Nautilus Efficiency


Long Title: Analysis of the locomotion efficiency of a Nautilus Pompilius Pompilius

 

There was an open-ended project assigned for my Thermal & Fluid Sciences Lab during my undergraduate studies. Having access to a pretty nice windtunnel, I decided to take this project as an opportunity to get creative and study one of my favorite sea animals: The Nautilus!

The nautilus is a funny little creature; in the race of evolution, it doesn’t even feign advancement. Relatively unchanged for the past 500 million years, it goes about its days just swimming around the Indo-Pacific seeking out the molts of lobsters and hermit crabs for its meals. The Nautilus are one of the few marine animals that uses jet propulsion as its sole means of active locomotion. I had come across a publication by O’dor et al. (1990) about the swimming speeds, jet pressure, and oxygen consumption of the Nautilus, and it gave me some inspiration.

Using the Reynold’s Number, one can show that the forces resulting from an adult nautilus swimming could be modelled by putting a smaller than average nautilus shell into a wind tunnel and taking the air flow velocity up to 50 mph (20 m/s). And that is exactly what I did. By measuring the drag forces, it was possible to use the coefficient of drag scaling to determine the drag forces experienced by a real Nautilus in the ocean at its usual swimming speeds. From there it was possible to use the data from O’dor et al. to determine the efficiency of the jet propulsion as a function of the their speed.

My results showed that the efficiency of the jet propulsion increases dramatically and roughly linearly with speed until it tapers off at about 75%. This is a 30% efficiency increase from when it is cruising at its idle speed of 5 cm/s, which is a result from natural respiration.  Overall, it was a fun project and a neat way to apply the concepts and techniques that I was learning at the time. Also, making the Nautilus model was a good challenge, since it had to securely house a mount for the sting balance, and ended up being a perfect souvenir. I’ll leave you with an image from the experiment, which is truly one of the most bizarre sights I’ve seen in a wind tunnel.

Smoke Trails of Nautilus in Wind Tunnel