A streamline is a field line in the velocity field of a fluid at some instant, ie. a curve which on all points along it is in the same direction as the velocity of the fluid at that point and time.
In a steady flow, ie where the velocity field does not change with time, the streamlines show the paths of the fluid elements (it is worth noting the difference that a streamline is a curve in space while a particle moves in space-time, and that its path is merely a projection onto space). In an unsteady flow this is not the case, and the streamlines will also change with time. In any case streamlines give a good view of the instantaneous properties of the fluid motion, in the same way that magnetic field lines help visualise magnetic fields etc.

Stream"line` (?), a.

Of or pert. to a stream line; designating a motion or flow that is free from turbulence, like that of a particle in a streamline; hence, designating a surface, body, etc., that is designed so as to afford an unbroken flow of a fluid about it, esp. when the resistance to flow is the least possible; as, a streamline body for an automobile or airship.


© Webster 1913

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