# Aerodynamics Questions and Answers – Compressible Flow through Diffusers

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This set of Aerodynamics Multiple Choice Questions & Answers (MCQs) focuses on “Compressible Flow through 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 subsonic wind tunnel in order to slow down the high flow velocity post test section to a lower velocity at the diffuser’s exit.

2. Diffuser is only applicable for incoming subsonic flow.
a) True
b) False

Explanation: Diffusers work for both subsonic and supersonic incoming flows. Although the design and shape of diffuser in both the cases vary. In both the cases, the work of the diffuser is to lower the speeds by least total pressure reduction.

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.

4. Why is ideal supersonic diffuser not feasible?
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. Which of these processes is involved in slowing down the air inside the subsonic diffuser?
c) Isentropic expansion
d) Isothermal compression

Explanation: The process of slowing down the incoming flow inside the diffuser is done using adiabatic expansion. The pressure is constant throughout, if this was not the case then reduced total pressure in front of the compressor would yield less flow velocity.

6. Which of these results in decrease of the flow inside the actual supersonic diffuser?
a) Test section
b) Convergent nature
c) Divergent nature
d) Shock waves

Explanation: In case of actual supersonic diffuser, there’s a series of oblique shock formations. It’s present in the convergent and the test section of the wind tunnel. The interaction between the viscous flow and the shock waves resulting in slowing down the flow velocity.

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

Explanation: When the flow interacts with the oblique shock waves inside the supersonic diffuser, the flow diffuses to a slower velocity. This results in a normal shock wave at the end of the diffuser. Thus the exit entropy is higher than the entropy at the inlet section.

8. Normal shock diffuser is more efficient than the oblique shock diffuser.
a) True
b) False

Explanation: The goal of the diffuser is to reduce the flow velocity with small total pressure loss. If we reduce the incoming supersonic flow through a series of oblique shock followed by a weak normal shock wave, leads to lower total pressure loss compared to reducing the incoming supersonic flow to subsonic using one strong normal shock.

9. How is the flow compressed in an ideal supersonic wind tunnel inside a diffuser?
a) Isentropic
c) Isochoric
d) Isobaric

Explanation: In an ideal supersonic diffuser, the air is slowed to subsonic speed and then expended to the atmosphere. This is done by isentropic compression as ideally there should be no loss in total pressure.

10. Which of these factors contribute to additional total pressure losses inside the 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: 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.

11. What is the formula to compute the efficiency of diffusers?
a) ηD = $$\frac {(\frac {p_{d_0}}{p_0})_{actual }}{(\frac {p_{0_2}}{p_{01}})_{normal \, shock \, at \, M_e}}$$
b) ηD = $$\frac {(\frac {p_{d_0}}{p_0})_{actual }}{(\frac {p_{0_2}}{p_{01}})_{oblique \, shock \, at \, M_e}}$$
c) ηD = $$\frac {(\frac {p_{0_2}}{p_{01}})_{normal \, shock \, at \, M_e}}{(\frac {p_{d_0}}{p_0})_{actual }}$$
d) ηD = $$\frac {(\frac {p_{0_2}}{p_{01}})_{oblique \, shock \, at \, M_e}}{(\frac {p_{d_0}}{p_0})_{actual }}$$

Explanation: The efficiency of diffuser is measured by the ratio of actual total pressure ratio $$\frac {p_{d_0}}{p_0}$$ to the total pressure ratio across hypothetical normal shock wave at test section Mach number $$\frac {p_{0_2}}{p_{01}}$$.

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

Explanation: The diffuser efficiency is given by the formula:
ηD = $$\frac {(\frac {p_{d_0}}{p_0})_{actual }}{(\frac {p_{0_2}}{p_{01}})_{normal \, shock \, at \, M_e}}$$
In case of normal shock diffuser, the numerator will be equal to the denominator, hence the efficiency is one.

13. What is the diffuser efficiency for hypersonic conditions?
a) ηD = 1
b) ηD > 1
c) ηD < 1
d) ηD = 1/2

Explanation: Usually for low supersonic test section Mach numbers, diffusers perform better than the normal shock diffuser, hence the numerator is greater than the denominator in the efficiency formula. This leads to ηD being greater than 1. But for hypersonic conditions, this is not the case and ηD < 1 as the normal shock recovery is not that great.

14. How is diffuser throat area At2 related to the nozzle throat area At1?
a) At2 = At1
b) At2 > At1
c) At2 < At1
d) At2 At1 = 1