Why Do Stream Tubes Form in Fluid Mechanics?

[mrmojorizing]

I don't understand why stream tubes exist. Stream tube is defined here: http://www.princeton.edu/~asmits/Bicycle_web/streamline.html

Why can't you have a stream line starting at a point going one way, and then the stream line at the neighboring point going some orthogonal direction, and thus not forming a tube?

 

[Studiot]

Welcome to Physics Forums

Why can't you have a stream line starting at a point going one way, and then the stream line at the neighboring point going some orthogonal direction, and thus not forming a tube?

Stream tubes and lines occur in laminar flow.
How would your example be laminar?

 

[RandomGuy88]

It is just a definition. A streamline is a line that is tangential to the instantaneous velocity vector. A streamtube is a collection of streamlines and by definition there is no flow perpendicular to a streamline so therefore there is no flow through the walls of the streamtube.

 

[boneh3ad]

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Stream tubes and lines occur in laminar flow.
How would your example be laminar?

Streamlines are not only present in laminar flow. In fact, the concept of streamlines is completely independent of the concept of viscosity, which means it is also independent of laminar/turbulent flow since they make no sense in an inviscid sense.

I don't understand why stream tubes exist. Stream tube is defined here: http://www.princeton.edu/~asmits/Bicycle_web/streamline.html

Why can't you have a stream line starting at a point going one way, and then the stream line at the neighboring point going some orthogonal direction, and thus not forming a tube?

RandomGuy88 said it pretty well. As he said, streamlines are defined as following along the instantaneous velocity vector. Imagine that two streamlines did somehow cross. That would mean the flow is moving in two directions at once! Clearly, that doesn't really make physical sense, hence streamlines cannot cross one another.

There are more mathematically robust answers, but they aren't as easy to visualize.