Making Tags Less of a Drag: Optimising Biologging Devices with Computational Fluid Dynamics

Post provided by WILLIAM KAY

Drag and Biologging Devices

A harbour seal tagged with a biologging device. ©Dr Abbo van Neer

A harbour seal tagged with a biologging device. ©Dr Abbo van Neer

Michael Phelps is one of the most decorated Olympic athletes of all time and the world’s fastest swimmer. And yet, he could swim faster. Wearing the Speedo LZR Racer supersuit Michael Phelps could reduce his hydrodynamic drag, or water resistance, by upwards of 40%. That could increase his swim speed by more than 4%! In competition, that’s the difference between silver and gold. But, if Phelps forgot to remove his “drag socks” – cumbersome footwear designed to increase water resistance for strength training – his speed would be dramatically reduced. He’d be lucky to walk away with bronze!

Professional swimmers have adapted to the use of performance enhancing technologies to decrease their drag, but that’s nothing compared to the adaptations made by wild animals. Creatures in the marine environment have evolved incredible adaptations to decrease drag, such as extreme streamlining in marine mammals and seabirds. This allows them to move underwater as quickly and efficiently as possible. Seals, for example, are pretty ungainly on land, but in the water they’re sleek and rapid. They have a body shape designed to maximise speed while swimming.

When we study marine animals we often use tracking devices, which can be attached using harnesses, glue, or suction-cups. These ‘biologging devices‘, or tags, are similar to Fitbits. Attaching them to animals allows us to record, amongst other things, all of the animal’s movements and behaviours. This information is crucial to understanding their ecology and for improving their conservation management.

The Downside of Biologging Devices

A magellanic penguin tagged with a biologging device. ©William Kay

A magellanic penguin tagged with a biologging device. ©William Kay

Using FitBits to see how dolphins or penguins spend their day seems great, right? But attaching external biologging devices to animals can lead to a substantial increase in drag, especially on such streamlined subjects. This can impede the animal’s movement or increase the effort required to swim, which means the animal has to work harder to perform normal behaviours, such as finding and catching food. This is an issue because by studying an animal using a tag, we’re affecting the very movements and behaviours that we wish to record. It’s a real ‘catch 22’.

We know that the drag of tags is the result of complex interactions between their size, shape, and positioning. So, what can we do to optimise biologging devices to minimise their drag?

Computational Fluid Dynamics

Step in Computational Fluid Dynamics (CFD). CFD is a virtual design tool that is able to simulate fluid flow. We can use it to study how much drag is created by an object, such as a biologging device, when it passes through water. CFD is routinely employed for improving the design of spacecraft or Formula One cars. Its use in ecological applications has been more limited though – mostly because of the required expertise in aerospace engineering. But our recent work has made inroads to overcome this.

Investigating hydrodynamic drag on a tagged seal. ©Hannah Bowen, Simon Withers and David Naumann

Investigating hydrodynamic drag on a tagged seal. ©Hannah Bowen, Simon Withers and David Naumann

Crucial Interdisciplinary Collaboration

In a recent collaboration, we teamed up with aerospace engineers at Swansea University to use CFD to explore how simple changes to biologging devices could minimize the drag impact that they had on swimming animals. Together we investigated how the impact of drag changed with differently sized or shaped biologging devices. We also looked at how drag varied depending on where the tags were positioned on the animal.

Minimizing drag is crucial for marine animals, especially those that swim at high speed or travel in fast currents, so it’s essential that biologging devices are designed with good hydrodynamics in mind. Knowing what factors are at play in this is very important and CFD is perfect for this because it lets you quickly and efficiently evaluate the impact of multiple factors in tag design.

Examples of hydrodynamic flow and pressure visualisation of two different tag designs using computational fluid dynamics. ©Hannah Bowen, Simon Withers and David Naumann

Examples of hydrodynamic flow and pressure visualisation of two different tag designs using computational fluid dynamics. ©Hannah Bowen, Simon Withers and David Naumann

In ‘Minimizing the impact of biologging devices’, we demonstrate this approach. We show how and why improving the shape of biologging devices can be more effective in improving the hydrodynamics than simply reducing tag size. In fact, biologging devices could actually be made slightly larger to accommodate an improved shape.

Interestingly, improving a tag’s design also reduces differences in the amount of drag caused by attaching the tags to different positions on the animal. This would allow you to attach the tags across a greater range of positions on an animal. It also reduces the impacts of accidental device positioning errors.

Having greater flexibility in tag positioning is a big advantage. With some animals it can be very difficult to get the tag positioned correctly. Take for example a cetacean that surfaces only briefly to breathe – you may only get one chance to attach the device. Tags are often deployed on cetaceans through the use of a long, wobbly pole, so you can’t be as accurate as when you’re tagging a carefully restrained seal or seabird. Designing tags so that their impact is minimal, regardless of their final position, means that the consequences are smaller if things don’t go quite as well as planned.

 An Instructional Guide for Future Research

We want other ecologists to be able to repeat our methods so we created a step-by-step guide for you to follow. This guide walks you through the process of modelling the drag impact of biologging devices. It takes you from importing the tag design and animal geometry files into the CFD software, all the way through setting up the computational environment and on to running the simulations.

An adult Weddell seal with an infrared camera logger. ©Mr Dominik Nachtsheim

An adult Weddell seal with an infrared camera logger. ©Mr Dominik Nachtsheim

The guide is written around the use of a standard CFD software package, ANSYS Fluent. This should (hopefully) encourage future interdisciplinary collaborations between engineers and ecologists, including novice CFD users. The guide is a useful tool to promote these techniques more widely throughout the biologging community by helping other researchers to assess the impact of their tags.

We’ve included a short review of the use of CFD in tag design until now in the article. There’s also a cautionary note on the limitations of the method, providing a platform for future research to build on.

We don’t expect our findings to be taken up as strict, formal guidelines, or that using CFD will become compulsory. However, we hope that this work, and especially the step-by-step guide the paper contains, will help to move the biologging community towards achieving best practice in tag design.

To find out more, read our Methods in Ecology and Evolution article ‘Minimizing the impact of biologging devices: Using computational fluid dynamics for optimizing tag design and positioning

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