Shock Collar

An F/A-18C Hornet fighter jet assigned to Strike Fighter Squadron (VFA) 113 breaks the sound barrier during an air power demonstration over the USS Carl Vinson aircraft carrier and Carrier Air Wing 17 in the Pacific Ocean in this U.S. Navy handout photo dated June 3, 2011 (hat tip, MSNBC photoblog).

What causes this? Let’s turn to the Prandtl–Glauert singularity

The Prandtl–Glauert singularity is the prediction by the Prandtl–Glauert transformation that a shock wave approaching infinite pressure is produced as an aircraft approaches the speed of sound. Because it is invalid to apply the transformation at these speeds, the predicted singularity does not emerge. Nevertheless, extreme atmospheric conditions are produced which may give rise to a vapor cone, shock collar, or shock egg of visible condensation.

The air pressure in the immediate vicinity of a transonic aircraft can be described as an “N-wave,” so named because a plot of pressure against time resembles the letter ‘N’. Observed from a stationary point as a shock wave passes, the pressure profile of the wave is composed of a leading compression component (the initial upward stroke of the “N”), followed by a pressure descent forming a rarefaction of the air (the downward diagonal of the “N”), followed by a return to the normal ambient pressure (the final upward stroke of the “N”). The rarefaction may be thought of as the “rebounding” of the compression due to inertial effects.

Since heat does not leave the affected air mass, this change of pressure is adiabatic, with an associated change of temperature. In humid air, the drop in temperature in the most rarefied portion of the shock wave (close to the aircraft) can bring the air temperature below its dew point, at which moisture condenses to form a visible cloud of microscopic water droplets. Since the pressure effect of the wave is reduced by its expansion (the same pressure effect is spread over a larger radius), the vapor effect also has a limited radius. Such vapor can also be seen in low pressure regions during high–g subsonic maneuvers in humid conditions.

Thanks, I needed that. Pretty exciting in real life…