Vortices and condensation

Pretty sky curls, by munir.

Wondering aloud on flickr how those curls got there (spin of an engine's exhaust? was the plane actually spinning?) and this morning in my inbox my brother Amgad, finishing up his Physics PhD, sent me an explanation given to him by a colleague:

On 29.01.2009, at 15:02, Gregory Bewley wrote:

Hi Amgad,

I don't know what caused that pattern. But here is some information anyway:

Strong vortices form at the tips of the wings. You can think of it as being caused by air spilling out from under the wing at the tip, and swept back behind the plane in a continuous scroll. The vortex core is parallel to the direction of flight.

Combustion of fuel produces water, among other things, and the water condenses behind the plane as the exhaust cools. Some of this moisture gets trapped in the cores of the vortices. This stuff tends to hang around longer than the moisture outside the cores of the vortices, since the stuff in the core doesn't mix much with the dry outside air, which eventually evaporates the moisture back into the gas phase.

So, the condensation in the vortex core is a pretty good marker of the position of the core, and on a good day you can see some interesting vortex dynamics as the two wing tip vortices interact with each other. Most often, you will see the parallel vortices become wavy, and then intersect and reconnect, forming a chain of vortex rings. So, my guess is that the pattern in the photo came about by the interaction of the tip vortices with each other, but i don't know how.

by the way, the exhaust of the jet engine does not swirl - if it did, this would be energy wasted that could have been used to propel the airplane forward.

In the same way, the wing tip vortices contain a lot of wasted energy, and airplane wings sometimes have fins on the wing tips to try to minimize the strength of the tip vortex.

Hope that helps,

Greg

This reminds me that I should probably have chosen to study a real science where they can explain real things, rather than the make-believe world of academic economics.