Fluid Mechanics
Specific speed of a centrifugal pump relates it with another pump having the

Geometrical similarity
Same efficiency
Same speed
Dynamic similarity

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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
Relative roughness
All of these
Absolute roughness

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