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?

Throat area has no effect

Normal shock is generated at the inlet

Normal shock remains upstream of diffuser

Normal shock is swallowed by diffuser

Correct Answer : Normal shock is swallowed by diffuser

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.

Recently Updated in Aerodynamics Questions

What is the entropy relation between the entry and exit of an actual diffuser?

s₁ = s₂

s₁ > s₂

s₁ < s₂

s₁ s₂ = 1

Correct Answer : Option (C) : s₁ < s₂

Explaination : The flow diffuses to a slower velocity when the flow interacts with the oblique shock waves inside the diffuser. This causes the diffuser to have a normal shock wave at the end. The entropy of the exit is therefore higher than the entropy of the inlet segment.

What is the diffuser efficiency for supersonic flow?

η_D > 1

η_D = 1

η_D = 1/2

η_D < 1

Correct Answer : Option (A) : η_D > 1

Explaination : The diffuser efficiency is given by the ratio of actual total pressure ratio across the diffuser to the total pressure ratio of a hypothetical normal shock wave at test section Mach number. For a supersonic test section Mach number, the diffusers perform better than the normal shock, thus the numerator of the ratio is greater than the denominator, hence η_D > 1.

Why is the diffuser throat area greater than the nozzle throat area?

To reduce speed

Rise in entropy within diffuser

Rise in entropy within nozzle

To maintain constant mass flow rate

Correct Answer : Option (B) : Rise in entropy within diffuser

Explanation : In a supersonic wind tunnel, there are two throats present. One is that of the nozzle which is known as the first throat which has sonic speed with corresponding area as A*. The second throat is that of the diffuser which is always greater than the first throat because there’s an increase in entropy due to the presence of shock waves.