# Gas Dynamics Questions and Answers – Supersonic Flow Generation

This set of Gas Dynamics Multiple Choice Questions & Answers (MCQs) focuses on “Supersonic Flow Generation”.

1. For a supersonic wind tunnel application which of the following nozzle is used?
a) Straight C-D nozzle
b) Contoured C-D nozzle
c) Convergent nozzle
d) Annular nozzle

Explanation: The flow exiting from the straight C-D nozzle has a uniform Mach number, however, it will diverge away from the nozzle axis as it leaves the nozzle. Whereas the contoured C-D nozzle can generate a flow that has uniform Mach number as well as which is uni-directional at the nozzle exit. Now since the desired flow field in supersonic wind tunnels has to be both uniform and uni-directional, contoured C-D nozzles are used for such applications.

2. By increasing the nozzle divergence angle the nozzle length ___________
a) Increases
b) Decreases
c) Remains the same
d) Depends on the requirement

Explanation: A small divergence angle is preferred to minimize the divergence losses for a nozzle, however, a smaller angle requires a nozzle length to be large for a specified Mach number leading to increased weight. While a large divergence angle will require shorter nozzle to generate specified Mach number but the flow inside the nozzle will tend to separate leading to performance losses.

3. How the condensation affects the flow Mach number and pressure at supersonic speeds?
a) The static pressure increases and the Mach number decreases
b) Both the static pressure and Mach number Increases
c) The static pressure decreases and Mach number Increases
d) Both the static pressure and Mach number decreases

Explanation: The changes in flow properties due to condensation depends upon the amount of heat released through condensation. At supersonic speeds, the flow temperature at divergent portion is low compared to the inlet temperature. This cause the air moisture to condense inside the nozzle and hence at supersonic speed with increase in condensation Mach number increase and in turn pressure decreases.

4. Among the following cases listed below, for which case the shock diamonds are formed with an oblique shock followed by an expansion fan behind the nozzle exit?
a) Pe < Pb
b) Pe > Pb
c) Pb = 0
d) Pe = Pb

Explanation: When the static pressure at the nozzle exit is lower than the ambient pressure, the flow is compressed by means of oblique shock waves to increase its pressure. This compressed flow is again expanded by expansion fans due to reflection from the free boundary. This combination of an oblique shock with an expansion fan forms the shock diamond.

5. The expansion in a contoured C-D nozzle occurs through _________
a) Thermal expansion
b) Nonisentropic expansion
c) Centered expansion
d) Continuous expansion

Explanation: In the contoured C-D nozzle the nozzle shape is optimized to improve its performance. Such nozzles have contoured throat due to which the flow gradually expands as it moves downstream. Whereas in case of the straight C-D nozzle the flow expands through a point having a centered expansion fan.

6. The flow through a C-D nozzle exhausts at pressure 2.3 bar to an atmosphere having the pressure of 1.25 bar. If the nozzle diameter is 49.38 mm, calculate the approximate location of the first shock diamond from the nozzle exit.
a) 56.2 mm
b) 44.8 mm
c) 34.7 mm
d) 70.4 mm

Explanation: The distance to the first shock diamond from the nozzle exit is approximated as;
x = 0.67D0 $$\sqrt {\frac {P_0}{P_a} }$$
Hence according to given data, x = 0.67 * 0.04938 * $$\sqrt {\frac {2.3}{1.25} }$$
x = 0.0448 m = 44.8 mm

7. For an ideal gas infinite number of shock, diamonds occur.
a) True
b) False

Explanation: In real gases, the turbulent shear layer is formed between the free boundary and the exhaust due to friction. This layer causes the viscous damping that gradually dissipates the wave structure and eventually, the nozzle exit pressure and back pressure becomes equal so that the shock diamonds are formed no longer. However in an ideal gas, there is no such phenomenon, hence the shock diamonds keep to form continuously.

8. At which location the incident shock, Mach disk, and the reflected shock meet?
a) Triple point
b) Jet boundary
c) Nozzle walls
d) Kink

Explanation: As the flow passes through centered expansion fan in nozzle under expansion, the streamlines turn parallel to the centerline and creating a normal shock (Mach disk) at some location downstream of the nozzle exit. Also as expansion fan reflect from free boundary it results in oblique shock which is again reflected at Mach disk. Hence the location at which the incident shock, Mach disk and reflected shock meet are termed as triple point.

9. In the nozzle, under expansion, the shock diamonds are formed due to which of the following flow phenomenon?
a) Compression waves followed by expansion waves
b) Compression waves followed by compression waves
c) Expansion waves followed by compression waves
d) Expansion waves followed by expansion waves

Explanation: In the case of a nozzle under expansion, the exit pressure is high than the back pressure. Hence the flow expands through an expansion fan to match with back pressure. However, the exit pressure now becomes lower than the back pressure and expansion waves reflect from the free boundary to form a compression fan (oblique shock) creating a shock diamond.

10. Which of the following statement is true for a C-D nozzle in terms of nozzle pressure ratio (NPR)?
a) Shock structure moves upstream with an increase in NPR
b) Shock structure remains same with an increase in NPR
c) Shock structure moves downstream with a decrease in NPR
d) Shock structure moves downstream with an increase in NPR

Explanation: For a given NPR in C-D nozzle, the sonic flow at throat expands continuously until it encounters oblique shock where the flow is fully detached from the wall. Now as the NPR is increased this distance increases as the flow has to expand more to decrease the pressure. This statement can be also be approximated by the relation;
x = 0.67D0 $$\sqrt {\frac {P_0}{P_a} }$$
That says that with an increase in static pressure at nozzle exit the distance of shock diamond from nozzle exit increases in direct proportion. Hence the shock structure moves downstream of the nozzle with an increase in NPR.

Sanfoundry Global Education & Learning Series – Gas Dynamics.

To practice all areas of Gas Dynamics, here is complete set of Multiple Choice Questions and Answers.

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