Divergent exhaust duct with static pressure loss?

[Founder]

Here's the case:

According to a book I'm reading about exhaust nozzle's the static pressure in a convergent-divergent duct, the static pressure first decreases as exhaust enters the convergent part of the duct. According to the book when the exhaust has reached the choke point (mach 1) at the throat it enters the divergent duct and static pressure still continues to fall. Shouldn't the static pressure rise as the velocity of the exhaust decreases...?

if not, does anyone have a better explenation to why the static pressure continues to decrease? And why does the velocity of the air continue to accelerate even though the duct becomes divergent?

Best Regards...
Tim

[Mad (Flt) Scientist]

The point of a con-di nozzle is to accelerate the flow in the convergent section until it 'chokes' at the narrowest part - i.e. becomes sonic.

The nature of the equations for flow in a duct switches when flow is supersonic - now increasing area continues to accelerate the flow and the pressure continues to drop. If you want a headache you could look up the maths behind it (I don't, so i won't)

A Con-di intake is the same in reverse - convergence to SLOW the flow at first (with due attention to getting the shocks where you want them) then divergence to further slow the flow once it hits M1.0.

[Founder]

Okey, now I understand... the book didn't mention anything about the change in properties that the air has when traveling at a sonic speeds...

Thanx =)

[Mad (Flt) Scientist]

If you want a non-maths way of thinking about it, the properties change because in subsonic flow the flow can 'see' changes in the downstream conditions and adjust accordingly. Once sonic the flow is essentially travelling 'blind' and can only react to the local and previous conditions, but not the ones in front. (It's not exactly a rigourous explanation, but it might help ?)

[Founder]

If you want a non-maths way of thinking about it, the properties change because in subsonic flow the flow can 'see' changes in the downstream conditions and adjust accordingly. Once sonic the flow is essentially travelling 'blind' and can only react to the local and previous conditions, but not the ones in front. (It's not exactly a rigourous explanation, but it might help ?)

It does help a lot =) I'm not that good at supersonic aerodynamics and I have to say that I'm having a hard time understanding some books where the author depends on the reader having such knowledge...

Can you reccomend any good reading? the ATPL course books from oxford doesn't say much about it... mostly only concentrates on flow over and under a wing... =(