Feasibility Study of a Density-Difference-Engine Underwater Glider

The final goal of this research is to study the feasibility of a ‘density-difference-engine underwater glider (DDE-UG)’, which makes the fullest possible use of the physical characteristics of pycnocline in the ocean. The ocean covers over 70% of the earth’s surface, and it has a great influence on global climate changes and ocean ecosystems. It is, thus, a matter of great significance to comprehend changes of the ocean environment and ecosystem. Japanese Ministry of the Environment has concluded in the ‘Marine Biodiversity Conservation Strategy’ in 2011 that it is necessary to realise a wide-range and long-term monitoring of ocean environments and ecosystems. The authors would like to propose a wide-range and long-term observation by using multiple autonomous underwater gliders, or ‘swarm-intelligent underwater robot system,’ as it is called. In recent years, various kinds of autonomous underwater vehicles (AUVs) and remotely operated vehicles (ROVs) are actively involved in the front lines of ocean researches. These underwater vehicles are usually operated alone or in a small group, and thus it is difficult to grasp ocean information over a wide area. Underwater glider is one of the most useful underwater vehicles. Typical underwater has no propulsive machinery such as thrustor, and it has a buoyancy control device and a kind of centre-of-mass shifting device. The authors have been developing an ocean-going underwater glider with independently controllable main wings, named ‘SOARER’, which can control its angle of incidence of both main wings independently. They demonstrated this enables to realise high performance of motion. The authors are also developing an ocean-going solar-powered underwater glider, named ‘Tonai60,’ so as to realise long-term operation by using photovoltaic power generation system. Besides sunlight, natural energy such as wind force, tidal energy and wave energy attracts our attention. This paper deals with the concept and initial design of such an underwater glider with a ‘density-difference-engine.’ It is well known that there often appears thermocline and/or pycnocline in the ocean, as physical characteristics. Cartesian diver, which demonstrates the principle of buoyancy, is one of famous educational materials in a scientific experiment. Popular Cartesian diver repeats diving and surfacing in an enclosed PET bottle by changing the pressure of water. Cartesian diver can be realised in a tank with density gradient too. An idea of controlling the attitude of underwater glider in accordance with depth enables to realise underwater cruising without any propulsive machinery. Test-bed vehicles of such an underwater glider with density-difference engine were developed, and tank test was conducted at a small-scale acrylic tank. This demonstrates possibilities of a DDE-UG.