Re: "Visible" Sonic shock wave

[William Beaty <billb@ESKIMO.COM>]

On Tue, 12 Jan 1999, David Simmons wrote:

> A colleague showed me a photo on the web today. It purports to show an
> F-4 Phantom at an air show "in the act" of breaking the sound barrier.
> It shows one large and a smaller "shock cone"? of perhaps condensed
> moisture radiating out from the aircraft.
>
> If this is really what is happening...
>
> I  Can anyone shed light on the cause? My guess is that the shock wave
> caused condensation in very humid air?

On high-humidity days, a sudden decrease in air pressure causes an instant
cloud to appear.  I've seen this happen above the wings of airliners
landing in the rain (also the wingtip vortices become white tubes of
cloud-stuff, and even the corners of the flaps all develop little white
vortex cores.)

Knowing this, I'd say that the white stuff in that photo is a cloud, and
marks the boundaries of a low-pressure region which follows the aircraft.
Since the borders are straight, I bet that these probably are shockwaves
caused by the air locally exceeding the speed of sound relative to certain
parts of the aircraft, even though the aircraft as a whole hasn't exceeded
the speed of sound.

> II Would this really be a transient event? Since the shock wave is
> formed continuously as long the aircraft is exceeding the speed of
> sound, wouldn't this effect be continuous?

Each parcel of air would see it as a transient event as the aircraft moved
past the unmoving parcel.  If you followed the aircraft, the white regions
would appear stable (although they might flicker a bit or even wink on and
off as the aircraft encountered regions of low and high humidity in the
atmosphere.)  Yet the white regions would be like visible waves, where the
wave moves fast and the medium (the humid air) does not move.

> You can view the picture at
>
>    http://www.artbell.com/images/jetbarrier.jpg
>
> And, no, I am not an Art Bell fan.

Ever listen to Firesign Theater albums in the '70's?  Their "Everything
you know is WRONG" album nails Art Bell perfectly!  Broadcasting from a
trailer in the High Desert!  Playing secret military training films about
what to do if an alien appears!  Nino the Mindboggler mentally melting a
watch for Dr. Dali at the Soviet Socialist Institute of Electrical Shocks
and Psychic Suprises!

Whoda thunk that twenty years later we'd be able to tune our radios late
at night and listen to some *REAL* gas music from Jupiter.  :)

((((((((((((((((((((( ( (  (   (    (O)    )   )  ) ) )))))))))))))))))))))
William J. Beaty                                  SCIENCE HOBBYIST website
billb@eskimo.com                                  http://www.amasci.com
EE/programmer/sci-exhibits          science projects, tesla, weird science
Seattle, WA   206-781-3320          freenrg-L taoshum-L vortex-L webhead-L