Question

The best estimate of whether the flow is compressible or not comes from ______

(a) Pressure

(b) Mach number

(c) Compressibility values

(d) Experiments

I had been asked this question by my school teacher while I was bunking the class.

My question is based upon Definition of Compressibility in division Compressible Flow – Preliminary Aspects of Aerodynamics

Answer 1

Right answer is (b) Mach number

For explanation I would say: Compressibility values gives a good estimate of the nature of fluid i.e. whether it is compressible or not. But it has exceptions like low speed flow over airfoil etc. The Mach number is the best parameter to judge the compressibility of the fluid.