Question

For which of these flows do we need a divergent duct to increase the velocity of the flow?

(a) Subsonic flow

(b) Supersonic flow

(c) Hypersonic flow

(d) Sonic flow

The question was posed to me in an interview.

This interesting question is from Area-Velocity Relation in chapter Quasi-One-Dimensional Flow of Aerodynamics

Answer 1

The correct option is (b) Supersonic flow

Easy explanation: The area-velocity relation is given by:

\(\frac {dA}{A}\)=(M^2-1)\(\frac {du}{u}\)

According to this formula, for supersonic flows the value of M^2-1 is positive since M > 1. Thus, with increasing cross-sectional area, the velocity increases. Increasing area is achieved by convergent duct.