Aerodynamics - Quasi-One-Dimensional Flow Quiz(MCQ)

1 .

What causes the flow properties to vary in quasi – one – dimensional flow?

A)

Normal shock

B)

Head addition

C)

Frictional drag

D)

Cross – sectional area

Correct Answer : Cross – sectional area

Explanation : In case of quasi – one dimensional flow the flow properties keep changing along the distance x due to the varying cross – sectional area (A) which is in contrast to the one – dimensional flow. In that, the area remains constant thus, the flow properties change fir to the presence of shock wave friction and heat addition or removal.

2 .

The governing equations for quasi – one – dimensional flow is used to find properties of flow in a nozzle.

A)

True

B)

False

C)

Can Not Say

D)

None of the above

Correct Answer : True

Explanation : The three governing equations for quasi – one – dimensional flow is used to compute properties such as pressure, enthalpy, density of a flow inside a nozzle or a diffuser. Apart from this the area – velocity relation also helps in understanding the physics of supersonic/subsonic flow.

3 .

According to the energy equation, which of these properties remain constant along the flow?

A)

Total entropy

B)

Total enthalpy

C)

Kinetic energy

D)

Potential energy

Correct Answer : Total enthalpy

4 .

What is the differential form of energy equation for quasi one – dimensional flow?

A)

dh + udu = 0

B)

dh – udu = 0

C)

dh – u²du = 0

D)

dh + u²du = 0

Correct Answer : dh + udu = 0

Explaination : The energy equation for quasi one – dimensional flow is given by :

h + u²2 = constant

On differentiating the above equation we arrive at the differential energy equation for the quasi one – dimensional flow :

dh + udu = 0

5 .

For a subsonic flow, what should be the value of dA to get flow acceleration?

A)

Zero

B)

One

C)

Positive

D)

Negative

Correct Answer : Negative

Explanation : According to the area-velocity relation for a quasi one-dimensional flow, when the flow is subsonic Mach number is less than 1 resulting in the quantity M²-1 being negative. Therefore for achieving accelerated flow, dA has to be negative.

6 .

For which of these flows do we need a divergent duct to increase the velocity of the flow?

A)

Sonic flow

B)

Subsonic flow

C)

Supersonic flow

D)

Hypersonic flow

Correct Answer : Supersonic flow

Explanation : The area-velocity relation is given by :

According to this formula, for supersonic flows the value of M²-1 is positive since M > 1. Thus, with increasing cross-sectional area, the velocity increases. Increasing area is achieved by convergent duct.

7 .

What is the minimum area of a duct known as?

A)

Throat

B)

Divergent duct

C)

Convergent duct

D)

Minimum area duct

Correct Answer : Throat

Explanation : In a nozzle which is designed to achieve supersonic speed, there’s both converging and diverging section which is used to accelerate the flow. There is a minimum area point in the duct where Mach number reaches 1. This minimum area place in a duct is known as the throat.

8 .

What happens to the velocity of the supersonic flow in the convergent duct?

A)

Increases

B)

Decreases

C)

Remains the same

D)

Changes periodically

Correct Answer : Decreases

Explanation : Since the supersonic flows have Mach number greater than 1, the value of M²-1 is positive. According to the area-velocity relation, for a convergent duct in which dA is negative, the velocity decreases.

9 .

The trends of velocity change in a duct for both subsonic and supersonic flow are identical.

A)

True

B)

False

C)

Can Not Say

D)

None of the above

Correct Answer : False

Explanation : For a flow in a duct, both supersonic and subsonic flows show opposite trends. For a subsonic flow (M < 1), the velocity increases in a convergent duct and decreases in the divergent duct. On the other hand, for a supersonic flow (M > 1), the flow increases in a divergent duct and decreases in the convergent duct.

10 .

Which equation is obtained when Mach number is equated to zero in area-velocity relation?

A)

Energy equation

B)

Bernoulli’s equation

C)

Momentum equation

D)

Continuity equation

Correct Answer : Continuity equation

11 .

For a flow in a duct, what is Mach number a function of?

A)

Sonic throat area to local duct area

B)

Local duct area to divergent duct area

C)

Local duct area to sonic throat area

D)

Local duct area to convergent duct area

Correct Answer : Local duct area to sonic throat area

Explanation : On observing the area – Mach relation for a flow inside a duct, we notice that the Mach number at a point in a duct is a function of ratio of local duct area to the sonic throat area.
M = f(A/A*)

12 .

The area – Mach number relation yields how many solution(s) for a given Mach number?

A)

2

B)

3

C)

4

D)

5

Correct Answer : 2

Explanation : The area – Mach relation which is the ratio of local area to throat area as a function of Mach number yields two solutions for a given Mach number. One is a subsonic value and the other is the corresponding supersonic value. That value that has to be chosen for a specific Mach number depends on the inlet and exit pressure of the duct.

13 .

What happens in case of a choked flow?

A)

When flow becomes supersonic at the throat

B)

Normal shock is created at the inlet of the nozzle

C)

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

D)

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.

14 .

In which of these conditions there’s no formation of shock wave inside a convergent – divergent nozzle?

A)

Overexpanded

B)

Fully expanded

C)

Underexpanded

D)

Fully underexpanded

Correct Answer : Fully expanded

Explanation : When the nozzle is fully expanded i.e. when the ambient pressure is equal to the exit pressure of the nozzle, there no formation of shock waves. In case of underexpanded nozzle, there are expansion waves formed at the tip of the nozzle exit and in case of overexpanded nozzle, there’s formation of oblique shock waves at the tip.

15 .

What happens to the mass flow rate if the reservoir pressure is doubled?

A)

Doubled

B)

Becomes half

C)

Remains same

D)

Becomes one – fourth

Correct Answer : Doubled

Explanation : The mass flow rate depends on the inlet stagnation pressure, temperature and throat area by the relation:

Thus, when the resrvoic pressure is doubled, the mass flow rate through the nozzle also doubles as it is directly proportional to it.

16 .

What is the exit Mach number if the convergent nozzle is choked?

A)

0

B)

1

C)

2

D)

2.5

Correct Answer : 1

Explanation : In case of a convergent nozzle, when the flow is choked the exit Mach number is 1. In order to check if the nozzle is operating at choking conditions, we compare the actual pressure ratio to the critical pressure ratio. When the actual pressure ratio is larger than the critical pressure ratio, the nozzle is considered to be choked.

17 .

How is flow deaccelerated In the diffuser?

A)

Adiabatic expansion

B)

Isentropic expansion

C)

Isentropic compression

D)

Adiabatic compression

Correct Answer : Isentropic compression

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.

18 .

Why can't build an ideal supersonic diffuser with no total pressure losses?

A)

Choked flow

B)

Varying throat area

C)

Presence of shock waves

D)

Varying cross – sectional area

Correct Answer : Presence of shock waves

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.

19 .

Total pressure loss in a normal shock diffuser is less than the oblique shock diffuser.

A)

True

B)

False

C)

Can Not Say

D)

None of the above

Correct Answer : 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.

20 .

Why is a variable – geometry diffuser used?

A)

Higher efficiency

B)

Inexpensive

C)

More mass flow rate

D)

Ease in manufacturing

Correct Answer : Higher efficiency

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.

21 .

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)

Throat area has no effect

B)

Normal shock is generated at the inlet

C)

Normal shock remains upstream of diffuser

D)

Normal shock is swallowed by diffuser

Correct Answer : Normal shock is swallowed by diffuser

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.

22 .

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

A)

Isentropic flow

B)

Presence of normal shock

C)

Shock wave interaction with walls

D)

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.

23 .

Why is the diffuser throat area greater than the nozzle throat area?

A)

To reduce speed

B)

Rise in entropy within diffuser

C)

Rise in entropy within nozzle

D)

To maintain constant mass flow rate

Correct Answer : 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.

24 .

What is the diffuser efficiency for supersonic flow?

A)

η_D > 1

B)

η_D = 1

C)

η_D = 1/2

D)

η_D < 1

Correct Answer : 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.

25 .

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

Correct Answer : 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.