This Lockheed-Martin F-16CJ Fighting Falcon belongs to the 16th Weapons Squadron at Nellis Air Force Base, seen here taking off for a night training mission for the USAF Weapons School. It’s arguably the most spectacular photo of the “Mach Diamond” phenomenon I’ve ever taken. For those of you not familiar with the physics involved in producing this visual effect, here’s a little explanation:
Under most conditions, the gases from an afterburning jet engine come out of the exhaust can at a much higher pressure than the surrounding atmosphere. Because of the higher pressure, the exhaust gases flow smoothly into the surrounding atmosphere, shoving that air out of the way as it expands.
When the aircraft is very close to the ground, as it would be during a takeoff roll, the atmospheric pressure is at its highest and the exhaust coming out of the engine is actually at a lower pressure than the surrounding air. Immediately, the atmosphere takes advantage, compressing the exhaust from all sides.
By compressing the exhaust quickly, the atmosphere has squeezed it so hard the exhaust pressure is suddenly back to a higher pressure than the atmosphere. It presses back–equal and opposite, remember?–and pushes at the surrounding air. It overreacts, expanding out so far that it returns to a lower pressure than the atmosphere around it, and gets compressed inward again. The cycle continues until the two pressurized gases finally equal out.
The compression/expansion dance between the exhaust and atmosphere creates shock waves within the exhaust flow. When the exhaust is getting compressed by the surrounding atmosphere, the shockwaves move toward that backward flow, adding extra pressure on it. That allows some of the extra fuel to ignite and glow, forming the discs or “diamonds.” Since the entire production is cyclical, it keeps going down the exhaust, putting on a stunning visual display.
Here’s a video clip of Swamp Fox Block 52s conducting night operations at McEntire Joint National Guard Base between Sumter and Columbia, South Carolina.
(Featured Image: Scott E Wolff)
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