Split Fin Physics

[John C. Ratliff]

Here's a side shot of the effectiveness of the scoop fins:

The straight blade fins are pretty good, but do have that "dead area," which is an area where the water does not provide any propulsion. It is an area just beyond the toes, before the blade bends and provides an angle to the direction of travel. The whole idea of the Jet Fin was to use that dead area, and provide a second blade for it to function. The scoop fin captures that water and channels it to the rear. The split fins do the same, because of the density of the water, but loose some efficiency as some of the water escapes between the blades.

SeaRat

 

[Puffer Fish]

M Bipartitus: Your friends work is very interesting and informative. I do like that the serious pro's are adding to the discussion. Hope we see more.

MDB, If you ever get the chance to see a T-38 up close, it's wings don't even remotely look like a "wing", and provide almost none of the classic teaching "lift".

Also, on any really large jet, the wings bend a significant amount (something around 7 ft with a fully loaded 747), which would mess up a Bernoulli effect.

What that very nice paper points out is that conventional wisdom and what is commonly taught is not what is actually happening.

What tends to happen when looking at the issue of fins is to focus on one aspect, say thrust, or minimizing turbulence, when it is the combination of stroke, recovery and matching the fin to the design of the motor (our legs) that end up making the final product.

And like the wing on a plane, common knowledge is that turbulence is bad, when some types of turbulence actually reduce drag.

 

[Blackwood]

John -

Can you post your photos to scubaboard directly? Where you have them is blocked on my webserver (I can't see them).

The part that is wrong has to do with the "flying" and "wing" concepts, as those apply to air and not to water. The reason they don't apply to water is that water is incompressible, and therefore there is no possibility of a "low pressure" area above the wing.

Low speed flight is considered incompressible as well. Incompressible doesn't mean that the pressure is constant. It means the density is constant.

 

[Blackwood]

MDB, If you ever get the chance to see a T-38 up close, it's wings don't even remotely look like a "wing", and provide almost none of the classic teaching "lift".

The wings of supersonic aircraft have dramatically different sections (small thickness-to-chord ratio, sharp leading edge, etc.). Compressible flow is fundamentally different, and really doesn't apply to this discussion. Note that most use significant flaps and slats to generate lift at low speeds.

And like the wing on a plane, common knowledge is that turbulence is bad, when some types of turbulence actually reduce drag.

Better stated: turbulence reduces some types of drag.

 

[John C. Ratliff]

I will have to look on my computer to get those photos leaded directly. That cannot happen until tonight.

John

 

[JanK]

@Puffer Fish
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SNIP
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[*]Figure 2 is a mess (IMO). It should have been omitted from the article. There are many inconsistencies between the key, figure legend, and text. Shameful really. Fortunately, it's more of a summary figure. There's no new data here...just a re-organization of the data already presented -- this is considered poor form by the way.
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SNIP
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Not to mention that the table 3 on the same page demonstrates an interesting way of spelling Newton :amazed:

But thank you for the link.

 

[John C. Ratliff]

John -

Can you post your photos to scubaboard directly? Where you have them is blocked on my webserver (I can't see them).

The part that is wrong has to do with the "flying" and "wing" concepts, as those apply to air and not to water. The reason they don't apply to water is that water is incompressible, and therefore there is no possibility of a "low pressure" area above the wing.

Low speed flight is considered incompressible as well. Incompressible doesn't mean that the pressure is constant. It means the density is constant.

It looks like you are correct and that my assumption was somewhat wrong. I'm currently reading up on the Bernoilli equation and the physics of flight. I found a very interesting article that could shed some light on our subject of fins and fluid dynamics:

Physics of flight - revisited

I guess what I'm trying to say is that there is a lot of discussion and representations of the dynamics of diving fins in the water that just are not correct. Here's and interesting discussion of the Bernoulli principal in Wikipedia:

Misunderstandings about the generation of lift
Main article: Lift (force)

Many explanations for the generation of lift (on airfoils, propeller blades, etc.) can be found; but some of these explanations can be misleading, and some are false. This has been a source of heated discussion over the years. In particular, there has been debate about whether lift is best explained by Bernoulli's principle or Newton's laws of motion. Modern writings agree that Bernoulli's principle and Newton's laws are both relevant and correct [27] [28][29].

Several of these explanations use the Bernoulli principle to connect the flow kinematics to the flow-induced pressures. In case of incorrect (or partially correct) explanations of lift, also relying at some stage on the Bernoulli principle, the errors generally occur in the assumptions on the flow kinematics, and how these are produced. It is not the Bernoulli principle itself that is questioned because this principle is well established[30][31][32][33].

Bernoulli's principle - Wikipedia, the free encyclopedia

One of the problems of generating thrust with some types of fins which are based upon the faster fish is that we don't have the musculature to push/pull these designs at the frequency which the fish use for propulsion (tuna, sharks, dolphins, etc.). So we need more of a fin structure which mimics a grouper or trout/salmon. This is what the scoop design does.

SeaRat

 

[mdb]

John:

Thank you for your continuing contributions to this discussion and references provided.

What are your thoughts on the Mares Avanti and Quattro designs, both being "scoops".

I was in Italy when the Avanti was first introduced and was diving with the Mares crew.
Their comments regarding the concept seem to mirror yours.

Have you ever been diving with these model fins?

 

[Blackwood]

I've used Avanti Quattro fins. Didn't think they were anything special.

 

[Puffer Fish]

The wings of supersonic aircraft have dramatically different sections (small thickness-to-chord ratio, sharp leading edge, etc.). Compressible flow is fundamentally different, and really doesn't apply to this discussion. Note that most use significant flaps and slats to generate lift at low speeds.



Better stated: turbulence reduces some types of drag.

Pretty much all planes use flaps and most larger ones have slats. Only supersonic planes I have flow are the T-38 (just barely supersonic) and the FB111 (really, really supersonic)...FB111 wings are fairly traditional shaped (sharper edged and a bit thinner) and uses wing angle. T-38's are pretty much flat metal wings

The point was that it flys without using what anyone would call a wing shape (and flaps are just changing the direction of the air..directing it downward for increased lift, at really slow speeds.

Actually turbulence is essential to reduce almost all types of drag. Ever hear of a golf ball? Fill in all the dimples and see what happens when you hit one. In water, riblets are a great example. Perhaps it would be better to say there is both good and bad turbulence, but the right type is essential to reduce drag.