpendicular to the direction of flow within the tube is considered to be uniform at a given instant.
$3$. When fluid flows through a physical pipe or tube at a very low speed (laminar flow),the entire pipe can be considered a single tube of flow.