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Maarten88 4 hours ago [-]
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 2 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.
mike_hock 2 hours ago [-]
> core tenant
And Lady Mondegreen.
aeternum 3 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 3 hours ago [-]
It’s probably operationally easier to keep surfaces smooth than to keep them a specific amount of roughness.
greggsy 2 hours ago [-]
It’s presumably easier to keep a smooth surface clear of bugs, dust and ice too.
swasheck 3 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
stackghost 2 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.
dilawar 2 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?
dilawar 2 hours ago [-]
Never mind. I didn't read the article (paywalled) and someone in the comments below answered this exact point.
3 hours ago [-]
colordrops 2 hours ago [-]
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 2 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.
dotancohen 7 minutes 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.
mlmonkey 7 hours ago [-]
> 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 4 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 24 minutes 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".
beering 6 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.
Swizec 6 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 4 hours ago [-]
Is that what those things are on random civics? Do they make any difference for regular street cars?
ungreased0675 4 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.
cpncrunch 5 hours ago [-]
Read the article….this is a completely different effect.
NetMageSCW 4 hours ago [-]
Tough behind a paywall.
nmstoker 1 hours ago [-]
And the Mig-29 too but according to the reply that's different
dathinab 7 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 6 hours ago [-]
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aaron695 4 hours ago [-]
[dead]
Groxx 4 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.
gregman1 59 minutes 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 6 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 5 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 4 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 3 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.
consp 22 minutes ago [-]
Doesn't the barrel remove this texture completely with the spin groves? Seems more of an issue than rough handeling.
mauvehaus 3 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.
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 3 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
I'll await the experimental measurements of fuel efficiency using real aircraft.
drpixie 3 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 3 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 3 hours ago [-]
Is this not useful in the speed regime of automobiles?
1 hours ago [-]
w10-1 3 hours ago [-]
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...
mike_hock 2 hours ago [-]
> You’ve read your last free article.
charles_f 2 hours ago [-]
You can worked around that in Firefox by switching to focused reading
tobadzistsini 3 hours ago [-]
This reminds me of the Dimple Car Experiment from Mythbusters.
zabi_rauf 3 hours ago [-]
Aren’t Turbulators doing similar thing i.e. its keeps the boundary layer for longer before it totally turns into turbulent layer?
2 hours ago [-]
philip1209 2 hours ago [-]
Fascinating.
I wonder what the implications for radar-absorbing finishes are. Could they be more aerodynamic already?
7 hours ago [-]
wizardforhire 3 hours ago [-]
This article and thread has got some major Tai’s Model vibes [1]
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 4 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 2 hours ago [-]
Thanks for clarifying.
6stringmerc 6 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 6 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 4 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.
bediger4000 10 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 7 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 7 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 4 hours ago [-]
Not to be that guy, but 38-53um is 1 order of magnitude smaller than 200um
toss1 4 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 7 hours ago [-]
"We apologize for the mistake in overturning a fundamental principle of aeronautical engineering, those responsible have now been sacked."
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.
And Lady Mondegreen.
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.
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.
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.