# UK taking physics and looking to study aero eng.

## Main Question or Discussion Point

hello all, i'm new here...an A-level student in the UK taking physics and looking to study aero eng. thats my profile atm.

to topic.........obviously when a supersonic flow travels through a normal shock it reduces to subsonic, but which is the cause and which is the effect? apparently the shock forms due to combined pressure waves (from constructive interference from supersonic, sonic, and subsonic regions) forming a large wavefront.

so from that, you could argue that the flow slows down and the shock forms as a result of that. rather than the shock slowing down the flow.

i may be arguing semantics here. or i might be wrong.

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Clausius2
Gold Member
Saoist said:
to topic.........obviously when a supersonic flow travels through a normal shock it reduces to subsonic, but which is the cause and which is the effect? apparently the shock forms due to combined pressure waves (from constructive interference from supersonic, sonic, and subsonic regions) forming a large wavefront.

so from that, you could argue that the flow slows down and the shock forms as a result of that. rather than the shock slowing down the flow.
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Welcome to PF.

In order to look for the cause and the effect, you may as well to imagine an unsteady process. Imagine there is an stream of supersonic flow everywhere. Imagine you instantaneously put an obstacle (such as an airplane nose) in the stream. Fluid particles travelling downstream will find suddenly a solid surface, crashing into it and causing a rising in local pressure. This pressure wave will try to travel upstream, but it is impossible because its proper wave speed is smaller than the convective fluid velocity. Once the steady state is reached against, in each time you will find a zone of crowded of particles behind the wave, which makes the downstream travelling particles to crash into this "traffic jam" and become slowered down.

So the logical secquence is:

I) the shock is formed by means of a solid surface submerged in a supersonic stream

II) the shock and the great density behind it slows down the fluid once it has passed trough the shock, which by the way has a thickness of the order of the inverse of the Reynolds number.