Ascher Loose Diamonds

Q.What determines the color of the afterburner flame ? On some pictures taken at night, the flame is rather blue-white, and during the day it is red-orange-yellow, like a blowpipe. What are the small "rings" in the flame ? You can see them on the Eurofighter, F-15, F-16 I think. Is it a pulse phenomen, like a pulsoreactor ?

A.Shock diamonds are formed when an underexpanded supersonic jet exits its nozzle. The nozzles on most all supersonic aircraft are convergent-divergent (Con-Di). The point of the nozzle is to accellerate the exhaust flow to as high a velocity as possible converting heat energy to kinetic and thus, thrust. But when you pour the coal to it in A/B the pressure, velocity and temperature goes way up and the area ratio required to have a perfectly expanded flow would be prohibative. So the flow leaving the nozzle is underexpanded - that is, its static pressure is still higher than the free stream. This means the exhaust flow must expand to match free stream conditions outside the nozzle. In supersonic flow this is done through a series of shock waves and expansion fans (opposite of a shock). Mach diamonds are the pattern formed by the shocks which succesivley turn the flow as it expands and decelerates. The more underexpanded the jet, the more Mach diamonds form. This is a classical compressible flow effect which can be found in any good fluids text. The best is "The Dynamics and Thermodynamics of Compressible Fluid Flow" by Ascher H. Shapiro, original copyright 1953. If you are careful you can see Mach diamonds in the jet of a high pressure air nozzle if you get the lighting and background right.

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