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Water flows through two identical tubes A and B. In a given time, a volume V0 passes through tube A and 2*V0 passes through tube B. Which of the following could be correct?

1. The flow in both tubes is steady (laminar).
2. The flow in both tubes is turbulent.
3. The flow is steady in A but turbulent in B.
4. The flow is steady in B but turbulent in A.

Correct answer: The flow is steady in A but turbulent in B.

Solution

For identical tubes, the flow speed is proportional to volume flow rate. Reynolds number Re = rho*v*D/eta increases with speed. B carries twice the flow of A, so B has the larger Re. Depending on the values, B can exceed the critical Re (turbulent) while A stays below it (steady/laminar). The combination 'turbulent in A but steady in B' is impossible because B is faster. Hence the consistent possibility is steady in A and turbulent in B.

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