# Aerodynamics Questions and Answers – Quasi-One-Dimensional Flow Diffusers

This set of Aerodynamics Multiple Choice Questions & Answers (MCQs) focuses on “Quasi-One-Dimensional Flow Diffusers”.

1. What is the role of the diffuser?
a) Increase the flow velocity after test section
b) Decrease the flow velocity after test section
c) Increase the flow velocity inside test section
d) Decrease the flow velocity inside the test section

Explanation: Diffuser is a duct used mostly in the supersonic wind tunnel in order to slow down the high flow velocity post test section to a lower velocity at the diffuser’s exit before exhausting it to the atmosphere.

2. How is flow deaccelerated In the diffuser?
a) Isentropic compression
b) Isentropic expansion

Explanation: The diffuser’s function is to slow down the incoming high speed flow to a lower subsonic flow. The aim is to reduce the velocity with small loss in total pressure. The ideal diffuser does this task with the help of isentropic compression so that there is no pressure loss.

3. Which of these properties remain constant in the ideal isentropic supersonic diffuser?
a) Total pressure
b) Velocity
c) Mach number
d) Mass flow

Explanation: Isentropic supersonic diffusers have a constant entropy throughout the diffuser duct. Since the entropy is constant, the total pressure along is the duct is also constant. Although this is an ideal case and in reality there are some pressure losses occurring due to the formation of shock waves.

4. Why can’t build an ideal supersonic diffuser with no total pressure losses?
a) Presence of shock waves
b) Varying cross – sectional area
c) Varying throat area
d) Choked flow

Explanation: Due to the presence of oblique shock waves on the convergent portion, the ideal supersonic diffuser is far from achievable. This contributes to the breaking of the isentropic flow characteristics according to which the entropy in the diffuser is constant. In fact, the flow in reality is viscous and there is an increase in entropy near the boundary layer.

5. Total pressure loss in a normal shock diffuser is less than the oblique shock diffuser.
a) True
b) False

Explanation: In case of an oblique shock diffuser, the total pressure drop across multiple oblique hocks followed by a weak normal shock is less than a strong normal shock in case of normal shock diffuser. This is why it is preferred to opt for an oblique shock diffuser that manages to slow down the accelerated flow with lesser pressure loss.
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6. Oblique shock diffuser has a better pressure recovery than a normal shock diffuser.
a) True
b) False

Explanation: In an oblique shock diffuser, the accelerated flow is slowed down by a series of oblique shock waves followed by slowing down of the flow by a weak normal wave at the end of the diffuser. Therefore, the static pressure at the exit of the diffuser is equal to p. The pressure recovery in case of an oblique shock diffuser is greater because of lower total pressure loss compares to the normal shock diffuser.

7. Which of these phenomena attenuates the advantages of greater pressure recovery in an oblique shock diffuser?
a) Abrupt change of convergent – divergent sections
b) Shock wave interaction with walls
c) Isentropic flow
d) Presence of normal shock

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.

8. Why is the diffuser throat area greater than the nozzle throat area?
a) To reduce speed
b) To maintain constant mass flow rate
c) Rise in entropy within diffuser
d) Rise in entropy within nozzle

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.

9. What is the diffuser efficiency for supersonic flow?
a) ηD = 1
b) ηD > 1
c) ηD < 1
d) ηD = 1/2

Explanation: 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 ηD > 1.

10. What is the diffuser efficiency of a normal shock diffuser?
a) 1
b) 0
c) Inifinty
d) $$\frac {1}{2}$$

Explanation: The diffuser efficiency compares the actual pressure ratio of a diffuser $$\frac {p_{d_{0}}}{p_0}$$ and the total pressure ratio across a normal shock $$\frac {p_{0_{2}}}{p_{01}}$$ . The formula is given by:
ηD = $$\frac {(\frac {p_{d_0}}{p_0})_{actual}}{(\frac {p_{0_2}}{p_{01}} )_{normal \, shock \, at \, M_e}}$$
The numerator will be equal to the denominator for a normal shock diffuser, hence the efficiency is one. For subsonic flow, the siffuder’s performance is lower resulting in efficiency which is less than unity. And for the hypersonic flow the efficiency is greater than unity since it has a better shock recovery.

11. What is the entropy relation between the entry and exit of an actual diffuser?
a) s1 = s2
b) s1 > s2
c) s1 < s2
d) s1 s2 = 1

Explanation: 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.

12. 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

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.

13. Why is a variable – geometry diffuser used?
a) Ease in manufacturing
b) Higher efficiency
c) More mass flow rate
d) Inexpensive

Explanation: In the case of fixed – geometry diffuser, the second throat area is kept larger than the first throat area so that there is no starting problem in the wind tunnel but it does not operate at its maximum efficiency.
Variable – geometry diffuser on the other hand can vary the throat area by hydraulic or mechanical means. The second throat area at the start of the operation is kept high enough so that the normal shock is able to pass through the diffuser and there is no starting problem. Once the wind tunnel starts working, its throat area is reduced so that it operates with a higher efficiency.

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