What is the difference between Critical Mach Number & Sonic Mach Number? Are they same?
The Critical Mach Number and the Sonic Mach Number are not the same, although they are related concepts in compressible flow.
🔹 1. Sonic Mach Number (M = 1)
It is the Mach number at which the local flow speed equals the speed of sound.
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Mach 1 (M = 1) is the transition point between subsonic and supersonic flow.
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It can occur at any point in the flow field, not necessarily over the entire body.
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This is a general term for any flow condition where velocity = speed of sound.
🔹 2. Critical Mach Number (M<sub>crit</sub>)
It is the freestream Mach number at which some part of the airflow over an airfoil first reaches Mach 1 (sonic condition).
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At this point, the body is still flying in subsonic conditions overall, but local supersonic regions begin to appear.
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Important in transonic flow regimes (typically between Mach 0.75 and 1.2).
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The critical Mach number depends on
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Airfoil shape
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Angle of attack
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Compressibility effects
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