What is the definition of the vorticity and its physical concept?

According to Crocco's relation, under what conditions will an inviscid fluid be irrotational?

What is the definition of the velocity potential?

Does a velocity potential exist for every possible flow? Explain your answer.

What is the perturbation velocity potential? How is it related to the full velocity potential?

How can a compressible subsonic flow be related to an incompressible flow?

What are the Prandtl$-$Glauert and the Karman$-$Tsien compressibility correction?

What is the definition of the critical Mach number?

A$)$ Will the critical Mach number increase or decrease as the lift produced by the airfoil increases?

B$)$ Will the critical Mach number increase or decrease as the airfoil thickness increases?

A$)$ What is meant by the drag$-$divergence Mach number?

B$)$ List two factors that contribute to the rapid increase in drag around Mach $1.$