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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

#101

Post by golddust-gecko »

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.

Re: A fundamental principle of aeronautical engineering has been overturned

#103

Post by lonely_wanderer »

plorg wrote: post #81
larusso wrote: post #80

Earlier quoted context omitted.

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.

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?

Re: A fundamental principle of aeronautical engineering has been overturned

#104

Post by nnevatie »

bonsai_spool wrote: post #88
otterdude wrote: post #55

Earlier quoted context omitted.

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

swoosh swoosh - that's the sound of things traversing backward in time.

Re: A fundamental principle of aeronautical engineering has been overturned

#106

Post by naasking »

Arodex wrote: post #89
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, di…

Water is also largely incompressible. The fluid dynamics are just too dissimilar to air to carry over simplistic assumptions.

Re: A fundamental principle of aeronautical engineering has been overturned

#110

Post by fnord77 »

> 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.
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