Re: shock diamonds in jet exhaust

[John Denker <jsd@MONMOUTH.COM>]

At 05:48 AM 4/18/99 -0700, Mike Wilson wrote:
> In watching the military jets take of on the nightly news I notice the
> exhaust exhibits patterns of bright exhaust, dark lines, bright exhaust,
> etc.

The pattern is called "shock diamonds".

Check out
  http://www.af.mil/photos/Oct1997/x1-1.jpg
  http://www.dfrc.nasa.gov/gallery/photo/X-2/Small/E-2822.jpg
  http://www.wvi.com/~lelandh/sr71engb.jpg
  http://www.dfrc.nasa.gov/gallery/photo/SR-71/Small/EC92-1284-1.jpg
  http://www.dfrc.nasa.gov/gallery/photo/SR-71/Large/EC92-1284-1.jpg (8 meg!)
for some examples.

> Am I seeing some type of interfernce pattern from light?

It's an air-wave phenomenon, not a light-wave phenomenon.

> Why do I only notice it in military jets?

It has to be supersonic, which implies very very loud, which is why you
won't see it just every day.  But Dutch rocket amateurs report making shock
diamonds in their back yards:
  http://www.iaehv.nl/users/nero/hybrid_engines.html

> Am I seeing some type of combustion pattern (I can't imagine what)?
> Does it depend on the use of after burners?

Combustion is not the main story.  Combustion is typically essentially
complete well upstream of the shock diamonds, and indeed you can have
shock-diamond-like structures with no combustion at all;  check out
  http://kosmos.aip.de/~cfendt/jet3.html
for an astrophysical example.  Cold air from a pressure tank will make
perfectly good shock diamonds, but you'll have to do some work to visualize
them because they won't glow in the dark.

--------------

I don't entirely understand the physics involved, but the basic idea is
this:  Shock waves in the jet bounce off the boundary between the jet and
the ambient air.  (Don't try to use a simple index-of-refraction argument
to explain the bounce.  Index of refraction doesn't make sense in a
supersonic medium.)

(Note that I am using the word "jet" with its original meaning, referring
to the plume of air, not as shorthand for "jet propelled aircraft".)

One thing is for sure:  If you see N diamonds it does *not* mean the jet is
Mach N.  It just means the shock bounced N times before becoming so mixed
with the ambient air that it became subsonic or just so cool as to be hard
to see.

You *can* estimate the Mach number by looking at the angle made by the face
of the diamond.

Cheers --- jsd