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Here is a cropped video of the space shuttle exhaust. There appears to be some throbbing/oscillatory behaviour to the exhaust plume.

It is often visible along with an intensity changes/fluctuation in the exhaust. As if the plume is fluctuating between being rarer and denser.

Why does this happen?

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  • $\begingroup$ I am not entirely certain, but it is not impossible that this is related to Shock Diamonds, which are created by the standing wave of a supersonic exhaust. $\endgroup$ – Polygnome Jan 24 at 14:20
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This is just a classic (but very large) example of a turbulent jet.

The SRB is even used as an example in this textbook.

Turbulent eddies of many sizes and shapes are observed due to the presence of the smoke as a tracer of the turbulent motion. There is a general tendency for the length scale of the turbulent motion to increase as the eddies move along the jet. Induced by the motion of the eddies, fluid from the surrounding environment is drawn into the jet through a process called turbulent entrainment. The extent of the turbulent zone increases and the concentration of the tracer decreases with distance from the source as the source fluid is diluted by the fluid entrained from the surroundings. The jet in this example produces a thrust (three million pounds) for the rocket engine.

Turbulent Jets and Plumes (sadly, the sample chapter cuts off before the picture)

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    $\begingroup$ Since the exhaust is ducted by the launch pad, there could be large scale turbulence/oscillations inside the structure as well. In the OP's video the oscillations seem strongest after there's some light showing up from the left, which might suggest that the shuttle is "blowing into the hole" like a musical instrument. The whole thing may be more complicated than a jet in free space. $\endgroup$ – uhoh Jan 24 at 23:47
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    $\begingroup$ @uhoh Agreed. With a cover over the duct (the pad's platform) it makes a very short resonant pipe. Because it's so short the Q of the resonances will be very low, so the resonant frequencies won't be narrowly constrained: the instantaneous frequency of any one mode can vary quite a bit, especially with a turbulent flow "blowing into the hole". $\endgroup$ – Tom Spilker Jan 25 at 5:25

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