Discussion
Consider the following statements about specialised tunnel facilities:
1. A water tunnel reaches a given Reynolds number at a much lower speed than air.
2. An automotive tunnel needs a moving ground plane to represent the road.
3. A smoke tunnel is the preferred facility for accurate drag measurement.
Which of the statements given above is or are correct?
Answer: A
Correct answer: 1 and 2 only
Statements 1 and 2 are correct: water is about fifteen times more viscous than air but eight hundred times denser, so the same Reynolds number is reached at roughly a fifteenth of the speed - which is why water tunnels suit slow, detailed flow visualisation. And an automotive tunnel needs a moving ground and rotating wheels, since a stationary floor grows its own boundary layer that a road does not have. Statement 3 is wrong: a smoke tunnel is a low-speed visualisation facility with no balance and no useful accuracy for drag.
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