Question

Gaseous flow over an airfoil is always compressible because of very high compressibility value of gases.

(a) Always true

(b) Always false

(c) True only for high speeds

(d) True for low speeds

I have been asked this question during an interview.

I'd like to ask this question from Definition of Compressibility in division Compressible Flow – Preliminary Aspects of Aerodynamics

Answer 1

The correct answer is (c) True only for high speeds

Explanation: Gases have high compressibility values which states that for a given pressure change, the density change can be higher giving a compressible flow. But low speed flows are an exception since the pressure change across the field is small. This dominates the high compressibility value giving an incompressible flow.