Why does the Soyuz parachute pulsate?

Question

During the recent return of Expedition 47 from the ISS, I noticed the main parachute of their Soyuz capsule pulsating. You can see it in this video:

"Flat":

"Domed":

My wife noted that it looks a little like a jellyfish, moments later I saw this tweet by Alexander Gerst:

The "jellyfish breathing" of the parachute feels like sitting in a small boat at sea. Not indicative of what is to come...

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Is the parachute specifically designed to do this, if so - why? What benefits does it provide?

If it's not specifically designed to do this, and it's just a side effect of some other design constraint, what causes it to happen, and is it detrimental to the overall performance of the parachute at all? Perhaps all parachutes of this design/scale behave this way, and I've just never noticed it before?

Answer 1

Thanks to the paper linked by Jan Doggen in the comments we can say that it is a natural outcome of the shape of the parachute.

In particular, the shape, together with the near absence of wind and the fact that the craft is descending near vertically determines that pulsating motion.

In the words of the paper:

The studies of [1, 2] demonstrated that during the motion of an object with a parachute the latter's canopy changes its shape and geometric dimensions periodically - it pulsates. In what follows, the effect of this phenomenon on the dynamics of the object-parachute system (OPS) is examined.

The paper then proceeds to derive the pulsating frequencies and the other characteristics of the motion.

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So to directly answer your own question

Is the parachute specifically designed to do this?

No, it is not.

What causes it to happen?

You'll find the detailed explanation in the references of the paper mentioned above (that apparently google scholar fails to find). But to keep it simple, it is the flow of air around the canopy.

The canopy has a hole on the top to allow some air to escape in a controlled fashion, this provides stability, as mentioned on another SE. The crucial point is that the hole is not sufficient for all the air below the canopy to escape (and for good reason, the parachute would offer no resistance, failing at its job!), the air then escapes laterally.

Under the conditions discussed in the linked paper (vertical descent at constant/terminal velocity) this results in a natural quasi-harmonic oscillatory movement.

Perhaps all parachutes of this design/scale behave this way, and I've just never noticed it before?

Given the paper, yes they do.

Answer 2

It is called parachute "breathing", and is quite common. It is a natural consequence of the flexibility of the parachute shape. It is initiated in the initial inflation, where the parachute will always over-inflate, and then oscillate around a mean inflation state. Hopefully with decreasing amplitude. If the breathing amplitude would be too large or growing, e.g. threatening inversion or damage, then measures are taken to damp the oscillations. Otherwise it is relatively harmless.

At supersonic speeds, "Heavy canopy breathing and high-speed flutter frequently result in material disintegration, seam separation. and canopy damage." (Knacke)