AdvancedTopic 79 of 105Thermodynamics & Fluids
Normal shock waves
Abrupt supersonic-to-subsonic transitions with entropy rise.
In depth
A normal shock is a thin discontinuity where supersonic flow becomes subsonic with a jump in pressure, density and temperature, and entropy increase (stagnation pressure loss). Rankine-Hugoniot relations; M₂²=(1+0.2M₁²)/(1.4M₁²-0.2) for air. Oblique shocks arise on wedges.
Key points
- Irreversible
- Stagnation temperature constant
- Inlet design avoids strong shocks
ExampleAt M₁=2, static pressure rises 4.5 times across a normal shock.
What is Normal shock waves?
Abrupt supersonic-to-subsonic transitions with entropy rise.
Why is it important to understand normal shock waves?
Normal shocks are abrupt jumps in pressure and temperature in supersonic flow, which must be known to avoid overestimating performance.
What happens across a normal shock?
Velocity drops from supersonic to subsonic, pressure and temperature rise sharply, and entropy increases, so the process is irreversible.
Why are shocks a problem for engines?
They cause large total pressure losses and can cause flow separation, which reduces the efficiency of intakes and compressors.
How is Normal shock waves used in practice?
Typical applications include predicting the pressure rise across a shock in a supersonic inlet, checking the loss in a supersonic compressor cascade and designing an intake to limit shock losses at high Mach number.
What should I learn before Normal shock waves?
Study "Isentropic nozzle flow" first (topic 78 of 105 in Thermodynamics & Fluids). It covers the groundwork this topic builds on.
What are the key points of Normal shock waves?
Irreversible. Stagnation temperature constant. Inlet design avoids strong shocks.
Can you give an example of Normal shock waves?
At M₁=2, static pressure rises 4.5 times across a normal shock.
What comes after Normal shock waves?
The next topic is "Navier-Stokes equations", which builds directly on this one.
How difficult is Normal shock waves?
It is classed as Advanced in Thermodynamics & Fluids (topic 79 of 105). It is an advanced topic. It assumes you are comfortable with the intermediate topics in Thermodynamics & Fluids.