为什么有些火箭尾焰是正弦型的?Why does the exhaust in some rocket engine tests have a sinusoidal shape?

【补两张图】

@ 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.

这些是马赫点和马赫圆盘。这是尾气与周围大气互相抗衡时产生的一系列斜激波/垂直激波。

@ 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):

马赫菱形,或冲击菱形,并不只在火箭引擎中出现。在飞机引擎中也可以看见。例如SR-71的马赫数为5的J58引擎,在全补燃后的照片(这些都是马赫圆盘)

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.

马赫菱形产生的原因是,推力能让引擎以超音速移动。当尾气被排出,尾气的速度和膨胀让尾气锥里的气压小于环境的气压。

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.

周围的大气压,然后就开始挤压尾气。根据理想气体法则,如果你增加一个气体的气压,那么它的温度也升高,所以尾气的温度到达了能再次点燃的温度。这让尾气中剩余的燃料再次被点燃。

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.

这就是马赫圆盘,超音速气流的温度到达了能再次点燃气体的温度。所以这些气体就开始膨胀,当这些气体的气压开始抵御周围大气压时,尾气就开始被冷却,燃料也无法继续燃烧了。于是尾气再次被周围大气压挤压,于是和之前一样,又有了下一个马赫圆盘。

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.

这会一直持续到尾气中剩余的燃料被用完,或者尾气的速度不再是超音速,或者气体与大气压的压差消失。【??】

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.

在上图中,您可以清楚地看到,从尾气柱往前看,尾气一直被压缩,直到最末端时在火焰末端的“参差不齐的尾巴”中降至亚音速。 同时,随着尾气柱的进行,周围的空气与排气混合,导致燃烧的效率越来越低(变成红色而不是蓝色)。

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.

马赫菱形只是锥形的马赫云盘。由于喷嘴膨胀其他的特点,这在火箭中更常见。相较于在飞机引擎中你看到的扁圆盘,气体在火箭引擎的碰嘴中更类似于锥形,压力波由此会将其塑造成“菱形”的形状。

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.

D0是碰嘴的直径,P0是尾气的气压,P1是周围的大气压。

Mach discs and diamonds are named after Ernst Mach, who first described the physics behind them.

马赫菱形和马赫圆盘由Ernst Mach命名,他第一次描述了这种现象后的物理原理。

【以及两个很棒的文章】

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