Sunday, 7 March 2021

Explaining and animating how cloakfish swim

In the two previous posts I wrote about parts of The Book that provide background information about how animals move on Furaha. There will be four double-page spreads showing such themes in The Book, about rusps, spidrids, tetrapters and cloakfish. 

I thought the cloakfish one would be easy, until I decided that it was high time I also made some progress towards a short CGI documentary I wrote about earlier, the one with cloakfish biodiversity as its main subject. That is a very big job and it is quite possible that I will fail on the programming side. But nothing ventured, nothing gained, so I went ahead and put some hours into Matlab programming. My strategy to design diverse cloakfish is to write editors that allow shapes to be designed with ease. The programmes then proceeds to make ‘3d meshes’, the basic working material for 3D design. How to get from meshes to nice photorealistic images is another story altogether. 

Click to enlarge; copyright Gert van Dijk

These are the editor screens. The user places control points here and there, which are then connected by a spline function. This results in nice smooth curves, useful for organic shapes. Panel A shows the body designer with a default shape. The inset shows a separate window controlling cross-sections of the beastie. By changing both shape and cross sections interesting forms can be produced. Panel B is the cloak editor. Apart from determining the shape of the cloak it also allows control over cloak movement, such as number of waves, wave amplitude, cloak curvature, thickness, etc. Panel C does something similar for the four front fins, and panel D shows the resulting output for the default shapes. 

Click to enlarge; copyright Gert van Dijk

With a few minutes’ worth of tinkering, you get this relatively slender cloakfish, probably a reasonably fast swimmer. 

Click to enlarge; copyright Gert van Dijk

Or this short and bulky ‘short-sleaved cloakfish’.

Click to enlarge; copyright Gert van Dijk

Or even this highly derived cloakfish, in which the cloaks are no longer ribbons, but shaped like penguin wings. They function in much the same way. 

 But the main point of this post was to show how the cloaks move. To do that, I had a look at the literature, and I found some papers on knifefish, but to my surprise more papers about artificial robot fish with similar fins. Apparently, people all over the world are working on robot fish, which is nice. Less nice is that some work at defence institutes, so what are they preparing for? Killer fish robots? Must we really? 

Click to enlarge; from: Liu, Curet. Swimming performance of a bio-inspired robotic vessel with undulating fin propulsion. Bioinspir. Biomim. 13 (2018) 056006 1748-3190/aacd26

Anyway, here is an example of robot ribbon fin design. The artificial cloak design followed exactly the same reasoning as my virtual cloakfish designs. Such papers make a distinction between ‘oscillation’ and ‘undulation’. If a cloak, ribbon or fin swings side-to-side as a whole, the word ‘oscillation’ is used, and when waves travel along the length of the cloak, it is ‘undulation’. But the distinction is not all that clear; it depends on the number of waves travelling along the cloak. If there is less than one wave on the cloak or fin at a time, then the movement largely concerns the fin as a whole, so the movement edges towards oscillation. Here is a YouTube film explaining the difference

Pure oscillatory sideways movement are useless, because they do not propel the beast forwards. The animation above shows such an almost pure sideways movement of the cloaks. The movement would push water away from the cloak, resulting in a force towards the attachment of the cloak (the ‘dagger’). In knifefish, with just one cloak underneath the body, this force ‘heaves’ the body up. With four cloaks, the dagger will not be going anywhere, so this is just a waste of energy. We want water to be pushed backwards. The cloaks push water backwards when its parts are at an angle to the direction of movement. These parts produce forwards thrust. 

Let’s equip our default cloakfish with exactly one wave per cloak. One half of that wave will be angled towards the left, and the other half towards the right. Both produce sideways forces, but these should cancel one another out. The robot designers reported that the robots swam nicely with just one wave per cloak. Mind you, the robots usually had just one cloak, like the knifefish. 

The animation also shows one other trick: if you look closely, you can see that the cloaks do not sway much at front, and the amplitude of the wave increases towards the back of the animal. I borrowed that from real biology, as at least rays and knifefish do this. 

Let’s now equip our knifefish with 2 waves per cloak. The parts of the wave that are useful for swimming are now at a steeper angle towards the direction of movement, nearing perpendicular to it. I thought that this should increase the propulsive force a lot, but work on the robot fish did not agree. The velocity did not increase much, but the robot was more stable, which is intriguing. Real knifefish have more than two fins on their ribbon fins at one time, so there must be an advantage in that. 

Click to enlarge; from: Blevins, Lauder. Rajiform locomotion: three-dimensional kinematics of the pectoral fin surface during swimming in the freshwater stingray Potamotrygon orbignyi.  The Journal of Experimental Biology 2012; 215, 3231-3241

This image is from a paper about ray fin movement. The fins as a whole move up and down (so they oscillate), while there are ripples along the edges of the fins (so they also undulate). Nature seems to like combinations better than separations. The edges of the fins curl up and down, so they do not move as if there are completely stiff rays in them. 

I decided to build that in too, so here is a ‘curly-cloaked cloakfish’. I like it. One odd thing about these cloakfish animations is that it is not immediately obvious how they work, when you see the movement. It is also not easy to find an angle, when you rotate the objects, from where it is easy to get an immediate overview just how the animal is built. That can be seen as a disadvantage, or, in reverse, as an advantage, because it underlines that we are looking at an alien shape. 

By now, cloakfish ‘evolution’ has progressed to allow a variety of body and cloak shapes. Shapes range from ‘long-sleeved’ cloakfish with long narrow ribbon fins with multiple waves along their surface, to very short star-shaped cloakfish with narrow wings that fly through the water. That is certainly enough material for two explanatory pages in The Book, and should be enough for a short documentary too. But that will depend on me improving my skills as regards merging and smoothing 3D meshes.


Unknown said...

you can do it! we have confidence in you.
-anthony docimo.

ps: thank you for the info on rays, knifefishes, and water robots as well as cloakfishes.

Abbydon said...

As always, that's another interesting post. Coincidentally, just last night I was reading about batoid motion and robotic fish as I was wondering whether a hybrid airship design (i.e. buoyancy and aerodynamics) is required for large interesting ballonts. This requires deliberate motion and fish might provide better inspiration than birds for this.

I did then wonder whether a torus shaped balloon with fins on the outside for motion and a filter feeding tube in the middle would work. This post reminded me that your cloakfish are similar to this and certainly can look a bit blimp-like. I wonder whether your design would work to provide sufficient propulsion in the air too. The body shape would have to be more like an aerofoil to produce lift as well but that wouldn't be a big change.

Sigmund Nastrazzurro said...

Anthony: thank you! Glad you liked it.

Abbydon: I don't have to tell you about ballont size problems. If I rephrase your question it may become something like this: can oscillating fins, undulating cloaks or a hybrid between the two propel a ballont/airship?
Well, we know that you can propel a zeppelin with propellors, so it is certainly possible to make a balloon into a 'dirigible', meaning something that can be steered. The basic engineering problems are probably, 1. the propulsion system has to overcome wind resistance, and that is something the old zeppelin designers already solved, so that one is doable; 2. get you get the same thrust with fins/cloaks? and, of course, 3. can you make it light enough to actually work?
If the cloaks are thick and hollow they could probably lift their own weight. Now to get them to move with force...

Mind you, this does not solve the problem of how small ballonts survive if only large ones can take to the air. If you read the comments of my old ballont posts, you will find that the commenters came up with some very nice ideas.

ed said...

are they fast? maybe there could be a dolphin-analogue called the torpoise

Andrew Broeker said...

It seems to me that the undulatory wavelength ought to vary considerably with swimming speed for best efficiency.

Spooktober said...

Interesting, nice to see how this is coming along

megapig said...

Could a cloakfish-like aquatic species evolve to come on land? what would an entire clade of land-cloakfish descendants be like and how could such fins become limbs?

Dugong WITH LEGS said...

Are there any limbless snake-like creatures on Furaha? Perhaps a hexapod that lost all its limbs as it became a predator of burrowers?

Sigmund Nastrazzurro said...

ed: I never envisaged anything with undulatory fins as fast, but manta rays can jump out of the water, so they they must be able to generate considerable power. I guess that the 'long-sleeved ones will not be very fast, but that ones that evolve towards oscillatory fins can be a lot faster.

Andrew Broeker: the papers on the robot fishes make the point that speed is directly related to the frequency of wave propagation, which makes sense, but the relation with the number of waves per cloak was more complex.

Spooktober: than you.

megapig: I never thought of that, and do not think their anatoy lends itself well to that. I'll keep them underwater, I think.

Dugong: Interesting; I wonder whether limbless slitherers are a universal design. I might sneak one i=one into the background of a painting one day.

Abbydon said...

I have sometimes pondered what advantages/disadvantages a limbless exoskeleton would produce and what the maximum size limit would be. I guess it would be a giant mealworm with ring segments covered in bristles/setae to help provide grip.

Noasaur said...

Bravo, Bravo. Love this blog. Helped me to make my own.

Sigmund Nastrazzurro said...

Abbydon: The maximum size probably depends on the choice of respiratory system. If you use tracheae, the animal probably will have to squeeze and relax rings to refresh the air. But if you provide it with an updated system, respiration should not be a big problem. The same goes for the exoskeleton; if you vary the degree of malleability, and equip the animal with internal attachments points for muscles etc., it could grow big. Exoskeletons do not have to be totally rigid nor exclusively external; they are not in many earth lineages.

Noasaur; thank you! Nice site, by the way. Are the writer and the painter one and the same person?

African heffalump said...

How big can a cloakfish get? Would undulating membranes work at like, blue whale size?

Sigmund Nastrazzurro said...

African...: in my mind's eye, i see cloakfish the size of orca's. On earth, filter feeders can obviously grow in size to become the largest animals in the history of the planet, so that mode of feeding will not restrict them. I cannot think of any mechanical reason why undulating membranes would only work up to a certain size either. In short, they could!
Perhaps there should be a giant cloakfish in the documentary...

Abbydon said...

Out of curiosity, did you ever come across a paper (or papers) that provides approximate simple equations for the forces produced by different types of swimming? It would be helpful to use that to compare speeds for different body shapes but I suspect it is all far too complex for that unfortunately.

Sigmund Nastrazzurro said...

Abbydon, No, I did not. I saw that one paper on robotic undulatory swimming calculated forces for rectangular parts of the membrane and then went on to combine them all into a sumed force. That should not be too difficult for cloakfish, and perhaps I should do that to see whether they would jiggle about or would need to use their front wings to counter such forces.

Otherwise, no. he books by the late Robert McNeill Alexander contain fairly digestible physics of moving animals. I think they are all easily found.

Noasaur said...

Sigmund-Nope. My profile picture is from a guy known as Joschua knuppe...

Sigmund Nastrazzurro said...

Noasaur; Aha! I met Joschua one at a Tetzoo conference.

Abbydon said...

I haven't finished Manton's book on arthropods yet and now I have another textbook to read... My bookshelf is looking quite varied these days. His "dinosaur speed calculator" shown on Wikipedia is interesting as it is invariant to gravity if you assume the organism linear size is inversely proportional to gravity (i.e. double gravity, half the hip height and stride length).

Anyway, back on topic, my interest was how well fish modes of swimming could be applied to ballonts. Simplistically, both thrust and drag are proportional to fluid density so that cancels out. The problem however is that the drag is due to the large balloon but the thrust is due to the small "fish" hanging underneath.

This means that there is some scale factor that reduces the speed of a ballont using fins relative to a fish. That presumably is dependent on the length or area of the fin depending on the swimming method used. I have a feeling that the undulating ribbon fin might be the best approach though the Air Swimmers radio controlled toys use the tail fin.

Odobenodiabolus infernus said...

Could ribbon fins become a slug-like "foot" for a theoretical mudskipper like amphibious species? Just a thought

Unknown said...

>Simplistically, both thrust and drag are proportional to fluid density so that cancels out. The problem however is that the drag is due to the large balloon but the thrust is due to the small "fish" hanging underneath.

Well, why not change it so the balloon and the fish are one and the same...or, if not the same, have the "fish" be extensions of the balloon -- the bits and pieces which couldn't be filled with gas or whatever. (like how in electric eels, all the vital organs are squished into what most people would assume is *just* the head of the eel)

Or like in ctenophores and salps, the parts of the body that aren't for the fluid (air, water, whatever) or for the skin, are for the propulsive structures.

just my two cents.

-anthony docimo

Abbydon said...

I just mean that the overall size is equivalent to a large fish but since most of that is just gas the available mass to provide other bodily functions is the same as a small fish. Effectively it would have the drag of a large fish but the thrust of a small fish. For example, the cloakfish images look a bit like a possible ballont but there would be a reduction in space inside for internal organs and muscles if it was full of gas.

Unknown said...

good point. maybe then limit the inside-the-balloon organs to scaffolding and air-generating tissues, and leave other things as outside-the-balloon organs, such as the cloak itself and the jaws.

-anthony docimo

sperm wheal said...

How many clades of "fish" are on furaha?

Unknown said...

offhand, I can think of...its either IV or VI..unless I'm wrong on both counts.

-anthony docimo.

ps: in hindsight, a tag search in the blog for "fishes" might have resulted in something useful.

Sigmund Nastrazzurro said...

Abbydon (22 March): Sorry about the delay. I was working on a paper and that requires integrated concentrated working on the statistics, data handling, graphics and writing, so I tend to do nothing besides until it is done.
Anyway, if the thrust in a ballont is prodeuced by an undulating ribbon, that need to not fixed to the 'cargo', but could also be fixed to the 'bladder', and perhaps more efficiently. Does that change matters?

Odobeno... Interesting, and possible. Originally, I had cloakfish that had their cloaks arranged like a cross, with two vertical and two horizontal cloaks, while others had four diagonal cloaks. The latter disappeared from view. In such a 'mud wrestling escape' scenario, the latter would have it much easier, as they could rest on two cloaks at once, which should make a very large difference. Actually, if the vertical/horizontal types would attempt this, they would very probably keel over until they rested on two cloaks. They might feel uncomfortable in that position, though.

Anthony / Abbydon /Anthony: I think we mean the same thing. Great minds...

Sperm wheal / Anthony: there are six!

cockzilla vs. dong said...

Are the "fishes" air breathers or water breathers? are any just derived hexapods?

Abbydon said...

I suspect the undulating ribbon could be fixed to the bladder (and the body probably wouldn't be hanging underneath like a gondala anyway) but the problem is how much mass can be devoted to the fin and associated muscles? Drag is proportional to the large cross-section but thrust is proportional to the smaller fin. That's the problem.

Just to stay on the fishy topic of this post, that probably means the bladder needs to act as a lifting body so that more mass can be used to produce thrust. This is akin to sharks which need to keep moving to maintain lift from their pectoral fins.