Module 2 Compressible Airflow

Question 1

What is the critical point?

Answer 1

Both v and a are inversely proportional when it comes to temperature. An increase in temp will lead to an increase of a and a decrease of v. The point where v and an intercept is the critical point, v = a.
This point will show the critical airspeed, the critical speed of sound, and the critical temperature.
M = 1

Question 2

What is the Speed Coefficient? M*

Answer 2

Is a ratio of airspeed ‘v’, to its critical speed of sound ‘ac’. It only changes with v.
M* cannot be a representative of whether the airflow is subsonic or not.

Question 3

Explain what happens to the airflow when it goes through a converging and diverging nozzle?

Answer 3

At the inlet of the nozzle, the airflow is subsonic, M<1, The airspeed increases, and the air pressure decreases through the converging section. This will increase M local.
It will continue to increase until M=1 at the throat, where the airflow is at its critical point and continues up.

In the divergent section, from the throat to the outlet of the nozzle. Mach number increases from 1 and airspeed and pressure increase and decrease respectively. Meaning at the outlet of the nozzle M>1

Question 4

What would happen if the inlet speed of the airflow of a converging and diverging nozzle was supersonic?

Answer 4

It would be reversed it would be M=1 at the throat and M<1 at the outlet. Air pressure would increase throughout the whole path.

Question 5

Why do shockwaves form?

Answer 5

A type of pressure disturbance can cause a shockwave in a supersonic system because the air particles travel at a speed greater than sound.

Question 6

What is the formation of a shockwave?

Answer 6

Airflow is greater than the speed of sound if there is a source of compression pressure disturbance the air particles travel faster than the pressure change and never catch up with the air particles. So the disturbance moves away from the source to form a strong wavefront and this is the front of the shockwave

Question 7

What happens to pressure, density, temperature, and velocity after a normal shockwave?

Answer 7

Pressure, density and temperature or suddenly increase after a shockwave and the airspeed will decrease.