What is stream tube in fluid mechanics?

In fluid mechanics, a stream tube is a conceptual model used to analyze the flow of fluid in a continuous, tubular region of space. It is defined as a collection of streamlines that confine the fluid flow, with the assumption that fluid particles within the stream tube do not cross its boundaries. This model helps in visualizing and studying the conservation of mass and momentum in fluid flows, particularly in incompressible and steady-state conditions. Stream tubes are crucial in applications such as pipe flow and aerodynamic analyses.