Abstract
Nature tackled the monitoring of surrounding fluid motion problem in many different, yet fundamentally similar, forms. Many terrestrial and aquatic species have developed hair and whisker sensory systems each specially suited to its environment and designed to observe the fluid motion surrounding the animal. Such an ability to passively monitor wakes of objects is of vital importance to surveillance and tracking in commercial, military, and scientific fields. This work is multidisciplinary combining efforts in the fields of biology, material science, electronics, and engineering to develop a whisker-like fluid motion sensor for tracking hydrodynamic wakes. The sensor inspiration comes primarily from the trichobothria of a wandering spider, Cupiennius salei, and vibrissae of harbor seals, Phoca vitulina.
The main contributions of this work are: (1) the development of a kinematic viscoelastic model to describe trichobothria motion (2) experimental study of pinniped vibrissae to determine material properties and response to free and obstructed flows (3) evaluation of a microelectromechanical haircell sensor (4) development of a bio-inspired whisker-like sensor for wake tracking and (5) development and testing of a Wake Information Detection and Tracking System (WIDTS).
A kinematic viscoelastic model is developed for the trichobothria of Cupiennius salei through force deflection tests. Force testing on trichobothria revealed the hair remains rigid under the loads and is supported by a membrane that provides the restoring and damping parameters of the kinematic system. A three parameter viscoelastic solid is in good agreement with the experimental results for all loading conditions of the hairs.
Tests conducted on whiskers of elephant seals, Mirounga angustirostris, harbor seals, Phoca vitulina, and California sea lions, Zalophus californianus, show that all three are underdamped with natural frequencies under 100 Hz. Force testing to examine the vibrissae material properties reveal the harbor seal whiskers to be the least stiff. Comparing the whisker's responses to flows, all three species demonstrate matching trends indicating at lower flow speeds the diameter perpendicular to the oncoming flow plays a larger in the vibrissae motion than the undulatory morphology. Both the smooth and undulated vibrissae from the California sea lion and harbor seal show similar ranges in frequency and amplitude when in an unobstructed flow.
Analytical modeling of a haircell sensor show the sensor was limited to accurately capture fluid forces in a single direction and proved fragile. For wake tracking, placing the haircell sensor on the surface of a moving body would likely generate unwanted noise caused by the boundary layer flow blocking important flow information.
A whisker-like fluid motion sensor is developed pulling from the spider trichobothria's membrane sensor and the sizing criteria of vibrissae. Exciting the sensor with a sinusoidal signal and measuring the change in output voltage across a load resistor provide measurements of the sensor deflection in two orthogonal directions. The design proved to be robust and is able to monitor fluid speeds and directions and well as capture the frequency shedding of cylinder wakes.
Lastly, the sensor is packaged in to arrays and a WIDTS system was developed for deployment on a trained harbor seal tracking a remote controlled submersible. Testing and numerical modeling show the WIDTS has a slight effect reducing the amplitude of wake signals reaching the seal, but the frequency information remained intact and well above the seal's threshold for capturing fluid motion. Preliminary field trials show the sensors are capable of detecting the sub's wake and indicate the side of the WIDTS the signal originated from.
