🔹 1. Sonic Mach Number (M = 1)

 It is the Mach number at which the local flow speed equals the speed of sound.

• Mach 1 (M = 1) is the transition point between subsonic and supersonic flow.

• It can occur at any point in the flow field, not necessarily over the entire body.

• This is a general term for any flow condition where velocity = speed of sound.

🔹 2. Critical Mach Number (M_crit)

 It is the freestream Mach number at which some part of the airflow over an airfoil first reaches Mach 1 (sonic condition).

• At this point, the body is still flying in subsonic conditions overall, but local supersonic regions begin to appear.

• Important in transonic flow regimes (typically between Mach 0.75 and 1.2).

• The critical Mach number depends on

  • Airfoil shape

  • Angle of attack

  • Compressibility effects