detroitflyer
Posts: 371
Joined: Fri Oct 27, 2006 11:01 am

Nozzle's Question For A Supersonic

Mon Apr 30, 2007 4:31 pm

I know for a fact that a nozzle for most current transport aircraft today are convergent, which make sense- the air shoots out at a faster speed the more it is compressed= more thrust

However for supersonic aircraft i hear they use a mix between the 2, called a convergent/divergent nozzle, which they describe as convergent till mach 1 and then divergent from then on. Why is that?? After airflow becomes supersonic, does physics reverse or something?? If so why??
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lehpron
Posts: 6846
Joined: Tue Jul 10, 2001 3:42 am

RE: Nozzle's Question For A Supersonic

Mon Apr 30, 2007 5:51 pm

Quoting Detroitflyer (Thread starter):
However for supersonic aircraft i hear they use a mix between the 2, called a convergent/divergent nozzle, which they describe as convergent till mach 1 and then divergent from then on. Why is that?? After airflow becomes supersonic, does physics reverse or something?? If so why??

Consider something called a Venturi tube (look it up sometime), it is a tube that is wide open on both ends and pinched in the middle such that the end with flow going in is the convergent section and the other is divergent. The small part in the middle is common called the throat.

Under subsonic conditions, air going in will speed up and be fastest at the throat, but the air pressure and temperature will actually drop (there is no compression) and in the divergent section, the flow will increase back up to how it was when it came in, more or less.

Under a supersonic/hypersonic condition, while flow accelerates towards the throat and while the temperature and pressure will still drop (overall still acting subsonic so far), if it reaches the speed of sound at the throat, a vertical shockwave forms. After a wave is created, instead of the subsonic condition where flow would slow down and temperature and pressure would rise again, the opposite happens due to the presense of shockwaves. Instead, the temperature and pressure continue to drop and the velocity of the flow skyrockets. Rocket engines and supersonic wind tunnels follow the same idea to get flow moving past sound.

Consider the Space Shuttle's main engines, in less than 15 feet of total nozzle distance, where the first third is the combustion chamber/convergent section, the flow accelerates from zero to 4km/s!

Hope this was helpful, if I was inaccurate, please correct.

[Edited 2007-04-30 11:08:24]
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oly720man
Posts: 5802
Joined: Fri May 21, 2004 7:13 am

RE: Nozzle's Question For A Supersonic

Mon Apr 30, 2007 7:59 pm

Quoting Lehpron (Reply 1):
if it reaches the speed of sound at the throat, a vertical shockwave forms. After a wave is created, instead of the subsonic condition where flow would slow down and temperature and pressure would rise again, the opposite happens due to the presense of shockwaves

You only get a normal shock if the downstream flow is subsonic.

A lot depends on the downstream pressure and the cross section of the divergent throat. The flow can either accelerate supersoncially or go subsonic again. I some cases, if the divergent throat is badly designed, or the flow separates, it can accelerate supersoncally then a normal shock occurs and the flow goes subsonic - and then you can get significant pressure pulses in the nozzle that can lead to mechanical failure.

Quoting Detroitflyer (Thread starter):
After airflow becomes supersonic, does physics reverse or something?? If so why??

It's not physics reversing it's physics having 2 solutions.


http://en.wikipedia.org/wiki/De_Laval_nozzle
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