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A Prandtl-Meyer expansion fan is

  • A.Isothermal
  • B.Isentropic, being built from weak waves
  • C.Strongly irreversible, like a shock
  • D.Adiabatic but not reversible

Answer: B

Each Mach wave in the fan is infinitesimally weak, and the entropy rise across a weak wave goes as the cube of its strength, so the total rise through the fan is negligible. The expansion is therefore isentropic to an excellent approximation, and the ordinary isentropic relations may be used straight through it.

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