Question

What is the diffuser efficiency for hypersonic conditions?

(a) ηD = 1

(b) ηD > 1

(c) ηD < 1

(d) ηD = 1/2

The question was asked in an online interview.

This key question is from Compressible Flow through Diffusers in chapter Compressible Flow through Nozzles, Diffusers and Wind Tunnels of Aerodynamics

Answer 1

Right choice is (c) ηD < 1

To elaborate: Usually for low supersonic test section Mach numbers, diffusers perform better than the normal shock diffuser, hence the numerator is greater than the denominator in the efficiency formula. This leads to ηD being greater than 1. But for hypersonic conditions, this is not the case and ηD < 1 as the normal shock recovery is not that great.