Question

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

(a) Subsonic flow

(b) Supersonic flow

(c) Hypersonic flow

(d) Sonic flow

This question was posed to me in examination.

My question comes from Governing Equations for Quasi One Dimensional Flow topic in division Compressible Flow through Nozzles, Diffusers and Wind Tunnels of Aerodynamics

Answer 1

The correct option is (a) Subsonic flow

Explanation: The area – velocity relation is given by:

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

According to this formula, for subsonic flows the value of M^2 – 1 becomes negative since 0 < M < 1. Thus, with decreasing cross – sectional area, the velocity increases. Decreasing area is achieved by convergent duct.