Defence X. Zheng: "Flow-sensing mechanisms and biomimetic potential of seal whiskers"

Promoters: Prof. Ming Cao and Dr. Ajay Kottapalli

Abstract: Seals have arrays of whiskers on their muzzle, which act as mechanosensors for interpreting flow information in their surroundings. Some Phocid seal species have unique undulating surface structures that resemble beads on a string, such as grey (Halichoerus grypus) and harbor seals (Phoca vitulina). The instabilities caused by the separation of the flow from a high-aspect-ratio bluff body usually cause it to vibrate when the oncoming flow is present or when it is being towed in still water. More specifically, a vortex street is formed in the wake of the bluff body due to the flow separation. As a result of this vortex shedding, both sides of the bluff body are subjected to intermittent alternate loads. A bluff body vibrates if its damping is small enough, and the vortex-induced vibration is caused by the reaction generated by shedding vortices. However, by measuring and comparing the vibrations of harbor seal whiskers (with undulations) and California sea lion whiskers (without undulations) using piezoelectric sensors at their bases, the harbor seal whiskers vibrate six times less than the California sea lion whiskers. Based on this phenomenon, the unique undulating surface of seal whiskers suppresses the VIV since both whisker species have similar Reynolds numbers, and the only difference is whisker geometry. Because the seal whisker can suppress VIV, self-motion does not cause significant vibration of the undulating whiskers, which results in a high signal-to-noise ratio for the undulating seal whisker due to its enhanced sensitivity to minute hydrodynamic stimuli caused by passing prey. As a result of the ultrasensitive flow stimulus sensing capability shown in behavioral experiments conducted on the harbor seal (Phoca vitulina), the seals were able to track the hydrodynamic trajectory of swimming fish as far as 180 meters away.

This thesis aimed to study seal whiskers in terms of their form and function. Therefore, investigations on 1) geometric parameters (involving formulated geometric frameworks, the geometry (length and thickness) distribution, and reshaped 3D distributions of seal whiskers), 2) mechanical responses (involving whisker vibrations in the flow and the natural frequency of the seal whisker), and 3) biomimetic potentials (involving the bioinspired fully 3D-printed MEMS cantilever sensor design and the optimized seal whisker structure) of undulating seal whiskers were conducted.

Dissertation