New on skin resistance

A Fundamental Principle of Aeronautical Engineering Has Been Overturned https://www.wired.com/story/a-fundamental-principle-of-aeronautical-engineering-has-been-overturned/?utm_source=firefox-newtab-en-us

Not sure if this discovery would have significant impact on hydrodynamics of hull design?

 

This has been understood for a long time. Early start of turbulent flow will create smaller whirlpools, therefore less energy loss.

 

"drag was dramatically reduced by up to 43.6 percent in the transition zone." Wondering if there's any aircraft or missiles that are already using the concept, maybe the classified stuff?

 

This appears to have come out in May of this year, so I doubt it has been used. I was fishing for links when you posted this, so I didn't see your post until after I posted on the same subject. oops.

 

It is not a new concept and definitely not classified. If you want an example related to boat design, check "Principles of Yacht Design" by Larsson and Eliason. They have data, equations, etc. on this topic. There are a lot of so called "new information" online which is nothing but a claim to make people read an article.

 

Yes, I basically agree with you. Mostly clickbait, but perhaps there was some substance. I had hoped that someone might point out where to apply the reduction.

The two main claims were that magnetic suspension of the test body eliminated the drag correction for the mounting hardware used in a conventional wind tunnel and a random roughness pattern that did not require the pattern to be oriented parallel to the flow. The first claim is a refinement in measurement. The second was characterization of a surface roughness profile.

Obviously the 46% claim is sensational, and if accurate, it applies to one aspect of overall resistance. Which aspect of drag was reduced by 46%? It did not seem that the random roughness profile claimed to be better than riblets, other than its insensitivity to orientation. Internet instant expertise allows me to point out that (at this time and subject to change at any time without reason) Wikipedia says "Tests on an Airbus A320 found riblets caused a drag reduction of almost 2%". It is possible that the claimed 46% change is 46% of the measured 2% due not to a change in drag, but a refinement in measurement. So are we looking at a calculated reduction of 46% 0f 2%, so without changing the drag at all, the number measured with magnetic suspension would be 0.0292 (3%).

 

I'm guessing that 46% would make a lot of difference at much slower watercraft speeds (drag >> 2%?) as compared to aircraft, assuming the same principles apply? Especially for water crafts that travel at slow speeds under six knots, maybe some barges, where there might be a significant difference in fuel consumption?

Are Longer Kayaks Really https://guillemot-kayaks.com/sea-kayak-recreational-kayak/petrel-play-petrel/are-longer-kayaks-really-faster?fbclid=IwAR2_2IYYTFuNvpSi4p0I4QVclTaaorowdczF8Mofk10dj9B9-S4hBY4UJtg

"If you were to take the drag number where the magenta curve crosses the 2 mph line and slide left to where that level crosses the green design optimized for 7 mph you will see that for the amount of effort it takes to make the 4.7' long boat go 2 mph you the 17.7' long 7-mph boat would only be going about 1.4 mph. The lower wetted surface area of the short boat is sufficient to make up for the fact that it will make a bigger wake at higher speeds."

 

Surface area treatments to reduce air resistance are indeed a very old concept, and I have many questions about the link myself!

What seemed different this time was the randomization aspect and that the disturbances are minute essentially invisible to the naked eye. No information on how such a randomized and minute surface can be created and remain attached (and remain unfettered from very fine particle debris), or the diameters and average distance between them? That let me to Guess that the process might be proprietary and possibly classified? Date of discovery and suppressed date of publication may be separated by a long time period, especially in matters of patents and National Security interest.

From the link, "Even if the separation at the rear of the object were completely eliminated, only about 20 percent of the observed reduction can be explained. In other words, the numerical analysis quantitatively confirmed that the main factor in the reduction of aerodynamic drag by DMR is not the suppression of delamination but the reduction of frictional drag itself.

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."

 

That reads like a sales brochure. "Principles of Yacht Design" has actual test results and numerical values. They use a value that is a percentage of the diameter of the body.

 

I wonder why the title is "principles of yacht design" if it also covers Universal principles that also apply to smaller boats? Non sequitur, seems like surface area treatments wouldn't be much of a concern to anyone who can afford or writes off, a yacht? Yacht owners probably aren't concerned about the cost of fuel compared to the other yacht expenses , and if they need more speed, they just go to larger engines, instead of fooling around with surface area of treatments?

 

It is a clickbait. There is nothing new. Search vortex generators, turbinators, Ludwig Prandtl, and Poiseuille’s Law. Been there for a long time.

 

Thank you for posting these work-papers,
I read them a little bit quickly and therefore superficially as I am a bit rookie with CFD , that is why I would have 2 questions for the engineers attending this forum:
1-I would like to know if what I understood is correct, I mean that the drag reduction is mentioned mostly for the transition part, which is probably a little part of the wing section chord ?
2-Is this issue is correlated with the concept of Turbulent Kinetic Dissipation?
Some time ago I posted something about TKD, but my post was sunk by a flow of posts from Carlos, that is why I gave up.
Thanks in advance
Cheers

 

Vortices are created by the friction between the fluid and the body interface. It takes energy to accelerate the fluid. The smaller the volume of fluid being accelerated, the less energy is required; therefore less drag.