Sharkskin increases drag

[Merman Storm]

For everyone thinking this is a good way to swim faster:

https://www.sciencedaily.com/releases/2016/03/160315120554.htm

Now, to be fair, they only looked at skin friction, not drag arising from the flow not following the contours of the shark's body. That may be where the real drag reduction occurs.

[Echidna]

but didn't swimmers using the sharkskin wetsuits swim considerably faster?
so much that FINA banned them from competitions.

If you time the same swimmer, once with a sharkskin and once without, and he is faster while wearing it, that means there must be a drag reduction, no matter if the mechanic is fully understood or not.

[Merman Storm]

That's why I said "they only looked at skin friction, not drag arising from the flow not following the contours of the shark's body. That may be where the real drag reduction occurs".
There is another drag reduction device known as vortex generators. They reduce drag from flow separation, which happens on non-streamlined shapes, like a swimmer. It may be the sharkskin pattern acts as vortex generators.
But until now many thought the sharkskin pattern reduced skin friction, like riblets do. But that's apparently not true.

[MermanOliver]

...
There is another drag reduction device known as vortex generators. They reduce drag from flow separation, which happens on non-streamlined shapes, like a swimmer. It may be the sharkskin pattern acts as vortex generators.
....


As far as I understand all the hydrodynamics articles in the science papers, that's exactly what sharkskin does. Introducing lots of tiny vortices which help reduce overall drag on the whole structure. The article (Spektrum der Wissenschaft, the German sister paper of Scientific American) from several years past (please don't make me look it up, it must have been around the time when I was still at the university, so at least 12 years ago) seemed to explain it exactly like that: Turbulence in small areas (I.e. the shark 'scales') helps smoothing the flow over the whole structure.

Has been quite a finding back then, when everyone thought the more laminar a flow is the better. ;)
Edit: I'm just an electrical engineer, perhaps someone with a degree in hydrodynamics can explain it better?

[Merman Storm]

"......perhaps someone with a degree in hydrodynamics can explain it better?"
Like, me? CalTech, class of '77, and I've been working in the aerospace industry ever since.
The idea that sharkskin would smooth the outer flow was the original hypothesis. They were the inspiration for riblets.
http://rsta.royalsocietypublishing.org/content/roypta/368/1929/4775.full.pdf
But man-made riblets do not look like sharkskin. Riblets are continuous along their length, where sharkskin riblets are broken at every scale junction. The article I linked in my first post looked at actual sharkskin, and found they don't work like riblets, they don't suppress outer flow turbulence, and they don't reduce skin friction drag. Surprise! So, if they are reducing the drag of the shark, (or a swimmer), it's not a result of the mechanism of smoothing the outer flow and reducing skin friction.
The only other mechanism that I can see is they are acting as micro-vortex generators, suppressing flow separation.
https://www.youtube.com/watch?v=dcNGwmV1Yio
Suppressing separation would allow a shark to turn faster and accelerate faster, just what is wanted for an ambush predator

[Mermaid Jaffa]

How bout the shark skin fabric that pro swimmers wear? Its supposed to make them faster innit?

[Mermaid Wesley]

It isnt actual real shark skin tho


Sent from my iPhone using Tapatalk

[Mermaid Jaffa]

I know that. Sharkskin fabric is manufactured to make it feel like shark skin and is supposed to make you swim faster. But what I don't understand is, how do you make fabric to do that?

[MermanOliver]

"......perhaps someone with a degree in hydrodynamics can explain it better?"
Like, me? CalTech, class of '77, and I've been working in the aerospace industry ever since.
...



:blush: Sorry. No offence meant. Didn't remember you work in that field.

[Merman Storm]

How bout the shark skin fabric that pro swimmers wear? Its supposed to make them faster innit?

"The only other mechanism that I can see is they are acting as micro-vortex generators, suppressing flow separation.
https://www.youtube.com/watch?v=dcNGwmV1Yio
Suppressing separation would allow a shark to turn faster and accelerate faster, just what is wanted for an ambush predator."
I would say that sharks have made an engineering compromise: Accept more drag during normal swimming in order to gain an advantage in maneuvering.

On a streamlined shape, like a shark, there is no separation during normal forward swimming, so a device to suppress separation will not give more speed. On a lumpy, not very streamlined shape like a swimming person, suppressing separation does reduce drag.

Another way to reduce the drag of a swimming person is to put them in a streamlined shape, like a flooded submarine. The speed record for those is 8 knots, faster than the monofin speed record at about 6 knots.
http://www.latimes.com/nation/la-na-sub-race-20150629-story.html

Consider what that means. Put a person in a submarine and the frontal area goes up, the surface area goes up, the volume goes up, yet the drag goes down. This shows how important streamlining is, and how poorly streamlined a swimming person is.

BTW, the human powered submarine races are run by the human powered vehicle association (HPVA). The founder of the company I work at, Paul MacCready (also a personal friend), helped start it. To some extent, HPV was a draw for me to tail swimming. In a way, you are turning yourself into a human powered vehicle when you don a tail.

"Sorry. No offence meant. Didn't remember you work in that field." None taken.

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