Which of these phenomena attenuates the advantages of greater pressure recovery in an oblique shock diffuser?

Isentropic flow

Presence of normal shock

Shock wave interaction with walls

Abrupt change of convergent � divergent sections

Correct Answer : Shock wave interaction with walls

Explanation : The viscous boundary layer inside the diffuser wall interacts with the shock wave. This creates an additional loss in total pressure which attenuates.

In real life, oblique shock diffusers have viscous flow. The presence of shock waves inside the diffuser leads to interaction with the viscous boundary layer of the diffuser walls which leads to additional pressure losses. There’s also friction involved which makes oblique shock diffusers far from the ideal diffusers which have no total pressure losses.

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.