GeographyFlyer
Topic Author
Posts: 15
Joined: Tue Dec 19, 2017 6:59 pm

### Shock waves

Hello,

it´s been a while since I started learning about shock waves, but I still don´t fully understand it. So I have a few questions.

1. After supersonic airflow encounters a shockwave, does it remain subsonic or does it return to supersonic?
2. Why do shock waves occur in Prandtl–Meyer expansion fan? Why is there infinite number of them?
3. What are the properties of air located between a bow shock and a blunt object/sharp object which sides have higher angle than the angle of possible oblique shock?

If there is anyone so kind to answer just some of my questions I would be really happy.

Thank you, GeographyFlyer.

mxaxai
Posts: 1153
Joined: Sat Jun 18, 2016 7:29 am

### Re: Shock waves

1.: Depends. Sometimes it remains supersonic (weak oblique shock), sometimes it slows to subsonic (strong oblique shock, normal shock, detached shock).
2.: There are no shock waves in an expansion fan. Shock implies compression, i. e. the opposite of expansion. There is an infinite number of Mach waves, however.
3.: A bow shock / detached shock leads to subsonic flow between the shock and the object. Typically, the bow shock becomes a weak oblique shock further away from the object (with the flow being deflected but remaining supersonic in that area). Unless we're talking about hypersonic flow (Mach>5), the properties of the air don't change dramatically - except for the compression by the shock, of course.

mxaxai
Posts: 1153
Joined: Sat Jun 18, 2016 7:29 am

### Re: Shock waves

CF here: https://farside.ph.utexas.edu/teaching/ ... de209.html
The figure at the bottom shows the various shock types present in a detached shock.

GeographyFlyer
Topic Author
Posts: 15
Joined: Tue Dec 19, 2017 6:59 pm

### Re: Shock waves

mxaxai wrote:
1.: Depends. Sometimes it remains supersonic (weak oblique shock), sometimes it slows to subsonic (strong oblique shock, normal shock, detached shock).
2.: There are no shock waves in an expansion fan. Shock implies compression, i. e. the opposite of expansion. There is an infinite number of Mach waves, however.
3.: A bow shock / detached shock leads to subsonic flow between the shock and the object. Typically, the bow shock becomes a weak oblique shock further away from the object (with the flow being deflected but remaining supersonic in that area). Unless we're talking about hypersonic flow (Mach>5), the properties of the air don't change dramatically - except for the compression by the shock, of course.

Thanks for the clarification!

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