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.
otterdude 13 hours ago [-]
The core tenant of the paper is that roughness reduces drag IN the transition zone. A very small region of the total flow.
Thats the region between laminar and turbulent flow. Laminar flow is typically 5x less drag than turbulent, and will be encountered about a Reynolds number of 500K-1M (ratio of inertial flow to viscous flow).
Surfboards will have a Reynolds number of 10^7 which is entirely turbulent.
A Cessna aircraft will have a Reynolds number of 1-5x10^6.
Maarten88 5 hours ago [-]
> Surfboards will have a Reynolds number of 10^7 which is entirely turbulent.
A fin, foil or daggerboard below the board/boat is operating well within the range of Reynolds' numbers where laminar flow is relevant.
mike_hock 12 hours ago [-]
> core tenant
And Lady Mondegreen.
bonsai_spool 8 hours ago [-]
Tenet is the word you mean
nnevatie 3 hours ago [-]
swoosh swoosh - that's the sound of things traversing backward in time.
10 hours ago [-]
zobzu 30 minutes ago [-]
as usual these things are presented as new and revolutionary but aren't actually.
the specific process and implemention however are usually newer or slightly different from before.
this is our sensationalistic based society - any iterative progress, or sometimes even copy, is explained as a revolution.
now show me a 737 using 40% less fuel - guess what - that wont happen - however, perhaps we'll get a slightly better process to create aircraft skins. keep in mind you cant re-sand a fuselage every week, it needs to work reliably with no maintenance.
Arodex 7 hours ago [-]
Water is fairly viscous, and when you try to pull through too fast you completely change regime due to cavitation.
In comparison, from my days studying aerodynamics for RC soaring, air has a wider range of "viscosities" (represented by the Reynolds number) depending on the scale of your aeroplane and the speeds you intend to go through the atmosphere. The aerodynamic ideal or what count as useful tricks (winglets, dimples) can be fairly different for a a golf ball compared to a RC airplane compared to a commercial jet compared to a fighter jet...
naasking 3 hours ago [-]
Water is also largely incompressible. The fluid dynamics are just too dissimilar to air to carry over simplistic assumptions.
warumdarum 2 hours ago [-]
Wasnt there something about building abblative vortexes that convert the friction into rotation and are then discarded at the edge of the surface?
aeternum 14 hours ago [-]
I wonder how quickly airlines will adopt sanded/rough wings. It's also interesting that the efficiency of winglets were known for quite awhile but only somewhat recently have nearly all airliners adopted them.
rogerrogerr 13 hours ago [-]
It’s probably operationally easier to keep surfaces smooth than to keep them a specific amount of roughness.
greggsy 13 hours ago [-]
It’s presumably easier to keep a smooth surface clear of bugs, dust and ice too.
swasheck 13 hours ago [-]
yeah. what are the effects of too much roughness? may be safer and easier to maintain at smooth than at a specific roughness spec
PaulRobinson 6 hours ago [-]
At least a decade.
I remember people could smoke on planes. On some airlines seat backs and bathrooms had cigarette ashtrays in them. Smoking was phased out between 1988 and 2000, with most airlines being smoke free in the mid-1990s.
But the ashtrays persisted well into the 2000s. Two reasons: they needed to refresh the cabins, which is on a longer maintenance cycle done every few years, and before that, they needed replacement seats and bathroom fittings without the ashtrays. That meant tests, regulatory approval, all sorts.
For ashtrays being removed.
Winglets are a similar story. They're an addition, but they needed test flying and type approval before they could be added to the maintenance cycle rotation and get added to aircraft.
This is a bigger change. Boeing and Airbus (and others), are going to need to design it, push it through CFD, build different variants, test fly them, get them through regulatory approval and then... well, existing aircraft are probably not going to get these. Too expensive, too hard.
What's going to make more sense is a new aircraft - even if it's a variant type like the 737-MAX or the A320-Neo or whatever - where they approve the type modification as a whole, but it's not a retrofit to an existing airframe, will help manufactures sell more aircraft, airlines don't need to ground existing fleet and over time the fuel efficiencies get involved.
bitdivision 6 hours ago [-]
The FAA still requires ashtrays in bathrooms interestingly. To avoid those who do smoke there using the trash and causing a fire:
Regardless of whether smoking is allowed in any other part of the airplane, lavatories must have self-contained, removable ashtrays located conspicuously on or near the entry side of each lavatory door, except that one ashtray may serve more than one lavatory door if the ashtray can be seen readily from the cabin side of each lavatory served
I don't think it's safe to generalize from this to a functional aspect of the plane. Removing the ashtrays serves no purpose, so there's no cost to letting it wait for a decade or two. Improving the aerodynamics does serve a purpose and might be done faster.
stackghost 12 hours ago [-]
Modifications to an approved type design, especially for commercial passenger aircraft, are an intensely bureaucratic and thus very expensive process. This is part of the reason why product cycles are long.
larusso 9 hours ago [-]
I thought that shark skin foil was a thing for years. Where they tried to emulate the micro roughness of shark skin.
plorg 9 hours ago [-]
The article says the investigators identify this as something fundamentally different than the shark skin effect.
lonely_wanderer 3 hours ago [-]
The article contrasts it with a manufacturing process known as “shark skin.” I wonder if it’s entirely identical to the skin of a shark or a marketing term?
larusso 1 hours ago [-]
Sorry the article is paywalled and I reacted to the comment about sanded surfaces. I remember seeing documentation or other nature documentation’s about the shark skin effect. I had the chance to touch a shark and a ray in an aquarium but never felt the shark skin foil. I assume the properties would be the same though.
dilawar 13 hours ago [-]
> and airfoils also benefit from micro-roughness for lowest friction.
I thought this was known to some extent that smooth surfaces are not always the best e.g. golf balls have dimples on them? No?
zobzu 27 minutes ago [-]
dimples are used for stability and lift, not for friction reduction / low cx
dilawar 13 hours ago [-]
Never mind. I didn't read the article (paywalled) and someone in the comments below answered this exact point.
Yeah I'm pretty sure I remember reading something in a pop science magazine 20 or 30 years ago when MEMS nano structures were all the rage and how they were gonna use mass arrays of them on airplane wings to somehow increase flow
greggsy 13 hours ago [-]
Not uncommon to hear bold claims with every new and emerging technology that isn’t well understood by the media or general public. The excitement over nanobots seems to have run its course (for now?).
Blockchain managed to find its way into every market imaginable.
Battery technologies have consistently delivered bold claims on an almost yearly cycle, but we have at least seen incremental improvements.
AI is obviously the worst offender in the current timeline.
> 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?
djeastm 15 hours ago [-]
From the article:
>This principle is fundamentally different from the effect of dimples on golf balls. Dimples reduce pressure resistance by intentionally turbulizing the airflow and suppressing backward separation. DMR, on the other hand, delays the transition, thereby suppressing not pressure resistance but the wall friction itself. They are opposite mechanisms.
degamad 11 hours ago [-]
mlmonkey did not say that this new observation was the same phenomenon as golf ball dimples, just golf ball dimples already disproved the "long accepted" belief that "smoother the surface, the lower the aerodynamic drag".
kazinator 10 hours ago [-]
Exactly; golf balls are one example of it not being accepted that smooth surfaces are always best for drag, regardless of how the new result works.
beering 16 hours ago [-]
TFA makes it clear that this is a very different phenomenon than golf ball dimples, and even goes as far as to say they are opposing.
coldtea 8 hours ago [-]
However the TFA doesn't make clear that this: "It's long been accepted that the smoother the surface, the lower the aerodynamic drag. That turns out not always to be the case" was already known to NOT always be the case (e.g. in golf balls).
Swizec 16 hours ago [-]
> Huh... I'd always heard that a golf ball's dimples help reduce drag?
Yep also vortex generators in cars have become common. So common that they've filtered down to after market parts you can put on a honda civic
Vortexes break up large air pockets and reduce drag.
SilverElfin 15 hours ago [-]
Is that what those things are on random civics? Do they make any difference for regular street cars?
ungreased0675 15 hours ago [-]
I put some (actual, as in from an airplane parts catalog) vortex generators on my hybrid. It slightly increased gas mileage and slightly reduced noise.
The less aerodynamic the vehicle, the more noticeable the result will probably be.
coldtea 8 hours ago [-]
>It slightly increased gas mileage and slightly reduced noise.
Between the day to day road conditions and different routes noise and the modest effect, I'd say this would be impossible to tell.
11 hours ago [-]
Ekaros 9 hours ago [-]
I read somewhere that it depends... Different shaped objects benefit from different surface effects. A rounded surface like ball benefits from dimples where as more straight surface like arrow would not. I have no idea but I could also guess that speed affects things.
cpncrunch 15 hours ago [-]
Read the article….this is a completely different effect.
And the Mig-29 too but according to the reply that's different
dathinab 17 hours ago [-]
yep
and a lot of "smooth" aerodynamic surfaces have "microscopic"/"very small" surface patterns to make the surface less perfect smooth as if it is too perfect smooth the air kinda "sticks" to it increasing drag (to say it in a very unscientific way)
pfdietz 17 hours ago [-]
[dead]
aaron695 15 hours ago [-]
[dead]
Groxx 14 hours ago [-]
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.
pwinwood 6 hours ago [-]
Same happened to me but I opened it in reader view in Firefox and it's fine!
gregman1 12 hours ago [-]
Same stuff! I’d rather prefer some archive link or something. Some websites are a bit aggressive these days.
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.
imoverclocked 16 hours ago [-]
… theoretically meets reality pretty quick in aviation. You’ll likely find a lot of red tape to modifying any particular aircraft until it has been tested or certified. Well, for certified aircraft anyway. Even in the experimental world you might find some (excuse the pun) resistance to sand blasting someone’s wing.
zonkerdonker 16 hours ago [-]
Based on the mechanism of flow attachment in the transition zone it seems like the overall airfoil profile would likely have to change to take full advantage of the reduced friction. I think its much more likely to see this technique played with somewhere like Formula 1, if it hasnt been already.
russellbeattie 15 hours ago [-]
> "...like Formula 1"
Or projectiles like bullets and missiles. A sniper bullet with nanoscale textured surface that's able to go x% farther due to reduced drag seems plausible.
TomatoCo 14 hours ago [-]
On a metal as soft as copper I imagine that texture'll last about 30 minutes after it's issued to the soldier.
mauvehaus 14 hours ago [-]
Once the box magazine is loaded, it's not like the individual cartridges are being handled heavily.
Deer season involves a lot of loading and unloading a rifle (for those of use without removable magazines who are also bad at finding deer), and the bullets don't come out of it appreciably worse for wear. And that's for lead-free soft annealed copper ammunition. If you aren't being aggressively careless with your ammunition, it's not getting a lot of friction and scratches.
consp 11 hours ago [-]
Doesn't the barrel remove this texture completely with the spin groves? Seems more of an issue than rough handeling.
They allude to this alternative tech in the article, and I think it will stay the dominant approach because the far finer dimensions of the new tech talked about in the article, even if integrated into a film using glass beads as they also did, appears to be intrinsically much more susceptible to rapid functional degradation. It's about or less than the thickness of dirt / grime / bug goo. But tests will tell.
xbmcuser 13 hours ago [-]
Paint and finish on an airplane has to account for a lot more than aerodynamics. So you need to build it from the ground up as that coating could be the difference between the surviving daily temperature fluctuation for 10000 trip vs 1000 trips
leptons 10 hours ago [-]
The physics of travelling at 600mph+ would affect the rough surface differently than at 60mph. Airplane wings experience erosion due to the high speed combined with particles in the air - dust, ice, volcanic ash, and rain/water. The erosion is a problem that sees significant mitigation. If the surface were made to be rough I'd expect some unexpected results, and it may even become a bigger problem. I do think the technique should be tested though.
golddust-gecko 4 hours ago [-]
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.
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.
matt-attack 2 hours ago [-]
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?
dotancohen 11 hours ago [-]
> The ... magnetic support balance system ... can levitate a streamlined model ... inside a wind tunnel without contact using electromagnetic force.
That's pretty cool. Presumably the varying magnetic field strength required to suspend the test article is also an indicator of varying forces on the vehicle.
10 hours ago [-]
felineflock 2 hours ago [-]
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.
adverbly 15 hours ago [-]
I'll await the experimental measurements of fuel efficiency using real aircraft.
drpixie 14 hours ago [-]
Me too. The number of "revolutionary" designs that are announced but disappear makes me cynical. Looking wings on real aircraft, unless freshly painted, they're pretty close to finely sanded :) If the airlines and engineers saw a significant performance degradation with wear, they'd be out there polishing and repainting wings.
On a similar note - How many times have you seen announcements about someones blended wing that is going to save 50% fuel? But there are very few blended wings in nature (eg. rays), and those are in a very slow-speed regime.
dnautics 14 hours ago [-]
The real obstacle to blended wing designs, I imagine, is more boring: airports are likely to be difficult to retrofit to support those, well for cargo anyways, and for passengers there's probably less appetite to board such a plane
dnautics 14 hours ago [-]
Is this not useful in the speed regime of automobiles?
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...
Soling20 3 hours ago [-]
Question, should both sides of a lifting foil be the same of the same texture?
tobadzistsini 14 hours ago [-]
This reminds me of the Dimple Car Experiment from Mythbusters.
fnord77 2 hours ago [-]
> 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.
12 hours ago [-]
zabi_rauf 13 hours ago [-]
Aren’t Turbulators doing similar thing i.e. its keeps the boundary layer for longer before it totally turns into turbulent layer?
mike_hock 12 hours ago [-]
> You’ve read your last free article.
charles_f 12 hours ago [-]
You can worked around that in Firefox by switching to focused reading
philip1209 13 hours ago [-]
Fascinating.
I wonder what the implications for radar-absorbing finishes are. Could they be more aerodynamic already?
qwertyuiop_ 16 hours ago [-]
Tell that to the ice build up on the wing.
joarv0249nw 7 hours ago [-]
I was thinking the same. Ice build upp will probably increase with a bigger and rougher surface area
18 hours ago [-]
13 hours ago [-]
wizardforhire 14 hours ago [-]
This article and thread has got some major Tai’s Model vibes [1]
Does this same principle make the moon orbit a little faster?
defrost 9 hours ago [-]
What order of aerodynamic drag does our moon in orbit experience?
rawgabbit 17 hours ago [-]
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.
clnhlzmn 15 hours ago [-]
You learned something different then because this finding is that some kinds of additional roughness delay the transition to turbulent flow which is pretty clear in the article.
A quick search looks to show the same general topic from more than a decade ago. I too have a recollection of this being discussed in the late 80s or early 90s. Maybe some folk wisdom that's just now getting quantified.
rawgabbit 13 hours ago [-]
Thanks for clarifying.
hexobhi 5 hours ago [-]
[flagged]
6stringmerc 17 hours ago [-]
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…
r3trohack3r 17 hours ago [-]
From the featured article:
> This technology is fundamentally different from the “rivulet (shark skin) process,” which is known as a typical aerodynamic drag reduction technology. The rivulet process mimics the fine longitudinal grooves in shark skin, and by carving grooves approximately 0.1 mm wide along the direction of airflow, it aligns the vortices that occur near the wall surface of turbulent airflow areas. DMR, on the other hand, delays the switch from laminar to turbulent flow by means of random and minute irregularities. The flow zones it affects and the mechanisms it employs are based on completely different concepts.
spacedoutman 14 hours ago [-]
>humpback whale fins
you might find this video interesting then, the fastest rc drone in the world and it uses humpback inspired props.
Why wait for heaven. There probably are mods for Kerbal Space Program with exactly that parts. Create your wingsuit there.
jdkee 13 hours ago [-]
Golf balls.
bediger4000 21 hours ago [-]
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.
staplung 18 hours ago [-]
The headline is perhaps overstating things a bit but they do discuss how this is different than e.g. rivulets
'''
This technology is fundamentally different from the “rivulet (shark skin) process,” which is known as a typical aerodynamic drag reduction technology. The rivulet process mimics the fine longitudinal grooves in shark skin, and by carving grooves approximately 0.1 mm wide along the direction of airflow, it aligns the vortices that occur near the wall surface of turbulent airflow areas. DMR, on the other hand, delays the switch from laminar to turbulent flow by means of random and minute irregularities. The flow zones it affects and the mechanisms it employs are based on completely different concepts.
'''
toss1 18 hours ago [-]
Yes, but this is not that.
Golf ball dimples are about 4 mm across and 0.2mm or 200μm (micrometers).
These features are several orders of magnitude smaller at 38 to 53μm diameter.
>>the first in the world to demonstrate that aerodynamic drag can be reduced by up to 43.6 percent simply by applying distributed micro-roughness (DMR), a surface roughness so fine and irregular that it cannot be distinguished by the naked eye. [...] Two types of DMRs were used in this experiment: A convex pattern made of glass beads with diameters ranging from 38 to 53 micrometers (μm) and a concave pattern applied by sandblasting. The height of the DMR coating is only 1 percent of the thickness of the boundary layer and is classified as a “smooth surface” from a hydrodynamic point of view.
nullhole 15 hours ago [-]
Not to be that guy, but 38-53um is 1 order of magnitude smaller than 200um
toss1 14 hours ago [-]
Not to be that guy ;-) , but the diameter of the golf ball dimples is ~4 mm or about 4,000 μm, whilst the diameter of the spheres is 38-53 μm, or about 0.04 mm.
Diameter-to-diameter seems like about 100x or two orders of magnitude?
Similarly, 200 μm is the golf ball dimple depth (oops, just noticed I dropped that key word), and they didn't give us a measurement of the depth of the dents caused by the spheres or sandblasting, but it would likely be significantly less than half the radius of the spheres?
Sorry about misleading with dropping the "depth" word.
doginasuit 18 hours ago [-]
"We apologize for the mistake in overturning a fundamental principle of aeronautical engineering, those responsible have now been sacked."
Rendered at 16:14:01 GMT+0000 (Coordinated Universal Time) with Vercel.
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.
Thats the region between laminar and turbulent flow. Laminar flow is typically 5x less drag than turbulent, and will be encountered about a Reynolds number of 500K-1M (ratio of inertial flow to viscous flow).
Surfboards will have a Reynolds number of 10^7 which is entirely turbulent.
A Cessna aircraft will have a Reynolds number of 1-5x10^6.
A fin, foil or daggerboard below the board/boat is operating well within the range of Reynolds' numbers where laminar flow is relevant.
And Lady Mondegreen.
the specific process and implemention however are usually newer or slightly different from before.
this is our sensationalistic based society - any iterative progress, or sometimes even copy, is explained as a revolution.
now show me a 737 using 40% less fuel - guess what - that wont happen - however, perhaps we'll get a slightly better process to create aircraft skins. keep in mind you cant re-sand a fuselage every week, it needs to work reliably with no maintenance.
In comparison, from my days studying aerodynamics for RC soaring, air has a wider range of "viscosities" (represented by the Reynolds number) depending on the scale of your aeroplane and the speeds you intend to go through the atmosphere. The aerodynamic ideal or what count as useful tricks (winglets, dimples) can be fairly different for a a golf ball compared to a RC airplane compared to a commercial jet compared to a fighter jet...
I remember people could smoke on planes. On some airlines seat backs and bathrooms had cigarette ashtrays in them. Smoking was phased out between 1988 and 2000, with most airlines being smoke free in the mid-1990s.
But the ashtrays persisted well into the 2000s. Two reasons: they needed to refresh the cabins, which is on a longer maintenance cycle done every few years, and before that, they needed replacement seats and bathroom fittings without the ashtrays. That meant tests, regulatory approval, all sorts.
For ashtrays being removed.
Winglets are a similar story. They're an addition, but they needed test flying and type approval before they could be added to the maintenance cycle rotation and get added to aircraft.
This is a bigger change. Boeing and Airbus (and others), are going to need to design it, push it through CFD, build different variants, test fly them, get them through regulatory approval and then... well, existing aircraft are probably not going to get these. Too expensive, too hard.
What's going to make more sense is a new aircraft - even if it's a variant type like the 737-MAX or the A320-Neo or whatever - where they approve the type modification as a whole, but it's not a retrofit to an existing airframe, will help manufactures sell more aircraft, airlines don't need to ground existing fleet and over time the fuel efficiencies get involved.
I don't think it's safe to generalize from this to a functional aspect of the plane. Removing the ashtrays serves no purpose, so there's no cost to letting it wait for a decade or two. Improving the aerodynamics does serve a purpose and might be done faster.
I thought this was known to some extent that smooth surfaces are not always the best e.g. golf balls have dimples on them? No?
Huh... I'd always heard that a golf ball's dimples help reduce drag?
>This principle is fundamentally different from the effect of dimples on golf balls. Dimples reduce pressure resistance by intentionally turbulizing the airflow and suppressing backward separation. DMR, on the other hand, delays the transition, thereby suppressing not pressure resistance but the wall friction itself. They are opposite mechanisms.
Yep also vortex generators in cars have become common. So common that they've filtered down to after market parts you can put on a honda civic
Vortexes break up large air pockets and reduce drag.
The less aerodynamic the vehicle, the more noticeable the result will probably be.
Between the day to day road conditions and different routes noise and the modest effect, I'd say this would be impossible to tell.
and a lot of "smooth" aerodynamic surfaces have "microscopic"/"very small" surface patterns to make the surface less perfect smooth as if it is too perfect smooth the air kinda "sticks" to it increasing drag (to say it in a very unscientific way)
> 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.
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.
Or projectiles like bullets and missiles. A sniper bullet with nanoscale textured surface that's able to go x% farther due to reduced drag seems plausible.
Deer season involves a lot of loading and unloading a rifle (for those of use without removable magazines who are also bad at finding deer), and the bullets don't come out of it appreciably worse for wear. And that's for lead-free soft annealed copper ammunition. If you aren't being aggressively careless with your ammunition, it's not getting a lot of friction and scratches.
> 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.
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.
And if so, couldn’t we just have a model iterate on different surface patterns and optimize?
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.
On a similar note - How many times have you seen announcements about someones blended wing that is going to save 50% fuel? But there are very few blended wings in nature (eg. rays), and those are in a very slow-speed regime.
I'm intrigued by the methodology of the wind tunnel: using magnets to more precisely measure and to avoid interference from guy wires...
I wonder what the implications for radar-absorbing finishes are. Could they be more aerodynamic already?
[1] https://en.wikipedia.org/wiki/Tai%27s_model
A quick search looks to show the same general topic from more than a decade ago. I too have a recollection of this being discussed in the late 80s or early 90s. Maybe some folk wisdom that's just now getting quantified.
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…
> This technology is fundamentally different from the “rivulet (shark skin) process,” which is known as a typical aerodynamic drag reduction technology. The rivulet process mimics the fine longitudinal grooves in shark skin, and by carving grooves approximately 0.1 mm wide along the direction of airflow, it aligns the vortices that occur near the wall surface of turbulent airflow areas. DMR, on the other hand, delays the switch from laminar to turbulent flow by means of random and minute irregularities. The flow zones it affects and the mechanisms it employs are based on completely different concepts.
you might find this video interesting then, the fastest rc drone in the world and it uses humpback inspired props.
https://www.youtube.com/watch?v=k9n1h0rn9No
''' This technology is fundamentally different from the “rivulet (shark skin) process,” which is known as a typical aerodynamic drag reduction technology. The rivulet process mimics the fine longitudinal grooves in shark skin, and by carving grooves approximately 0.1 mm wide along the direction of airflow, it aligns the vortices that occur near the wall surface of turbulent airflow areas. DMR, on the other hand, delays the switch from laminar to turbulent flow by means of random and minute irregularities. The flow zones it affects and the mechanisms it employs are based on completely different concepts. '''
Golf ball dimples are about 4 mm across and 0.2mm or 200μm (micrometers).
These features are several orders of magnitude smaller at 38 to 53μm diameter.
>>the first in the world to demonstrate that aerodynamic drag can be reduced by up to 43.6 percent simply by applying distributed micro-roughness (DMR), a surface roughness so fine and irregular that it cannot be distinguished by the naked eye. [...] Two types of DMRs were used in this experiment: A convex pattern made of glass beads with diameters ranging from 38 to 53 micrometers (μm) and a concave pattern applied by sandblasting. The height of the DMR coating is only 1 percent of the thickness of the boundary layer and is classified as a “smooth surface” from a hydrodynamic point of view.
Diameter-to-diameter seems like about 100x or two orders of magnitude?
Similarly, 200 μm is the golf ball dimple depth (oops, just noticed I dropped that key word), and they didn't give us a measurement of the depth of the dents caused by the spheres or sandblasting, but it would likely be significantly less than half the radius of the spheres?
Sorry about misleading with dropping the "depth" word.