Imagine if you had to track your lunch based on the disturbance it had caused in the air around it 30 seconds ago as detected by your beard. Obviously somewhat more difficult to imagine for women, but still, even the most hirsuite of gentlemen would have to admit that they might find themselves going hungry more often than not. Well, seals, male or female, don’t seem to find that sort of task very difficult at all.
The question of how exactly they manage to accomplish this is one that has been on the minds of members of MIT’s Department of Mechanical Engineering. Heather Beem, a graduate of the PhD program, and her dissertation director Michael Triantafyllou have recently published the results of their research into the marvelous sensing abilities of the bewhiskered harbor seal in the Journal of Fluid Mechanics.
One of the qualities of these mysterious whiskers is that they are able both to remain still while the seal moves through water and to vibrate in response to the turbulence in the water caused by the movement of other bodies in the water. That is a pretty tall order. They are able to perform in this way because of their unique form; rather than being the straight, tubelike structures we had imagined, their form follows a perfect sine wave.
Now, just in case you don’t keep the latest copy of this journal on your nightstand, let me explain how this relates to 3D printing. In order to closely study the way that this form affects their movement, Beem 3D printed a replica of a seal whisker but at a much larger scale. That ‘super-whisker’ was then tested in a 30 meter long tank of water while attached to a moving track. This sinusoidal form results in much smaller disturbances as it moves through water, effectively quieting the vibrational ‘noise’ that otherwise would be generated. Professor Triantafyllou explained:
“It’s like having the ability to stick your head out of a car window, and have there be no noise, so that your ears don’t ring: It’s a quieting effect.”
That was just one part of the equation, however. The other job of the whisker is to vibrate in response to turbulence caused by moving objects. Interestingly enough, the very properties of the whisker’s form that allowed it to move silently through the water are also what cause it to react strongly to the wake of external movement. In the 30 meter tank, the 3D printed whisker was moved along its track while a cylinder was moved ahead of the whisker’s path. As the eddies generated by the cylinder were passed through by the whisker, it began to vibrate at a matching frequency, something that would give a seal an idea of the direction, size, and shape of the object generating those eddies. Beem summed up the conclusions of this research:
“The geometry of the whisker allows for this phenomenon of being able to move very silently through the water if the water’s calm, and extract energy from the fish’s wake in order to vibrate a lot. Now we have an idea of how it’s possible that seals can find fish that they can’t see.”
Professor Triantafyllou outlined one area in which this understanding could have an impact:
“We already have a few sensors that can detect velocity [for underwater vehicles], but now that we know better what they can do, we can use them to track sources of pollution and the like. By having several whiskers on a vehicle, like the seal, you can, for example, detect a faraway plume, and track it all the way to the end.”
Not too shabby. It may be time that we stop being so impressed with bees’ knees…and start giving a little more respect to seals’ whiskers. Let’s hear your thoughts on this research in the 3D Printed Seal Whisker forum thread on 3DPB.com.