Fish exploit vortices in flowing water...
 

Now it gets a bit more interesting… Back in March 1995, ‘Scientific American’ published a fascinating article titled “An Efficient Swimming Machine” it focuses on detailed research into how fish swim and the results of tests of a mechanical fish that was built for the purpose, nicknamed “RoboTuna”!  Amongst many interesting discoveries, the research establishes that fish somehow exploit vortices in water to help them swim.

Over the years several research papers have been published about fish swimming.  One that I found particularly interesting is a paper by James C. Liao in 2004 which looks at muscle activity of a trout swimming in a vortex street.  This paper and others surmise that fish somehow exploit the vortices to reduce the amount of energy they need to combat the momentum of the flowing water.  There needs to be a relationship between the size of the fish, the size of the rock and the speed of the water flow for the conditions to be right, but if they are, the fish can hold its position with very little effort.  Apparently they do this by slaloming between the vortices, synchronising their body movements to match the flow of the vortices.

Pulling yourself up by your boot straps...

There is reference to this phenomenon in the Scientific American article, as well as the Ichthyology Web Resources website, (although crucially, I believe they have made a critical error and show the vortices forming the wrong way round).  It’s also mentioned by Viktor Schauberger in his ‘The Stationary Trout’ study.  The diagram below is taken from the book ‘Living Energies’ by Callum Coats who kindly gave me permission to use it. Living Energies is a very comprehensive study - the result of 15 years' research by Coats into Schauberger's work.  Among many fascinating revelations it details Viktor Schauberger’s explanation of how vortices form around the body of the trout: helping it to hold its position in a flowing river.

As previously discussed, most stationary objects in flowing water generate vortices and it looks like a trout does as well.  What is interesting to note here, is that due to the shape of the trout, the vortices start to form along the rear of its body. Viktor Schauberger explains (this is my simplified version) that the pressure of the vortices pushes the trout upstream, much like squeezing a wet bar of soap. By opening its gills the trout can increase the intensity of the vortices it generates, and hence how much energy it can extract. In essence the trout can remain almost stationary in the flowing water with just a gentle sway to keep itself aligned with the flow. This is sheer genius, but it gets better.

As far as I can tell from over a year of studying the leading research in this area, the latest understanding of a trout holding its position in flowing water, is based on two general scenarios, both characterised by a swaying movement:

a) A trout holding its position, in flowing water (that doesn’t have any vortices to exploit), can create vortices itself and remain almost motionless in the flowing water apart from a gentle sway to align itself with the flow of the water.

b) A trout holding its position, in flowing water behind an object that’s making vortices (a vortex street), slaloms between the vortices with a much more pronounced swaying movement which is synchronised to the flow and frequency of the vortices.

Although it appears that everyone agrees ‘one way or another’ that the trout exploits the vortices to hold its position; to me that doesn’t explain the rest of its movements like moving forward and darting away with such speed. Nor does any of the research I can find, explain how the trout extracts energy from the vortices… which is fundamental to understanding how it swims.

So how does the trout swim?

At the time of writing (early 2007), the commonly held view in resources like the wikipedia, most encyclopaedias and some of the research papers, appears to be that the tail is the main thruster. They explain that the backward force of the tail flapping against the water, in conjunction with the fins, moves the fish forward.

I believe this view is confusing cause and effect, I think that whilst flapping its tail may be one of its techniques, (and may be part of the thrust generating process) it’s not the most impressive, the most important, or the most powerful one: I think that it’s a result of the real technique that the trout uses to extract the energy from the vortices.  I think that as the trout slaloms between the vortices with its body and tail weaving from side to side – it’s not that the tail is generating the main thrust, it’s that the tail is flapping from side to side as the trout weaves through the vortices to extract energy – first to the right, then to the left and so on… much like a cat weaving between your legs.

This seems to be supported by several research papers including the very comprehensive work of James C. Liao which reveals that a trout can hold its position in a vortex street without using any of its thrusting muscles!  Apparently it only starts to use its thrusting tail fin muscles when the vortex street becomes too weak to provide it with enough energy to fight the current.  Even more startling results were obtained with experiments with a dead trout. It was tethered on a line to an object in flowing water and even though it was dead, it still adopted a swaying movement in the vortex street and was actually drawn upstream! 

It seems clear from the evidence that the trout extracts energy from the vortices and I believe that this is the main technique it uses to swim.  Using the example of a trout holding its position in flowing water (where there are no existing vortices to exploit) as discussed, the trout uses the combination of its body shape and its gills to create and intensify vortices forming against the rear part of its body, I believe it only needs to sway gently from side to side or open its gills slightly, to further intensify the vortices, extract more energy and move forward.

When it wants to move away quickly, it opens its gills and also makes a sharp, energetic sideways twist of its body so that it forms into a near semicircle. This causes more of an in-rush and a powerful vortex is generated that the trout extracts energy from and shoots away.  So in this scenario, the sharp twisting movement causes a flap of the tail but it’s not the tail that powers it away so quickly (although it may have some thrusting input), it’s the vortex. Somehow…

The scales create a ‘one way surface’.  As the water flows past the trout from its snout at the front, to the tail at the back, there is almost zero friction as it passes the shiny, slippery scales  (which are also covered by a slime excreted by the trout).  If the water were to flow from the back to the front however, it would encounter massive friction as it catches under the scales, each one acting as a tiny ‘sail’ for the flowing water.

If one looks again at the direction of the vortices that the trout slaloms between, you’ll notice that as the trout presses itself against the vortex, the water flowing along the rear of its body, is actually flowing upstream.  You’ll also notice that in all cases of swimming in a vortex street, the trout lines itself up this way. This is also true for the water revolving in vortices that the trout creates with its own body, again, it’s revolving upstream against the rear part of the fish. The trout extracts energy from the vortex much like a sailing boat does from the wind.

As the trout curves its body around the vortex, it has the effect of opening up the scales to capture more energy from the vortex which translates directly into forward thrust. The trout may also use its fins to capture upstream water flows and it may use its tail to provide some thrust but overall, I believe that the main tool is its body with a simple ‘one way’ scaled surface that offers ultra-low friction one way, and massive friction the other.

So, all the fish has to do to remain in position in a vortex street, is to gently ride the vortices, gently swaying from one side to the other curling round to catch each of the vortices and have them run down the side of their bodies and push them upstream as in the diagrams.

Simple, elegant and very efficient.  When it wants to dart away, it chooses upstream because it’s easier to form a strong vortex against the current. A sharp twist in the water and it spins itself away upstream to either create another vortex itself or hop a free ride from others it can sense in the water... The pinball effect.

It has taken a couple of years but finally I understand Viktor Schauberger’s comment that has stuck in my mind ever since I read it: “fish don’t’ swim, they’re swum”

Zero Friction?

The lessons we can learn from fish don’t stop here.

In the research paper “An Efficient Swimming Machine”, there is mention of the British Zoologist James Gray who created a stir in 1934 by calculating the power a dolphin would need to swim at 20 knots. The inescapable conclusion apparently, (based on calculations of rigid model dolphin) was that a real dolphin was ‘technically too weak’; it would need some way to reduce its drag by a factor of about 7 in order to swim at those speeds.  Since then there’s been a lot of research into dolphins and it’s now known that they shed their skin approximately every 2 hours and that the shedding process helps reduce drag but personally I doubt skin shedding accounts for the reduced drag alone.

Looking again at the example of the stationary trout: it can be seen that as a result of the vortices that form around the rear of the trout’s body, a significant percentage of the trout’s overall body surface will have water flowing in the same direction (i.e. upstream). This can mean zero (and possibly negative) friction for those parts; which in turn means a very low overall drag factor in relation to its total surface area. I would be very interested to hear from someone who knows how to make even a rough calculation in this regard.

Although Dolphins are not fish and don’t have scales, I believe their shape lends itself to create vortices around the rear of its body, just like a trout and hence significantly reduce its drag factor. It’s a sublime and elegant solution. If it’s correct, and a way could be found to use this principal to in the design of ships’ hulls, it could have hugely beneficial results.

Conclusion

This has been a mammoth journey of discovery for me. My initial idea was that fish exploit vortices to shoot about in water and that we could perhaps exploit the techniques a trout uses for our own water borne vessels to help reduce our environmental impact. When I started this topic I had no idea that research had already been done into this area and that it would support my initial hypothesis. 

If the final part of my hypothesis is correct, for now there are at least two important concepts that might be able to be employed in the design of ships, I will include them in the Design Suggestions section soon, along with a few rough sketches.  However, to make sure the Intellectual Property is in the Public Domain, I’ll state here that the ideas will be based on the shape of the hull likened to that of the shape of a fish, and a one-way surface akin to the surface of a fish’s body.

Of course ships are massively bigger than fish (and only partially submerged) and there are lots of things I don’t know, like if it’s possible to get water to move in a similar way on such a large scale. Also, I understand that in some cases generating vortices can increase drag. There are other considerations as well like (according to Victor Schauberger), the way flowing water breaks into a vortex is dependant upon temperature and velocity. Unfortunately, he’s not around to ask anymore and I don’t know the answer to these things, but someone out there might?

I was about half way through writing up this topic when I was made aware of a paper “Leading-Edge Vortex Lifts Swifts” which was published in Science in December 2004. It shows how a Swift generates a vortex at the leading edge if its wing which it uses to generate lift. If you look at the rotation of the vortex, it to, is pushing the bird ‘upstream’ just like a fish in a vortex street… using the ‘one way’ properties of feathers I propose. 

Viktor Schauberger’s comment in full, is actually: “fish don’t’ swim, they’re swum and birds don’t fly, they’re flown”.

I’ll start a “Birds Flying” topic at some point.