Why does air condense when a jet approaches the sound barrier?

[Clausius2]

This is my second post after that of "Shuttle Main Engines" in which I use again the question: what is happening here?

First of all, open the .jpg file below.

It shows a F-4 Phantom breaking the sound of barrier. I'm interested on what happens in the condensated flow. First of all, why is it condensated?. I know the flow shown is a transonic flow. So that, the shock wave is almost being formed in the zone in which Ma=1. In fact, it seems to be two shock waves being formed.

Behind the shock, the Pressure and Temperature raise a lot. But I have not found reasons for why this rising could provoke a condensation of the air in water vapor. On the other hand, do you think the borders of the condensed flow are the shocks waves themselves?. Are there any reason for thinking so?

Any comments appreciated.

 

[LURCH]

Not sure about this statement:

...Behind the shock, the Pressure and Temperature raise a lot. But I have not found reasons for why this rising could provoke a condensation of the air in water vapor...

Wouldn't pressure and temp rise sharply just ahead of the shockwave, where energy is being put into the air more rapidly than it can be dispersed (by soundwaves)? If so, then temp and pressure drop sharply behind the shockwave, causing moisture in the air to condense.

 

[Clausius2]

Not sure about this statement:



Wouldn't pressure and temp rise sharply just ahead of the shockwave, where energy is being put into the air more rapidly than it can be dispersed (by soundwaves)? If so, then temp and pressure drop sharply behind the shockwave, causing moisture in the air to condense.

I don't think so, Lurch. See http://tigger.uic.edu/~kenbrez/html/shock_wave.html ;

<quote:The gases behind a shock wave are at a much higher temperature, pressure and density than the gases in front of the shock wave>

This is a direct consequence of the 2nd principle of Thermodynamics. Saying P and T has to be higher behind is the same thing that saying entropy has to be increased through the shock.

 

[Rothiemurchus]

Surely the pressure is disproportionatly higher than the temperature.
Also, the aircraft is close to the ground and there could be lots of
dust in the air which would help condensation of
water vapour.

 

[GENIERE]

IMO the condensate occurs because the pressure is below the partial pressure of water vapor in the surrounding air.

Check this site for neat pictures.
http://chamorrobible.org/gpw/gpw-20040817.htm

And this site for info.

http://www.galleryoffluidmechanics.com/conden/pg_sing.htm


...

 

[Clausius2]

IMO the condensate occurs because the pressure is below the partial pressure of water vapor in the surrounding air.

Check this site for neat pictures.
http://chamorrobible.org/gpw/gpw-20040817.htm

And this site for info.

http://www.galleryoffluidmechanics.com/conden/pg_sing.htm


...

Thanks. I'm quoting it from your last link:

"Near Mach 1, the Prandtl-Glauert singularity has amplified all pressure perturbations. As a result, the regions of expansion ( low pressure ) above the wings and cockpit correspond to much lower pressures than we would expect in an incompressible flow. As in other condensation problems, the lowered bulk pressure results in a lowering of the temperature causing condensation of the ambient water vapor. "


May someone elaborate a bit more this?. I've never heard about that. Has anybody some idea about what is it meaning?