Passive wake detection using seal whisker-inspired sensing

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2015This thesis is motivated by biological experiments, which display the harbor seal's ability to track the wake of an object several seconds after it swam by. In this work, I elucidate the basic fluid mechanisms that seals may employ to accomplish this detection. Key are the unique vortex-induced vibration (VIV) properties resulting from the geometry of the harbor seal whisker. First, force measurements and flow visualizations on a rigid whisker model undergoing 1-D imposed oscillations show that the geometry passively reduces VIV (factor of >10), despite contributions from effective added mass and damping. This suggests that harbor seal whiskers would detect details of the oncoming flow with reduced background “noise". Next, a biomimetic whisker sensor is designed by mounting the model on a four-armed flexure, allowing it to freely vibrate, and using strain gauges to measure deflections at the whisker base. Finally, this whisker device is towed behind an upstream cylinder with larger diameter. In the wake, the whisker oscillates with large amplitude and at the Strouhal frequency of the upstream cylinder. A slaloming motion among the wake vortices drives this interaction, and it enables detection of the upstream wake.NDSEG Fellowship, MIT Presidential Fellowship, Link Foundation Fellowship, and the National Research Foundation Singapore through the Singapore MIT Alliance for Research and Technology's Center for Environmental Sensing and Modeling interdisciplinary research program. Support was also provided by the Office of Naval Research through grant N00014-13-1-0059, monitored by Dr. Thomas Swean, Jr., the MIT Sea Grant Program, and the William I. Koch Chair in Marine Technology