Acoustic Minimization of Ocean Twilight Zone Vehicle, Mesobot

Abstract

The ocean’s twilight zone (OTZ) is one of the most unexplored regions of the Earth’s oceans. The OTZ is defined as the region of the water column between 200 and 1,000 meters in depth. It plays a vital role in the global carbon cycle, pushing carbon from the surface layer into the deep ocean. It has a very diverse population of fauna, known and unknown, that migrate up and down the water column to feed and reproduce. The migration pattern occurs based on the amount of radiated sunlight into the water column. The mid-water column vehicle, Mesobot, was designed to mimic the migration patterns of mesopelagic organisms. Unmanned Underwater Vehicles (UUVs) have become a staple of ocean exploration for years, going where man is not able to. Although much quieter than noise from shipping traffic, the noise radiated from Mesobot could present potential for error in observation, tracking, and sampling. In this thesis, I have analyzed the effect of commutation methods and propeller design on the acoustic noise radiated from a single BlueRobotics T200 thruster. The propeller design choices are a standard three-blade propeller and a three-blade toroidal propeller. The commutation methods analyzed are trapezoidal control and field-oriented control. After analyzing four different alternatives, quantitative evidence was found to recommend using field-oriented control as the commutation scheme to minimize the radiated noise from the thrusters on Mesobot. The radiated noise from the thurster was dominated by motor noise, and no conclusive evidence was found to recommend the three-blade propeller over the toroidal propeller.