What happens in case of a choked flow?

When flow becomes supersonic at the throat

Normal shock is created at the inlet of the nozzle

When the exit pressure is reduced to a point where the flow becomes subsonic at throat

When flow is sonic and mass flow remains constant at the throat despite reducing exit pressure

Correct Answer : When flow is sonic and mass flow remains constant at the throat despite reducing exit pressure

Explanation : When we reduce the exit pressure, there comes a point when the Mach number becomes 1 at the throat i.e. the flow is sonic. The mass flow remains constant and the flow becomes frozen upstream of the throat. This condition where after reaching sonic flow at the throat, and despite reducing the exit pressure, the mass flow remains constant is called choked flow.

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.