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Transonic wind tunnels use slotted or perforated test-section walls chiefly to

  • A.Support the model
  • B.Increase the test-section Mach number above three
  • C.Reduce the noise of the drive fan
  • D.Relieve blockage and wall interference

Answer: D

Near Mach one a solid-walled tunnel chokes and the wall interference becomes intolerable, because small area changes have very large effects. Ventilated walls let the flow spill in or out and largely remove both problems, which is what made transonic testing practical from the 1950s.

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