Underwater propulsion

Navigation of unmanned underwater vehicles in coastal zones, tight spaces and close to structures such as ports, ship hulls and pipelines remains a difficult challenge. Currently Autonomous Underwater Vehicles (AUVs) use a variety of techniques for motion control, including single thrusters with diving planes or hydrofoils, robotic wrists, or a moving mass. However, these techniques provide limited maneuverability. The objective of this work was to understand the mechanics of elongated fin propulsion for swimming and motion control of underwater vehicles. This bio-inspired propulsion is used by several fishes that swim by undulating a thin and elongated median fin that allow them to perform forward and directional maneuvers. In the first chapter we present the literature review as well as the mathematical formulation using thrust vectoring approach to achieve forward and turning maneuvers. In the second chapter, we used a robotic vessel with elongated fin propulsion to determine the thrust scaling and efficiency. Using precise force and swimming kinematics measurements with the robotic vessel, the thrust generated by the undulating fin was found to scale with the square of the relative velocity between the free streaming flow and the wave speed. In addition, a hydrodynamic efficiency is presented based on propulsive force measurements and a model on the power required to oscillate the fin laterally.

Designing a propeller for optimum performance on a human powered underwater vehicle presents a significant engineering challenge. The propeller must be highly efficient to utilize the inherently low power output of a human. Also, the propeller must be correctly matched to the maximum sustainable torque of the propulsor. This thesis experimentally investigates a minimum induced loss propeller design program and its application to a human powered underwater vehicle. The design program is based on the vortex theory of propellers. The work includes experimental measurements of the velocity and rotational rate of three propellers designed with the minimum induced loss propeller design program. This positively verifies the output of the design algorithm. Also, the research, through the use of an underwater ergometer, determines the maximum power and torque sustainable by a human pedaling underwater. Final results of the research show that the design algorithm overestimates the blade section angles by 25% because the design program neglects the influence of the wake of the vehicle.

An experimental investigation of an oscillating hydrofoil propulsion system working behind a model of a small submersible was conducted. Tests were carried out both for a single foil and for systems of two foils. The tests with two foils considered various possible interactions between the foils. In order to understand the influence of trailing vortices of the foils on efficiency of the propulsion system, the two foils were tested for three different combinations of their relative wing spans. All of the above tests were carried out for two types of foil motion, one in which the pitch distribution was prescribed and the other in which the pitch depended on the motion characteristics.

Some Autonomous Underwater Vehicles have recently been designed to mimic the locomotion of underwater animals. A new way of propulsion which uses Oscillating Fin Thrusters (OFTs) has been implemented on the AUV Morpheus, with the Nektor module. In particular, first a low level adaptive controller has been developed with the purpose of studying the characteristics of the OFT. Then, a new vehicle using Morpheus' base has been built in order to implement this module and test it. This required for the Lonworks network to be interfaced with QNXnet to create a multi communication protocol vehicle. Concerning the high level control, some proportional controllers and a 6-degree of freedom adaptive controller have been implemented and tested on the new vehicle. The results from these tests have shown that the Nektor module is suitable for the Morpheus, providing high-maneuverability features unavailable when using more standard propulsion systems.