Fluid Mechanics
Boundary layer separation occurs when the

Velocity of sound is reached
Pressure reaches a minimum
Valve is closed in a pipeline
Cross-section of the channel is reduced

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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

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

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