Question

why there cannot be flow across a streamtube

Answer 1

Streamlines forming a streamtube. These streamlines form a tube that is impermeable since the walls of the tube are made up of streamlines, and there can be no flow normal to a streamline.

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Answer 2

Answer:

Streamlines forming a streamtube. These streamlines form a tube that is impermeable since the walls of the tube are made up of streamlines, and there can be no flow normal to a streamline (by definition).