Question

Select the incompressible flow out of the following.

(a) High speed gaseous flow

(b) M=5

(c) M=3

(d) Liquid flow

I had been asked this question in an interview.

This intriguing question originated from Definition of Compressibility topic in section Compressible Flow – Preliminary Aspects of Aerodynamics

Answer 1

Right answer is (d) Liquid flow

The best I can explain: The high speed gaseous flow is a compressible flow due to high compressibility of gases. Liquid flow is incompressible due to low compressibility of liquids. And M=5 or M=3 is compressible flow since the incompressible flow has M<0.3 only.