Expert: Paul Soderman - 9/22/2005

Question
Dear Paul,
Good Day!
My question is related to fighter jets. When these jets e.g. F-16 make a steep turn or make a steep nose up postion while being having high engine power, a cloud like thing around their wings specially at the place where wings joins main fuselage. Kindly tell me why air transforms into the form of cloud like and why it happen so.
does the same phenomena occur with other aircrafts as well?
kind regards
Kuki

Answer
Hi Kuki

Good question.  Natural flow visualization around aircraft can be caused by a number of phenomena.  The most common is the cloud of water condensation that occurs in regions of low pressure and low temperature.  In humid air, the cloud can form at many places on the aircraft where the local flow is accelerating around a wing or wing-fuselage junction or other component.  Another possibility is the generation of local shock waves that diffract light and also cause condensation.  Local shocks can occur where the flow accelerates to transonic speeds.  The following web site (www.galleryoffluidmechanics.com/conden/pg_sing.htm) shows a spectacular condensation cloud on a F-14 Tomcat that is attributed to the Prandtl-Glauert singularity, a large temperature pertubation drop related to compressible flow neat Mach 1 that is explained in the links.  Although the Prandtl-Glauert singularity occurs near Mach 1, the condensation could does not require a shock wave in this case.

Paul