CH5: Flows in Ducts & Wind Tunnels

Question 1

For an ideal Laval nozzle, if the flow is subsonic throughout, where is the maximum speed attained?

Answer 1

In the throat.

Elements of Gas Dynamics, Roshko, Pg. 125

Question 2

For an ideal Laval nozzle, if the flow becomes supersonic, it must be asymmetrical, with subsonic conditions _______ of the throat, and _______ conditions downstream of the throat.

Answer 2

upstream (aft)
downstream (post throat)

Elements of Gas Dynamics, Roshko, Pg. 125

Question 3

True or False
For an ideal Laval nozzle, once the flow becomes sonic in the throat, the mass flow rate can increase no further.

Answer 3

True.

Elements of Gas Dynamics, Roshko, Pg. 125

Question 4

True or False
For an ideal Laval nozzle, when the sonic condition is attained in the throat, it can be increased by further lowering the exit pressure.

Answer 4

False.
Once sonic conditions are attained in the throat, lowering the exit pressure does not further increase the speed.

Elements of Gas Dynamics, Roshko, Pg. 125

Question 5

For a Laval nozzle, if flow is subsonic throughout, then A* is _________.

Answer 5

Fictitious and does not actually occur in the flow.

Elements of Gas Dynamics, Roshko, Pg. 125

Question 6

For a Laval nozzle, if the flow is sonic or supersonic throughout, then A* is the _______.

Answer 6

Throat Area

Elements of Gas Dynamics, Roshko, Pg. 125

Question 7

The area Mach number relation given by Eq. 5.2 in Roshko, is only valid for _______ flows.

Answer 7

Isentropic flows.

Reason: rho0/rho used in the derivation is the isentropic relationship.

Elements of Gas Dynamics, Roshko, Pg. 126

Question 8

For a CD nozzle where the exit pressure is such that a normal shock exists inside the nozzle; if pressure is lowered, the shock will move from ________ to ________.

Answer 8

the throat towards the exit.

Elements of Gas Dynamics, Roshko, Pg. 129

Question 9

What is the effect of shock-boundary layer interactions with respect to pressure recovery?

Answer 9

A lower recovery will take place when shock-BL interactions are considered in a wind tunnel.

Elements of Gas Dynamics, Roshko, Pg. 131

Question 10

True or False
For a supersonic wind tunnel contraction section, the length of said section has little effect on the boundary layer thickness at the throat.

Answer 10

True.

Elements of Gas Dynamics, Roshko, Pg. 136

Question 11

In a supersonic wind tunnel, why is the test section usually given some slight divergence (with respect to the incoming flow)?

Answer 11

To account for boundary layer growth.

Elements of Gas Dynamics, Roshko, Pg. 137

Question 12

For an ideal closed-loop supersonic wind tunnel, the energy of the system is ________.

Answer 12

Constant

Elements of Gas Dynamics, Roshko, Pg. 137

Question 13

For an ideal, closed-loop supersonic wind tunnel, the work performed by the compression is _______ to the heat removed by the cooler.

Answer 13

Equal

Elements of Gas Dynamics, Roshko, Pg. 137

Question 14

For an ideal closed-loop supersonic wind tunnel, the compressor-cooler combination adds ________ net energy to the circuit. The function of the compressor-cooler combination is to __________.

Answer 14

no
remove entropy that is constantly being produced by the dissipative processes in the flow.

Elements of Gas Dynamics, Roshko, Pg. 137

Question 15

True or False
When deriving the area relation, the assumption of isentropic flow is essential.

Answer 15

True

MCF, Anderson, Pg. 199

Question 16

True or False
For M = 1, Eq. (5.15) yields dA/A = 0, which mathematically corresponds to a minimum or maximum area distribution. The minimum in area is the only physically realistic solution.

Answer 16

True

MCF, Anderson, Pg. 200

Question 17

True or False
If the flow is subsonic throughout, then A*<At.

Answer 17

True

GD, Roshko, Pg. 127

Question 18

True or False
If there are shocks in the nozzle, the flow can still be considered isentropic.

Answer 18

False.

Roshko, Pg. 129

Question 19

True or False
Underexpansion means that the pressure of the exhaust is large than the ambient.

Answer 19

True.

Chapter 5 lecture slides, Pg. 18

Question 20

True or False
The term “pressure recovery” is analogous to a pressure drop.

Answer 20

False. It means a pressure rise.

Chapter 5 lecture slides, Pg. 28

Question 21

For a general