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

Same efficiency
Geometrical similarity
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

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

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