Category: Physics

Ammonite propulsion of underwater robots

Ammonite statue showing creature inside its shell
Image by M W from Pixabay

Intending to make a marine robot that will operate under the ocean? Time to start learning, not just engineering and computing, but the physics of marine biology! And, it turns out you can learn a lot from ammonites: marine creatures that ruled the ocean for millennia and died out while dinosaurs ruled the earth. Perhaps your robot needs a shell, not for protection, but to help it move efficiently.

If you set yourself the task of building an underwater robot, perhaps to work with divers in exploring wrecks or studying marine life, you immediately have to solve a problem that is different to traditional land-based robotics researchers. Most of the really cool videos of the latest robots tend to show how great they are at balancing on two legs, doing some martial art, perhaps, or even gymnastics. Or maybe they are hyping how good they are running through the forest like a wolf, now on four legs. Once you go underwater all that exciting stuff with legs becomes a bit pointless. Now its all about floating not balancing. So what do you do?

The obvious thing perhaps is to just look at boats, submarines and torpedoes and design a propulsion system with propellers, maybe using an AI to design the most efficient propellor shape, then write some fancy software to control it as efficiently as possible. Alternatively, you could look at what the fish do and copy them!

What do fish do? They don’t have propellors! The most obvious thing is they have tails and fins and wiggle a lot. Perhaps your marine robot could be streamlined like a fish and well, swim, its way through the sea. That involves the fish using its muscles to make waves ripple along its body pushing against the water. In exerting a force on the water, by Newton’s Laws, the water pushes back and the fish moves forward.

Of course, your robot is likely to be heavy so will sink. That raises the other problem. Unlike on land, in water you need to be able to move up (and down) too. Being heavy, moving down is easy. But then that is the same for fish. All that fishy muscle is heavier than water so sinks too. Unless they have evolved a way to solve the problem, fish sink to the bottom and have to actively swim upwards if they want to be anywhere else. Some live on the bottom so that is exactly what they want. Maybe your robot is to crawl about on the sea floor too, so that may be right for it too.

Many, many other fish don’t want to be at the bottom. They float without needing to expend any energy to do so. How? They evolved a swim bladder that uses the physics of buoyancy to make them naturally float, neither rising or sinking. They have what is called neutral buoyancy. Perhaps that would be good for your robot too, not least to preserve its batteries for more important things like moving forwards. How do swim bladders do it? They are basically bags of air that give the fish buoyancy – a bit like you wearing a life jacket. Get the amount of air right and the buoyancy, which provides an upward force, can exactly counteract the force of gravity that is pushing your robot down to the depths. The result is the robot just floats under the water where it is. It now has to actively swim if it wants to move down towards the sea floor. So, if you want your robot to do more than crawl around on the bottom, designing in a swim bladder is a good idea.

Perhaps, you can save more energy and simplify things even more though. Perhaps, your robot could learn from ammonites. These are long extinct, dying out with the dinosaurs and now found only as fossils, fearsome predators that evolved a really neat way to move up and down in the water. Ammonites were once believed to be curled up snakes turned to stone, but they were actually molluscs (like snails) and the distinctive spiral structure preserved in fossils was their shell. They didn’t live deep in the spiral though, just in the last chamber at the mouth of the spiral with their multi-armed octopus like body sticking out the end to catch prey. So what were the rest of the chambers for? Filled with liquid or gas, they would act exactly like a swim bladder providing buoyancy control. However, it is likely that, as with the similar modern day nautilus, the ammonite could squeeze the gas or liquid of its spiral shell into a smaller volume, changing its density. Doing that changes its buoyancy: with increased density the buoyancy is less, so gravity exerts a greater force than the lift the shell’s content is giving and it suddenly sinks. Decrease the density by letting the gas or liquid expand and it rises again.

You can see how it works with this simple experiment.

You don’t needs a shell of course, other creatures have evolved more sophisticated versions. A cuttlebone does the same job. It is an internal organ of the cuttlefish (which are not fish but cephalopods like octopus and squid, so related to ammonites). They are the white elongated disks that you find washed up on the beach (especially along the south and west coasts in the UK). They are really hard on one side but slightly softer on the other. They act like an adjustable swim bladder. The hard upper side prevents gas escaping (whilst also adding a layer of armour). The soft lower side is full of microscopic chambers that the cuttlefish can push gas into or pull gas out of at will with the same effect as that of the ammonites shell.

This whole mechanism is essentially how the buoyancy tanks of a submarine work. First used in the original practical submarine, the Nautilus of 1800, they are flooded and emptied to make a submarine sink and rise.

Build the idea of a cuttlebone or ammonite shell into your robot and it can rise and sink at will with minimal energy wasted. Cuttlefish, though, also have another method of propulsion (aside from undulating their body) that allows it to escape from danger in a hurry: jet propulsion. By ejecting water stored in their mantle through their syphon (a tube), they can suddenly give themselves lots of acceleration just like a jet engine gives a plane. That would normally be a very inefficient form of propulsion, using lots of energy. However, experiments show that when used with negative buoyancy such as provided by the cuttlebone, this jet propulsion is actually much more efficient than it would be. So the cuttlebone saves energy again. And a rare ammonite fossil with the preserved muscles of the actual animal suggests that ammonites had similar jet propulsion too. Given some ammonites grew as large as several metres across, that would have been an amazing sight to see!

To be a great robotics engineer, rather than inventing everything from scratch, you could do well to learn from biological physics. Some of the best solutions are already out there and may even be older than the dinosaurs, You might then find your programming task is to program the equivalent of the brain of an ammonite.

– Paul Curzon, Queen Mary University of London

More on …

Thanks to the Dorset Wildlife Trusts at the Chisel Beach Visitor Centre, Portland where I personally learnt about ammonite and cuttlefish propulsion in a really fun science talk on the physics of marine biology.

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This page is funded by EPSRC on research agreement EP/W033615/1.

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Pots fixing problematic acoustics

Surface waves
Surface Waves, Image by Roger McLassus, CC BY-SA 3.0 via Wikimedia Commons.

Pots are buried in the walls of medieval churches and monasteries across Europe: in the UK, Sweden, Denmark and Serbia. Why? Are they just a weird form of decoration? Actually, they are there to fix problematic acoustics.

The problem

First of all, what do we mean by ‘problematic’ acoustics? When sound waves move around a room they reflect off the walls in a way that creates strange sound effects when they meet their reflections.

It happens because of what are called ‘standing waves’. Imagine dropping a pebble into a bath. The ripples create patterns in the water where they interfere with those that have bounced off the sides. As the two ripples pass in opposite directions if the movement pushing the molecule up from one ripple exactly cancels out the movement pushing down from the other and keeps doing so, then at that point the molecules remain still. On either side the two ripples reinforce each other rather than cancelling out giving the peaks and troughs of the combined wave. The result is the ripples appear to stop moving forward: a standing wave.

Sound waves are like water waves except that the air molecules vibrate from side to side rather than up and down as water molecules do. The same effects therefore happen when sound waves meet and standing waves can form. This is bad for two reasons. Standing waves take more time to die away after the sound source has been silenced than other sounds. Worse, the sound’s volume varies around the room depending on whether it is a point where the waves cancel out (no sound) or where they enhance each other (loud). That’s ‘problematic’ acoustics!

Standing Wave.By Lucas Vieira – Own work, Public Domain, from WIKIMEDIA

These acoustic problems ultimately come about because of what is known as ‘resonance’. That is where a sound repeatedly bounces back and forth across a space at a particular frequency. Frequencies that are directly tied to the room’s dimensions cause most problems. Called the ‘resonant frequencies’ they involve a whole number of wave troughs and crests fitting in the space between the walls. That is what leads to standing waves as the original and reflected wave coincide exactly. The lowest resonant frequency of a wave is also called the ‘fundamental frequency’. It’s the one where a single wave (a single trough and crest) fits in the space.

There are three different types of resonances developed in a room from sounds bouncing of the walls: called axial, tangential and oblique modes. Axial modes result from a sound bouncing back and forth between two facing walls. Tangential ones happen when the waves reflect around all four walls. Oblique modes are the most complicated and result from sound bouncing off the roof and floor too. Of all these, it turns out the worst are the axial modes. To improve the acoustics of a room you need to absorb the sounds at these resonant frequencies. But how?

The solution

OK, now we know the problem, but how do we deal with it? A solution is the ‘Helmholtz resonator’, named after a device created by Hermann von Helmholtz in the 1850s as part of his studies to identify the ‘tones’ of sounds. A Helmholtz resonator is just the phenomenon of air resonating in a cavity. It is the way you get a tone from blowing across the mouth of an empty bottle. The frequency of the tone is the resonant frequency of the bottle. If you change the volume of the air cavity or the length or diameter of the neck of the bottle you change its resonant frequency and so the tone.

A Helmholtz resonator actually absorbs sound at its resonant frequency and at a small range of nearby frequencies. This happens because when a sound strikes the resonator’s opening, the air mass in the neck starts to vibrate strongly at that resonant frequency and tries to leave. That makes the pressure of the air in the cavity lower than the outside. As a result it draws the air back into the cavity. This process repeats but energy is lost each time, which causes the wave, of this particular resonant frequency, to dissipate. That means that specific sound is absorbed by the resonator. Helmholtz resonators also reradiate the sound that is not absorbed in all directions from the opening. That means any energy that wasn’t absorbed is spread around the room and that improves the room’s acoustics too.

So back to those pots in the walls of medieval churches. What are they for? Well they would have acted as Helmholtz resonators so they presumably were designed to remove low-frequency sounds and so correct the acoustic of the vaults and domes. Ashes have been found in some of the pots. That would have increased the range of sound frequencies absorbed as well as helped spread the unabsorbed sound. St Andrew’s Church in Lyddington, Rutland, built in the 14th Century, has some of the finest examples of this kind of acoustic jars in the UK. Helmholtz resonators obviously predate Helmholtz, actually going back to the ancient Greeks and Romans. The pots in churches are thought to be based on the ideas of Roman architect Vitruvius. He discussed the use of resonant jars in the design of amphitheatres to improve the clarity of the speakers’ voices.

Designers of acoustic spaces like concert halls now use a variety of techniques to fix acoustic problems including Helmholtz resonators, resonant panels and tube traps. They’re all efficient ways for absorbing low-frequency sounds. Helmholtz resonators though have the particular advantage of being able to treat localized ‘problematic’ frequencies.

Those church designers were apparently rather sophisticated acoustic engineers. They had to be, of course. It would have been a little unfortunate to build a church so everyone could hear the word of God, only to have those words resonate with the walls rather than with the congregation.

– Dimitrios Giannoulis, Queen Mary University of London

Magazines …

This article was originally published on the CS4FN archive website and can also be found on pages 8 and 9 of issue 4 of Audio! Mad About Music Technology, our series of magazines celebrating sound and tech.

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