Stream line

In a flow, the stream line is a line whose local direction corresponds to the direction of the local flow velocity. 

In steady flows where a velocity independent of time exists at every point within the flow space the stream line describes the path of a given liquid particle (particle trajectory or path line).

Stream lines exhibit no breaks and can never intersect with one another, as otherwise two different flow velocities would have to occur at one and the same point. See Fig. 2 Vane cascade

As it is not possible to have normal velocity components along a solid wall, such as an impeller vane or pump casing, it follows that all the body contours of the flow space are simultaneously stream lines.

Strictly speaking, the Bernoulli equation which is often applied in fluid mechanics, ​​​​​​​is only valid for various points on a common stream line. However, often the equation is also used for a flow space designated the "thread of stream or filament of flow" which is enclosed by all the stream lines passing through a continuous line.

In the case of fluid flow in pipes, the entire liquid content of the pipe is usually considered to be a filament of flow (see Flow velocity in a cross-section).

The circular projection of a stream line into the meridional section plane (section through the axis of rotation) of an impeller is referred to as flow line.