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Probably not! This phenomenon is not scale-independent. There is a parameter called the Reynolds number (Re for short) which is the ratio of intertial forces to viscous forces. For a dandelion seed Re is small which is the key to the stability of the vortex.

For a parachute, the Re number is much higher which makes the dynamics of the flow chaotic (called turbulence). There is a critical Re number beyond which There is no way keep the vortex stable, or as they call it, the vortex bursts.




You're most likely right, but there is at least one caveat which might be able to help if we're lucky. (I'm a fluid dynamicist, though not an aerodynamicist.)

The Reynolds number is only part of the picture. You also need a measure of the strength of the turbulence. A common measure is the "turbulence intensity", which you can think of as the standard deviation of the velocity divided by the mean of the velocity. (Though that's only exactly true in "isotropic turbulence".)

In certain circumstances you can compensate for a higher Reynolds number with a lower turbulence intensity. The bristles of the dandelion may have a turbulence reduction ability, so perhaps this is already being done. I'm not certain how to reduce the turbulence level further as in this case it's mostly an ambient property which is beyond the control of the dandelion. Some sort of honeycomb structure upstream of the bristles might help, or it might hurt; it depends on the details.

Here are some examples:

Pipe flow can remain laminar for higher Reynolds numbers if the turbulence intensity is low enough. Though special turbulence control approaches (e.g., eliminating vibrations which could trigger transition to turbulence) laminar pipe flows have been observed at a Reynolds numbers of about 100000, about 50 times higher than the typical Reynolds number where laminar flow ends.

Here's a quote from a review article:

https://www.annualreviews.org/doi/abs/10.1146/annurev-fluid-...

> The impression gained from presenting data in this way is that there is a transition between two definable states. One is the relatively rare but well-defined state of motion, laminar flow, and the other is the more common and ill-defined state of turbulence. Experimental evidence suggests that the laminar state can be achieved in pipe flows over a wide range of Re with the record standing at Re = 100,000 by Pfenniger (1961). Reynolds himself managed to achieve Re = 13,000, and Ekman (1911) later improved on this to ∼50,000 using Reynolds’ original apparatus. [...] Achieving laminar flows at high values of Re is an indication of the quality of an experimental facility and gives some confidence that the observations will not be contaminated by extraneous background disturbances such as entrance flow effects, convection, and geometrical irregularities.

Matching the turbulence intensity of two wind tunnels is often necessary to make the results comparable between the two wind tunnels. In the first volume of Sidney Goldstein's "Modern Developments in Fluid Dynamics", there's a plot showing (if I recall correctly) the Reynolds number at which the "drag crisis" occurs as a function of turbulence intensity. This basically means that the drag coefficient can be very sensitive to the turbulence intensity, at least in special circumstances.

(Why I wrote this: In my dissertation, I have an entire section about how turbulence intensity is too frequently neglected in analyses, particularly for the problem I'm studying for my PhD.)


I agree with the importance of the freestream turbulence intensity, but at high Re numbers, it's extremely hard to control it.

It can be shown mathematically, using a technique called parabolized stability equations (PSE), that small disturbances amplify rapidly thorough non-linear interactions in the frequency space. Hence, although it's possible to create a laminar flow at high Re number in the lab, it's extremely hard to achieve in nature.

One interesting case of this is the Rutan's Voyager airplane in the 80s. It was designed to have a laminar flow over its wing to reduce drag. It worked quite well until it faced rain drops at some point which messed up the aerdynamics of the wing and caused the airplane to stall. At that point, they had to add vortex generators on the wing to prevent the stall.


Thanks for the interesting example. You're right that this is unlikely to redeem a scaled up dandelion, but I thought it was still worth mentioning as it's often overlooked.

I work in internal and multiphase flows, and changing the turbulence level is much easier there than in aerodynamics.

I'll also have to look at the parabolized stability equations as I am not familiar with them. If you have a preferred reference, I'd be interested.


The HondaJet was also designed to take advantage of superlaminar flow over the wings and body. It's why the turbines are mounted on pods held high above the wings, instead of closely slung underneath like a traditional design.


That was super interesting. Thank you for commenting!


Could it work at higher speeds, like a craft in re-entry from space? Like a dandelion pre-chute.


I don't know much about high speed flows, but I imagine the answer is no.

While the gas temperatures would be higher due to viscous heating, which would increase the viscosity of the air, the increase in viscosity is much lower than the increase in velocity. So the Reynolds number would still increase. I very strongly doubt the turbulence intensity would be lower in this case too.

Plus, I imagine the bristles would be quickly ablated away.


That gave me an idea for the bristles being fritzy ends of cables, slowly being wound out and sacrificed to the wind. Like an ablative shield.

Not saying that it would be good for anything, it just produced a cool image in my head. :-)


Perhaps many small dandelion apertures could be used in a larger chute (not necessarily the whole surface), rather than trying to make a single large seed-chute.

Perhaps I should have tried to patent that ...




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