Fluid Mechanics
A stream line is

Fixed in space in steady flow
A line connecting the mid points of flow cross-section
Always the path of particle
Drawn normal to the velocity vector at every point

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Fluid Mechanics
The Prandtl mixing length is

Useful for computing laminar flow problems
Zero at the pipe wall and is a universal constant
Independent of radial distance from the pipe axis
Independent of the shear stress

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