Question

What is a critical Mach number?

(a) Mach number at which transonic flow is achieved on the surface

(b) Mach number at which sonic flow is achieved on the surface

(c) Mach number at which wake formation does not exist

(d) Mach number at which the coefficient of lift is minimum

I got this question by my school principal while I was bunking the class.

Asked question is from The Supercritical Airfoil topic in division Subsonic Compressible Flow over Airfoils : Linear Theory, Linearized Supersonic Flow of Aerodynamics

Answer 1

The correct choice is (b) Mach number at which sonic flow is achieved on the surface

The explanation is: When freestream air flows over an airfoil at suppose Mach number = 0.3, there’s a point on the top surface of the airfoil where the pressure is minimum and the Mach number is maximum such as 0.43. When this freestream velocity is increased, there’s a certain Mach number for which sonic flow is achieved at the top surface. This is known as the critical Mach number.