Fluids- Choked Nozzle Flow and Shock Waves Flashcards
For a convergent nozzle, what happens for decreasing back pressure from the stagnation pressure before the nozzle?
At the same stagnation pressure, there is no flow. Below stagnation pressure causes acceleration of flow for M<1 producing sub-sonic jet. If back pressure reduced sufficiently, M=1 at the throat because critical conditions have been reached. If reduced further, M can’t be >1 inside nozzle so the drop in pressure occurs outside nozzle in a supersonic jet.
When does back pressure equal the exit pressure of the nozzle?
When the exit flow is subsonic
Supersonic exit flow but the back pressure is precisely adjusted to be the exit value.
Otherwise shock waves are present in the flow outside the nozzle.
What does it mean to say a flow is choked?
Sonic at the throat.
How can you increase the mass flow rate through a choked nozzle?
Increase the throat area
Increase the inlet stagnation pressure
Decrease the inlet stagnation temperature
How can you tell in a question if the flow through a converging nozzle has reached sonic conditions?
Find the critical pressure. If this is greater than the back pressure, the flow has reached sonic conditions by the exit.
Formula for mass flow rate of choked flow
Equal to ρcucAt
Subscript c means critical conditions
At means throat area
What does adiabatic mean for stagnation conditions?
They are constant
What happens in a hydraulic jump?
There is supercritical flow before a boundary where kinetic energy is turned into static pressure energy (not isentropically) and the height of the liquid increases until subcritical flow and the height remains constant. Analogous to shock waves.
What are normal shock waves?
The shock waves that occur in a plane normal to the direction of the flow. The flow process through the shock wave is highly irreversible. They are thin surfaces in the flow which flow properties change over the thickness of a wave which is typically less than a few millimetres.
How do Ma, stagnation enthalpy, pressure and temperature and area change across a shock wave?
Ma goes from above 1 to below 1. In a thin control volume around the shock wave the area is effectively constant. Stagnation enthalpy and temperature are constant by stagnation pressure decreases.
Where to find relation between temperature, density, static pressure and stagnation pressure either side of shock wave?
In terms of γ and M on page 7 of lecture notes and page 67 LBOTF
What can be said about static pressures either side of shock wave?
It always increases across a shock wave
Where to find M ratio across shock wave
Page 9 of notes or formula sheet
Where are shock tables and what can they be used for?
Page 70 LBOTF used for γ=1.4 diatomic gases.
Instead of using the equations can just look up values
How to approach problems with moving shock waves
Treat it like fluid moving through stationary shock wave so if shock wave is moving right, fluid is moving through it left
