Themes > Science > Physics > Fluid Dynamics > Fluid Dynamics > Some things to know about streamlines
  • Because the fluid is moving in the same direction as the streamlines, fluid can not cross a streamline.
  • Streamlines can not cross each other. If they were to cross this would indicate two different velocities at the same point. This is not physically possible.
  • The above point implies that any particles of fluid starting on one streamline will stay on that same streamline throughout the fluid.

A useful technique in fluid flow analysis is to consider only a part of the total fluid in isolation from the rest. This can be done by imagining a tubular surface formed by streamlines along which the fluid flows. This tubular surface is known as a streamtube.

A Streamtube

And in a two-dimensional flow we have a streamtube which is flat (in the plane of the paper):

A two dimensional version of the streamtube
The "walls" of a streamtube are made of streamlines. As we have seen above, fluid cannot flow across a streamline, so fluid cannot cross a streamtube wall. The streamtube can often be viewed as a solid walled pipe. A streamtube is not a pipe - it differs in unsteady flow as the walls will move with time. And it differs because the "wall" is moving with the fluid


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