In steady, 1D compressible flow with friction in a constant-area duct (Fanno flow), as friction acts along the duct on an initially supersonic flow ($M > 1$):

(A) Mach number decreases towards 1, while static pressure and temperature increase

(B) Mach number increases further into the supersonic regime

(C) Total pressure remains constant along the duct length

(D) Entropy decreases along the flow direction

Answer Key:

A

### Solution: In Fanno flow, friction always drives Mach number towards sonic condition ($M = 1$). For supersonic inlet flow, friction decelerates the flow towards $M=1$, increasing static pressure and temperature, while total pressure drops. Correct: A.

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