Why do you see 'rings' behind an afterburner?

Question

I saw a Eurofighter Typhoon the other day and when my hearing had recovered I looked at some photos of it and wondered why you see a series of 'rings' behind the afterburners? Here is a picture of what I mean:

http://www.flickr.com/photos/11444813@N03/2815794886/in/pool-bournemouthairfestival

is it a photo artefact?

Answer 1

The exhaust is supersonic relative to the surrounding air as it exits the aircraft. When supersonic flow meets up with subsonic flow in air, the interface between the two creates a shock wave. Depending on the relative velocity of the two flows, there may be multiple shock waves. When viewed from the side, these shocks form a classic diamond shape similar to the waves off the leading edge of a supersonic wing. The shock creates a change in static pressure that makes the exhaust appear differently where it crosses each shock - sort of like the shock waves that become visible when aircraft fly through clouds / water vapor.

ADDED: There is no correlation between the shock wave pattern and the number of compressor stages. The pattern does not appear until the afterburner is ignited, although the compressors may be running at the same mechanical speed / conditions with no afterburner - and of course there are no compressors within the afterburner itself.

Answer 2

mdGreg C is right, they're shockwaves

Answer 3

Shockwaves. In a way, they behave similarly to the "lens effect" that light experinces when passing through convex lens; where some places have all light rays crossing each other thus making the area very britght and areas where rays diverge so much theres dim light. For jet exhaust...I guess...there are still streams (despite the high, directional velocity) of gases that go in different directions due to deflections by compressed outside air (wall-of-sound) until the gases (their molecules) are deflected to converge almost regularly to a kind of focal point before they diverge away to region of uncompressed air...

...its complicated but from the very, very little I've understood about fluid dynamics/fluid mechanics/etc, I'd say its extremely different from the way light behaves in air.

Answer 4

Not a photo artifact.

They are shockwaves that become visible in high velocity jet exhaust.

Sometimes called 'Mach diamonds"

Answer 5

Read

http://www.aerospaceweb.org/question/pro...

Answer 6

These Are Shock Waves.

EDIT: Ray, How Do you Account for them In the SSMEs (Space Shuttle Main Engines).?

EDIT: Of Interest:

http://en.wikipedia.org/wiki/Shock_diamo...

EDIT: SphinxXXVII, the Pic of the SR-71 Reminded me of Hearing Them (and U-2s) Taking Off In Palmdale (Not Edwards AFB) and Engine Tests At Lockheed Long Ago.