In streamline flow, the motion of a particle after it passes a particular point is the same as the motion of the particle that preceded it at that point. The path that a particle takes is called a stream line . Every particle that passes any particular point will follow the stream line that goes through that point. A bundle of stream lines, like the ones here, is known as a stream tube .
Fluid never crosses the surface of a stream tube.
In turbulent flow, the motion of a particle after it passes a particular point may be quite different from the motion of the particle that preceded it at that point. Turbulent flow is characterized by randomness or irreproducibility of the motion of individual particles. It usually occurs in fluids moving at high speeds. As you might expect, friction is far greater in turbulent flow. We will concentrate most of our attention on streamline flow.