alt Hacker NewsA fundamental principle of aeronautical engineering has been overturned
Comments
The actual paper: https://www.cambridge.org/core/journals/journal-of-fluid-mec...
> It's long been accepted that the smoother the surface, the lower the aerodynamic drag. That turns out not always to be the case.
Huh... I'd always heard that a golf ball's dimples help reduce drag?
It's almost certainly my adblocker playing poorly with their "subscribe to read" stuff, but I had to lol at the failure mode. When I load the page, I get the splash image/headline, and below it:
> Subscribe to listen [9 minutes]
> Aerodynamic drag is a major “barrier” in high-speed airplanes, automobiles, and bullet trains. This is because a design with less aerodynamic drag allows the aircraft to move at higher speeds with less energy.
And then just comments and links to other articles. No indication at all that there's more to the article beyond (apparently) an audio recording.
This might explain some of the "didn't read the article" comments? Not that it doesn't happen anyway tho.
If the application method is as rudimentary as sandblasting, it sounds rather simple to retrofit to existing aircraft. If it works as they state it does, it's a virtually free same-day fuel efficiency boost.
However, I did not see what the actual net improvement was. When they talk percentages, they are talking only about "in the transition zone". They say the coefficient improves throughout, but in theory, it could be almost irrelevant if the overall improvement throughout the profile is close to 0. It also sounds like a very difficult level of precise degradation to maintain for any period of time in real world conditions, since it would be easy to clog or abrade further.
I feel like this part is either a mistake, or a whole story in itself:
> This premise was based on the results of a 1940 study by Ichiro Tani, a Japanese scientist who demonstrated the relationship between surface roughness (an indicator of the state of the machined surface) and turbulent transition, arguing that surface roughness, which was unavoidable with the manufacturing technology of the time, prevented laminar flow from being realized.
> However, in 1989 Tani reinterpreted the experimental data on rough-surfaced pipes obtained by fluid engineer Johann Nikulase in the 1930s, suggesting that “roughness may not necessarily only promote turbulent transition and increase fluid resistance.”
So if true, this means that Tani was working on the same problem for 49 years.
Evidently [he died in 1990](https://www.wikidata.org/wiki/Q24868684), so it's at least possible.
Interesting finding, but hardly fundamental. My fluids lectures taught that there's form drag ("pressure drag" in the article) and skin friction drag. The two trade off with each other depending on Reynolds number. Keeping the flow laminar reduces skin friction drag (suggesting smooth skin), but keeping the flow attached for longer (e.g. by inducing turbulence, or injecting air...) reduces form drag (at a cost of increased skin friction due to turbulence).
Reads like they've discovered a neat way to delay flow separation while maintaining laminar flow, but the underlying principles have not changed. "Smooth thing low drag" was never a rule and only works at certain scales.
> The ... magnetic support balance system ... can levitate a streamlined model ... inside a wind tunnel without contact using electromagnetic force.
I'll await the experimental measurements of fuel efficiency using real aircraft.
I’d be curious to know if this sort of thing could’ve been predicted through computer modeling. And if not, does that mean, we have a gap in our fundamental fluid equations?
And if so, couldn’t we just have a model iterate on different surface patterns and optimize?
fyi: the paper cited in the wired article is at https://arxiv.org/abs/2603.23843
Any chance golf ball dimples in the wings would make it better?
In a golf ball, the dimples create a turbulence in a layer of air around it but results in higher lift due to smaller vortex and less drag.
Klaus Savier is a longtime efficiency experimentalist, and opted for unpolished paint circa ~1990. His initial goal was weight reduction but numbers showed the finish had aerodynamic benefits.
I'm intrigued by the methodology of the wind tunnel: using magnets to more precisely measure and to avoid interference from guy wires...
Question, should both sides of a lifting foil be the same of the same texture?
This reminds me of the Dimple Car Experiment from Mythbusters.
Aren’t Turbulators doing similar thing i.e. its keeps the boundary layer for longer before it totally turns into turbulent layer?
Fascinating.
I wonder what the implications for radar-absorbing finishes are. Could they be more aerodynamic already?
Tell that to the ice build up on the wing.
This article and thread has got some major Tai’s Model vibes [1]
> Experimental results showed that the critical Reynolds number at which the turbulent transition begins increased from approximately 1.9 × 10⁶ to 2.2 × 10⁶ for the DMR-coated model, and drag was dramatically reduced by up to 43.6 percent in the transition zone.
balls! (golf balls)
Uhh. I was taught that in university in the late 80s. Some surfaces have a lot of friction and if you add surface imperfections the turbulent airflow actually reduces drag.
Does this same principle make the moon orbit a little faster?
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I wrote about this ages ago, in that shark skin is an evolutionary adaptation worth study because water is thicker than air, but when air compounds, blah blah blah. Basically think of making a composite mold with directional tiny tiny dorsal fin looking surface. If you rub your hand on it the wrong way it cuts you open. Could even be scaled for large cargo ship hulls.
Next up: my personal wing invention which uses leading edges modeled on humpback whale fins, because the use case / stall profile is better.
Sigh, I’m going to have a great time in Heaven chatting with Leonardo da Vinci…
Golf balls.
This article is kind of false. Keeping an object's boundary layer attached is known to reduce drag, even if the flow is turbulent. Golf ball dimples are a successful attempt to keep boundary layers attached.
Any competitive sailor or foil-racer knows that the underwater surface has the least friction and best laminar flow when sanded with fine-grid sandpaper, around 1000 to 1500 grid.
It always surprised me that this was not true in air and airplane wings were supposedly best when glossy. So now it turns out that this is indeed not true, and airfoils also benefit from micro-roughness for lowest friction.
Now the surprising question to me is how is it possible that something so simple was not known in this very well-researched and well-funded field. It probably was known, just not by the paper-publishing researchers.