Fluid Mechanics
The Prandtl mixing length is

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

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Fluid Mechanics
In case of hydraulically smooth pipe, the resistance to flow depends only on the Reynolds number, whereas for a hydraulically rough pipe, the resistance to flow is governed by the relative roughness. Two pipes are said to have the same hydraulic roughness, when they have equal values of

Friction co-efficient for flows at equal Reynold number
Absolute roughness
Relative roughness
All of these

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