The Mechanics and Why the Flow is Choked?

The trends of the properties can be examined by looking in equations (9.24) through (9.34). For example, from equation (9.24) it can be observed that the critical point is when $M=1$ . When $M < 1$ the pressure decreases downstream as can be seen from equation (9.24) because $fdx$ and $M$ are positive. For the same reasons, in the supersonic branch, $M>1$ , the pressure increases downstream. This pressure increase is what makes compressible flow so different from ``conventional'' flow. Thus the discussion will be divided into two cases: One, flow above speed of sound. Two, flow with speed below the speed of sound.