Question

What is the discontinuity called when the mach number jumps from above 1 to below 1?

Answer 1

The discontinuity is called as jump discontinuity.

A function is said to be discontinuous at apoint if the Lefthandlimit ≠Right hand limit≠The limit that exist at a point.

For example if we consider limit about  a point, for the function to be continuous at a point the lefthandlimit =Righthandlimit =limit which exist at that point, if any one of the three doesnot exist the function is said to be discontinuous about that point, In the above case the the value of one point let it be LHL is above 1 while the limit value at RHL is less than 1 so there is discontinuity at a given point and since the difference varies more the discontinuity is called as jump discontinuity.