Why can shock waves condense water vapor?

[jaumzaum]

Why can a shock wave condensate water droplets in the air and produce the visible vapor cone that we see when objects move faster than the speed of sound. Also, does this condensation happens only when the object is moving with a velocity greater than sound velocity? I don't understand the relationship between these two phenomenons

 

[Delta2]

I think its because the temperature and the pressure of the air in the shockwave have great fluctuations, so they might get to values of specific (temperature , pressure) pair that is required for water vapors to condensate.

 

[A.T.]

Why can a shock wave condensate water droplets in the air and produce the visible vapor cone that we see when objects move faster than the speed of sound.

https://en.wikipedia.org/wiki/Vapor_cone

Also, does this condensation happens only when the object is moving with a velocity greater than sound velocity?

The shockwaves can form with the plane still below the speed of sound, because the air flows faster at some points. You also can get condensation trails at the wingtips, where the vortices produce a pressure drop during some maneuvers.

And it happens also in shockwaves from explosion:
https://en.wikipedia.org/wiki/Condensation_cloud

 

[Lnewqban]

Please, see what happens at low speeds:

 

[boneh3ad]

Why can a shock wave condensate water droplets in the air and produce the visible vapor cone that we see when objects move faster than the speed of sound. Also, does this condensation happens only when the object is moving with a velocity greater than sound velocity? I don't understand the relationship between these two phenomenons

Shocks do several things to the air passing through them: they increase pressure, increase temperature, increase density, and decrease velocity relative to the shock. Two of those things tend to compete with each other when it comes to condensation: higher temperature tends to cause things to evaporate but higher pressure would tend to make them condense.

As it turns out, the increased temperature is much more important and so shocks do not cause water to condense. You actually need the opposite effect to occur. The temperature needs to drop below the dew point, and it turns out that you actually need an expansion, not a compression. What you typically see (e.g. in the example of the F-18 with a vapor cone) is the result of a Prandtl-Meyer expansion where the flow is locally supersonic and accelerates further, lowering the dew point a little but lowering the static temperature substantially. The result is vapor. The vapor cone abruptly terminates when it encounters the shock that forms to recompress the flow.

https://en.wikipedia.org/wiki/Vapor_cone

The shockwaves can form with the plane still below the speed of sound, because the air flows faster at some points. You also can get condensation trails at the wingtips, where the vortices produce a pressure drop during some maneuvers.

And it happens also in shockwaves from explosion:
https://en.wikipedia.org/wiki/Condensation_cloud

Note that a pressure drop would tend to encourage water vapor to remain a vapor. It’s the temperature drop that ultimately overcomes this.

 

[nettleton]

Increases in pressure and temperature depend on the Mach Number of the shock, so that there is no general solution to the problem of condensation occurring in the flow behind it. There is a further problem in the definition of speed of sound in what may be an inhomogeneous mixture of air and water vapour.

 

[boneh3ad]

Increases in pressure and temperature depend on the Mach Number of the shock, so that there is no general solution to the problem of condensation occurring in the flow behind it. There is a further problem in the definition of speed of sound in what may be an inhomogeneous mixture of air and water vapour.

Your answer contradicts observations of the phenomenon, though. Vapor cones always form in regions where the flow is accelerating and they always terminate along sharp lines with larger angles (relative to the free stream) than where they formed. This is classic behavior for Prandtl-Meyer expansion waves (which emanate at the Mach angle, $\mu = \arcsin(1/M)$ and oblique shocks, which have angle $\beta > \mu$).

In the photo above, you see one over the cockpit where the flow becomes supersonic and continues expanding before the curvature turns concave. The second cone occurs when that subsonic flow downstream of the cockpit accelerates again and forms another expansion wave before terminating at the termination shock at the trailing edge of the plane.