Like many mammals, such as bats and dolphins that use echolocation to locate their prey, harbor seals don’t rely on sight to follow their prey. Instead, the seals have specially shaped whiskers that can sense the movement of water as prey swims away.
Using that as a foundation for its research, a team at MIT is designing a sensor that mimics a harbor seal’s sensing ability during an underwater hunt. The sensor is expected to measure the exact size and trajectory of an object moving through a fluid, making it useful for tracking schools of fish or finding sources of pollution.
The whiskers have an elliptical cross-section that varies in diameter along the length of the given whisker. This shape gives it a wavy edge that minimizes vortex shedding, or the generation of eddies, from a seal’s own movement as it travels quickly through the water. In effect, the seals can detect turbulence from its fleeing prey without being influenced by the turbulence of their own swimming, giving them the ability to detect the whereabouts of their prey, even when they are up to 30 seconds late to the chase.
Conversely, if the seal had a perfectly cylindrical whisker, vortex shedding and turbulence would cause the whiskers to wave around as it swims, impeding the seal’s ability to sense turbulence from its fleeing food. Motion from the prey is picked up by the whiskers and sensed by nerves in the seal’s cheeks. Depending on the magnitude of the turbulence response and the amount of whiskers that are affected, the seal can detect the size of its prey and sense its path.
Watch the vortex shedding from a cylindrical rod moving through a fluid in the video below:
When thinking about sensor designs, the team created a wavy-edge transducer with an elliptical cross-section similar to the harbor seal’s whisker, but on a large scale. This is intended to minimize noise generated by the actuator’s own motion through the water. During testing of sensor designs, researcher Heather Beem attached the wavy rod to a motion-frequency sensor and set it up on a linear rail at a fixed distance behind a cylinder. Then, the two pieces were moved along the rail.
The cylinder naturally generated eddies as it traveled through a bath of water. This turbulence was picked up by the wavy transducer, causing it to move at an amplified frequency with a response time that depends on its distance from the cylinder (since turbulence has to travel through the water before it gets to the transducer). The signal was amplified with minimal noise from any motion of the wavy rod.