Streamline Motion:

The flow of liquid is said to be streamline (motion) if

(a) A liquid particles move along with fixed paths known as streamlines, and

(b) The velocity of the particles passing by a given point, one after the other, on a streamline remains unchanged in magnitude and direction at that point.

You may ask: What is a streamline? A streamline is defined as the curve whose tangent at any point gives the direction of the liquid velocity at that point. In other words, streamlines are the curves parallel to the direction of the fluid velocity at all points. Refer to Figure (a) which shows the flow of liquid through a straight tube. In steady flow, the streamlines such as PQ coincides with the line of flow.

One of the most important properties of streamlines is that the two streamlines never cross each other. Further, in a streamline motion, it is assumed that the entire thickness of the stream of the liquid is made up of a large number of plane layers, one flowing over another. Such a flow is, therefore, also called laminar flow.

Figure: (a) Streamline and (b) Turbulent Flow of Liquid