Page "Lift (force)" Paragraph 29

Returning to the picture from the previous section, the flow approaching an airfoil can be divided into streamtubes, which are defined based on the area between two streamlines.

By definition, fluid never crosses a streamline in a steady flow.

Assuming that the air is incompressible, the rate of flow ( e. g. liters or gallons per minute ) must be constant within each streamtube since matter is not created or destroyed.

If a streamtube becomes narrower, the flow speed must increase in the narrower region to maintain the constant flow rate.

This idea is called " conservation of mass ", and for incompressible flow mass is conserved within each streamtube.