Question

What happens if the second throat area is larger than the starting value when the wind tunnel starts and the reservoir pressure value is opened?

(a) Normal shock is swallowed by diffuser

(b) Normal shock remains upstream of diffuser

(c) Throat area has no effect

(d) Normal shock is generated at the inlet

I had been asked this question in an international level competition.

I need to ask this question from Quasi-One-Dimensional Flow Diffusers in portion Quasi-One-Dimensional Flow of Aerodynamics

Answer 1

Right option is (a) Normal shock is swallowed by diffuser

Best explanation: When the wind tunnel is initially started, there’s a pressure difference that is generated rapidly creating a transient flow which is often very complex. The starting process usually leads to the formation of the normal shock wave which is propagated throughout the duct (from nozzle to the diffuser).

If the throat area of the diffuser is not large enough, the normal shock remains upstream of the diffuser and the wind tunnel is unable to start properly. On the other hand, when the second throat area is larger than the starting value, the normal shock is able to pass through the diffuser/swallowed by the diffuser resulting in proper functioning of the wind tunnel.