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.

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.

> 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?

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

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.

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.

And the Mig-29 too but according to the reply that's different

> 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.

Read the article….this is a completely different effect.

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)

Same stuff! I’d rather prefer some archive link or something. Some websites are a bit aggressive these days.

… 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.

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.

What I've seen is a more structured texture applied with plastic films. https://www.lufthansa-technik.com/en/aeroshark One company claims up to 4% less fuel use. https://mako.aero/insights/delta-partners-with-mako-to-test-...

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

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.

Is this not useful in the speed regime of automobiles?

You can worked around that in Firefox by switching to focused reading

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.

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.

>humpback whale fins

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

Why wait for heaven. There probably are mods for Kerbal Space Program with exactly that parts. Create your wingsuit there.

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. '''

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.

"We apologize for the mistake in overturning a fundamental principle of aeronautical engineering, those responsible have now been sacked."