REPORT

To fix submarines, we’re learning from stingrays

By Jack Flanagan

Earlier this year, a research duo by the names of Borazjani and Daghooghi published the first known paper that took what we know about locomotion in the air into the seas.

The LEV, or leading edge vortex, defines what we know about how birds, insects and our own airborne technology flies through the air. A vortex, for starters, exists in a state in which the centre is low pressure and the outside is high pressure. This brings the medium – be it air, water or potentially solids – from the outside inward.

Animals fly by making use of this vortex. They create pockets of high pressure in front of themselves and low pressure behind and the “separation bubble” – the space occupied by a wing or tail. The resultant pressure on the air propels them forward. This is done by using what’s called an “airfoil”: basically, a wing of any sort. You can see the different effects caused by different, organic and inorganic, airfoils below.

Airfoils

The LEV in flight is named after the anatomy of an airfoil. As the air hits the wing, it separates and comes in contact with what’s called the “leading edge” – the point at which the high and low point of the wing meet. A vortex is then created as described above, and there you have a Leading-Edge Vortex, or an LEV. The separate points of air create a force which can then be used to propel the object. This is essentially the most important aspect of flight for insects and birds.

The fins of fish are similarly shaped, and use a similar method for movement – so we can take what we know about locomotion above water and apply it below as well. At least, that was the idea behind this latest study.

As a fish’s body undulates, or swings back and forth, it creates an area of low pressure in front of and to the sides of its body. Areas of high pressure mirror the points of low pressure. What you have created is a vortex through which the animal moves, keeping the low pressure in front and high pressure behind – propelling it through the water. It’s like the fish is sucking itself through the water.

In fact, not just its body but the fish’s tail also creates this vortex effect, creating low pressure in front of the tail and high pressure behind it. This force alone makes up 10-20% of the fish’s movement, according to recent, as-yet unpublished, research. Given this result and the amount we already use LEVs in our lives – such as planes and helicopters – it isn’t surprising that this new information has caused rumours about advances in underwater vehicles.

mack-tail

The question is: what animal should we base this tech on? While fish might be attractive in the popular imagination, the complexity of their movements might make adaption difficult. It’s not impossible, but it’s not the most attractive option. Meanwhile, the stingray – which sweeps through the water in a similar way to how a bird moves through the air – makes a more attractive prospect.

The stingray creates an LEV similar to that of a bird: it ‘flaps’ with its fins, which act as its airfoils. The size of stingrays, creatures that can reach truly massive lengths, allow it to undulate along its entire body – creating a wave.

The wave is a point of high pressure surrounded on either side by low pressure. The ray uses the muscles in its body to create the wave, and lets it roll from the head down. The sliding patch of high and low pressure are what create the vortex, through which the marine creature passes. So the principle is the same, but the execution is different.

This is not how submarines currently operate. In fact, compared to this sleek and energy-saving design, submarines are huge energy wasters. All propulsion comes from a fixed point – ie, the engine. The engine creates a zone of high pressure just behind the vehicle: the vortex required to move. But the rest of it is dead weight; friction between water and metal creates a substantial drag, which slows it down as well as burning through fuel.

Unfortunately, a stingray-submarine may or may not look like that creature that inspired it. Tellingly, the LEV principle exists in birds, insects, fish and the ray – so goodness only knows how the eventual war-machine will turn out. Even the unexciting – and sessile – sea dollar is built for LEV motion. So it might even look like a UFO.

This isn’t the first good idea of Mother Nature’s that we could benefit from. A shark’s skin is covered in what are called placoid scales, each one fitted with tiny spins to guide water over the skin (as well as prevent hangers-on like parasites).

While sometimes nature can be a bit slapdash in its engineering, billions of years of incremental adjustments and improvements can really deliver amazing results.