@ Walter J. Tingle 的回答
Those are Mach diamonds & Mach discs. They’re a series of oblique & normal shocks as the exhaust equalizes its pressure with the surrounding atmosphere.
@ Jeffrey Naujok 的回答
Walter Tingle is absolutely correct, those are Mach diamonds and Mach discs.
Walter Tingle 说的是对的，他们是马赫菱形和马赫圆盘。
Which, if I’m honest, is about as useful as saying they’re woogles and squiggles.
Short of an hour or two on Wikipedia and some physics books, that doesn’t really tell you anything, which is a shame, because why they’re there really gives some insight into high-velocity physics in flame fronts that’s really quite fascinating.
Mach diamonds, or shock diamonds, don’t only happen in rocket engines. They can also be seen in jet engines, for example this picture of the SR-71’s mach 5 capable J58 engines at full afterburner (which is actually a series of Mach discs):
Mach diamonds happen because the thrust leaving the engine is moving at supersonic speeds. As they leave the engine, this speed and expansion causes the air pressure inside the exhaust cone to drop below ambient air pressure.
The surrounding air pressure, then, begins to squeeze the exhaust. According to the ideal gas law, as you raise the pressure of a gas, the temperature also increases, and the exhaust temperature climbs up to ignition temperature again. This causes un-burned fuel to ignite within the exhaust plume.
That’s what the Mach discs are, the point in the supersonic flow where the temperature has reached high enough to re-ignite the gasses in the stream. This causes expansion, and once again the air pressure drops below ambient, the exhaust cools, and burning ceases. Then the air pressure begins to squeeze the exhaust again and you get the next Mach disc.
This continues down the length of the exhaust column until either the fuel is exhausted, or enough ambient air mixes into the plume to provide further oxygen, or, more typically, the exhaust drops below supersonic speeds and becomes turbulent.
In the above picture, you can clearly see that the exhaust is compressed as the column proceeds, until it reaches the very end where it drops to subsonic speed in a “ragged tail” on the end of the flame. At the same time, ambient air is mixing with the exhaust causing the burning to become less and less efficient (turning more and more red instead of blue) as the column proceeds.
Mach diamonds are just Mach discs as a cone, and are more common in rocket thrust because of the shape of the expansion nozzle. Rather than the exhaust column forming a flat disk, like you see in a jet engine, the exhaust expansion at the nozzle is more cone shaped, and thus the pressure waves in the exhaust follow that cone shape, creating the more “diamond” shaped mach disc.
All this is a little simplified, as there are also supersonic shock waves (expansion fans and compression fans) traveling within the exhaust cone. These expand from the nozzle, and “reflect” from the boundary of the supersonic flow and the ambient air. The expansion fan, turned inwards, becomes a compression shock wave, and the point at which that shockwave compresses the flow is where the Mach disc forms. This can actually be predicted with a pretty simple function:
所有这些都略微简化了，因为在排气锥内还传播着超声波冲击波（膨胀风扇和压缩风扇）。 它们从喷嘴膨胀，并在超音速气流周围空气的边界“反射”。 向内旋转的膨胀风扇变成压缩冲击波，并且该冲击波压缩气流的点就是马赫圆盘形成的位置。 实际上，可以通过一个非常简单的函数来预测：
where D0 is the diameter of the nozzle, P0is the pressure in the exhaust column, and P1 is the ambient pressure.
Mach discs and diamonds are named after Ernst Mach, who first described the physics behind them.