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A fundamental principle of aeronautical engineering has been overturned

wired.com

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Re: A fundamental principle of aeronautical engineering has been overturned

#81

Post by plorg »

larusso wrote: post #80
Maarten88 wrote: post #34

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 surprisi…

I thought that shark skin foil was a thing for years. Where they tried to emulate the micro roughness of shark skin.

The article says the investigators identify this as something fundamentally different than the shark skin effect.

Re: A fundamental principle of aeronautical engineering has been overturned

#84

Post by Ekaros »

mlmonkey wrote: post #9

> 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?

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.

Re: A fundamental principle of aeronautical engineering has been overturned

#85

Post by coldtea »

beering wrote: post #15
mlmonkey wrote: post #9

> 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?

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.

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).

Re: A fundamental principle of aeronautical engineering has been overturned

#86

Post by coldtea »

ungreased0675 wrote: post #24
SilverElfin wrote: post #22

Earlier quoted context omitted.

Is that what those things are on random civics? Do they make any difference for regular street cars?

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.

>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.

Re: A fundamental principle of aeronautical engineering has been overturned

#87

Post by Liftyee »

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.

Re: A fundamental principle of aeronautical engineering has been overturned

#88

Post by bonsai_spool »

otterdude wrote: post #55
Maarten88 wrote: post #34

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 surprisi…

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 air…

Tenet is the word you mean

Re: A fundamental principle of aeronautical engineering has been overturned

#89

Post by Arodex »

Maarten88 wrote: post #34

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 surprisi…

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...

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