Module 2 - Compressible Airflow Flashcards
According to the energy equation, what is the relationship between the temperature and the speed of the airflow?
Temperature increases as the speed of the flow decreases
What is the stagnation temperature? What is its relationship with the speed of sound and speed?
It is the temperature when;
v=0
a=max
What is the critical point? What are the characteristics of M, a, T and v?
- It is the point where v=a.
- Airflow speed is equal to the speed of sound
- At this point M=1
- Critical temperature is the temperature at this point
How can the critical speed of sound be determined?
By using the formula for calculating the speed of sound and using the critical temperature
What is the Speed coefficient M*?
It is the ratio speed of the airflow to the critical speed of sound
What does M* indicate? What is the relationship between M, M* and a? (if a= M=1)
If the airflow is subsonic or supersonic
M < M* < 1 < M* < M
M* is always closer to M=1 than the actual M at that point
When the change in area of the flow path is negative in a subsonic flow what will happen to pressure and velocity?
What is this situation called?
Change of speed will be positive
Change of pressure will be negative
Called a converging nozzle
When the change of area of the flow path is positive in a subsonic flow, what will happen to pressure and velocity? What is this situation called?
Change of speed will be negative
Change of pressure will be positive
Called a diffuser
When the change of area of the flow path is negative in a supersonic flow, what will happen to pressure and velocity? What is this situation called?
Change of speed will be negative
Change of pressure will be positive
Called a diffuser
When the change of area of the flow path is negative for a supersonic flow, what will happen to pressure and velocity? What is this situation called?
Change of speed will be positive
Change of pressure will be negative
Called a diverging nozzle
In a converging diverging nozzle what occurs at the location where change in area is 0?
How does the outlet speed change as the inlet speed increases?
- This is the minimum area, called the throat.
- The critical air properties will be present
- M=1, a(c), T(c), ρ(c), p(c)
If the inflow air was subsonic the outflow air will be supersonic
If the inflow air was supersonic, the outflow air will be subsonic
What is a shockwave?
An accumulation of pressure waves that form a front, called a shockwave
What are the dimensions and air property chanegs that occurs across a shockwave? What about v and M?
Shock is about 1mm thick (very thin)
- Pressure, temperature and density all increase behind a shockwave
- Mach will be supersonic before the normal shock and subsonic after the normal shock
- Velocity of the airflow will reduce behind the shockwave
How is a shockwave formed?
- When airflow is supersonicthe particels travel faster than the speed of sound
- If there is a compression disturbance the disturbance will propogate at the speed of sound
- Since the source of the compression disturbance is fastert than the disturbance itself.
- Since the ‘change’ can never catch up to the source the air porpeerties are not uniform and not continious through the field of fluid flow
- The wave fronts from the source all build up and accumulate to formt a front called a shockwave
What energy principal applies to airflow across a shockwave?
The shockwave is an irreversable adiabatic process, there is no heat transfer
