Supersonic Waves

Question 1

Is the airflow in a c-d nozzle a reversible adiabatic process? If so, what does it mean?

Answer 1

• Yes

- Because it is an adiabatic process, the pressure at the inlet, p1, is higher than pressure at the outlet p2.

Question 2

What is a reversible adiabatic process called?

Answer 2

Isentropic process.

Question 3

Regarding an irreversible process, what is not constant?

Answer 3

• Entropy

- Thermodynamic property

Question 4

What is an oblique shockwave? Describe how it is formed (4)

Answer 4

• Supersonic flow over a concave corner (FIG 10-3).
• A shockwave is formed at the corner with an angle to the incoming airflow.
• Angle between the shockwave and the direction of the incoming airflow is less than 90 degrees.
• This shockwave is called an oblique shockwave.

Question 5

What is a special oblique shockwave?

Answer 5

A normal shockwave which is when the angle is 90 degrees.

Question 6

What happens to air pressure as it passes through an oblique shockwave?

Answer 6

Air pressure increases when it passes through an oblique shockwave.

Question 7

What is the deflection angle?

Answer 7

Angle between the oblique shockwave and the air flow direction before the shockwave.

Question 8

What is the shock angle?

Answer 8

The air flow direction before the shockwave.

Question 9

What happens to the supersonic airflow after it passes through the oblique shockwave?

Answer 9

Changes direction. Flows in the parallel direction to the deflected surface.

Question 10

What happens if the deflection angle of a surface is smaller than the maximum deflection angle at M1?

Answer 10

• Two possible values of β, meaning there are two possible oblique shockwaves
• Smaller one of those two β, indicates the oblique shockwave is a weak shock, i.e M2 > 1. Airflow after this oblique shockwave is supersonic.
• Greater one of those two β indicate that the oblique shockwave is a strong shock, M2 < 1. Airflow after this oblique shockwave is subsonic.

Question 11

What is assumed when the incoming Mach number is a high Mach number supersonic flow?

Answer 11

Assume that airflow is sonic, M2 = 1 behind the oblique shockwave for a high Mach number supersonic flow when the deflection angle is equal to maximum deflection angle.

Question 12

What is the strong shock angle when the deflection angle is “0”? What about the weak shock angle?

Answer 12

• β = 90º, i.e. a normal shockwave.

- Weak shock angle depends on the value of the incoming Mach number M1.

Question 13

Where do strong shockwaves occur?

Answer 13

Internal supersonic flow, i.e. inside turbine or at the exit of a jet engine.

Question 14

Where do weak shockwaves occur?

Answer 14

In external supersonic flow, e.g. airflow over aerofoil or fuselage of an aircraft.

Question 15

What happens if the deflection angle is greater than the maximum deflection angle at a M1, θ > θmax?

Answer 15

• Oblique shockwave detached from leading edge of the deflected corner.
• Bow shockwave formed at the leading edge of an aerofoil, if the Mfs reaches Mdet, i.e. Mfs = Mdet.

Question 16

Describe how an expansion fan/expansion region is formed? (4)

Answer 16

• Compressible airflow expanse when it travels into a volume increasing location or encounters a pressure-decreasing disturbance.
• Pressure expansion wave will be formed at the entrance of the disturbance, e.g. airflow travels past a surface, which is deflected downwards.
• If the air is supersonic airflow, the expansion wave cannot propagate with the airflow.
• Thus a series of expansion waves at different stages of the expansion will be formed, known as EXPANSION FAN, or EXPANSION REGION, when a supersonic airflow passes a convex corner of a surface.

Question 17

What happens to air pressure and airspeed when a supersonic flow is deflected away from the relative airflow?

Answer 17

• Air pressure decreases

- Airspeed increases

Question 18

What is an expansion wave?

Answer 18

Mach wave that forms at the deflected surface is known as a Mach wave.

Question 19

Is the expansion process an irreversible adiabatic process? Why/why not?

Answer 19

No heat is being transferred to and from the airflow theoretically, so this expansion process is an isentropic process - reversible adiabatic process.

Question 20

What happens to the pressure and airspeed when the deflection angle is great?

Answer 20

Lower the pressure higher the airspeed going to be after the expansion region when deflection angle is greater.

Question 21

When will the expansion region be at its limit?

Answer 21

When air expands so much that pressure decreases to “0”, the airspeed will reach the highest possible value, and the expansion region will be at its limit.

Question 22

What is the value of extreme limit of deflection angle for air?

Answer 22

130.45º

Question 23

Can air expand beyond the θ = θmax?

Answer 23

No

Question 24

What is the line at the end of θmax called?

Answer 24

Slipstream

Question 25

What is stagnation region?

Answer 25

The area between the end of the deflection angle, θ, and the slipstream.

Question 26

What is the pressure at the slipstream? What does it mean for airspeed?

Answer 26

• 0

- Means that airspeed reaches its maximum value.

Question 27

What is the pressure and airspeed at the stagnation region?

Answer 27

• Pressure reaches its maximum value.

- Speed is 0.