The marine environment in the sea is facing problems such as eutrophication, acidification, and oxygen deficiency. The need is growing for ocean sensing to benefit marine safety, environmental monitoring and climate research. An unmanned autonomous vehicle that can assist surveying the sea by acquiring long-term placed bound measurements, reporting pollution, hydrographic and meteorological data could be a helpful and cheap alternative to a commercial research vessel. The researchers are now developing an autonomous sailing drone, which will be used as an experimental platform for autonomous data acquisition in the sea. The application drives requirements on the vehicle, where it needs to have robustness, reliability, and energy efficiency. The requirement is to develop a vehicle that can survive in any weather conditions all year under free conditions. Utilization of wing sails in autonomous sailing drones has been an object for research in maritime robotics for over 15 years. Researches on wing sails have been conducted by many researchers. However, difficulties in designing wing sails for autonomous sailing drone has been identified. Because of working in the long-term autonomous oceanography, the rigid sail's simple design and good aerodynamic performance are required.
The present work aims to suggest and design a curvy twin sail for sailing and make an evaluation of the curvy twin sail performance. For curvy twin sail design, the curvy twin sail designed parameters is established. The curvy twin sail is designed with different camber value (ranging from 5% to 13%), and the spacing (ranging from 0.05C to 0.25C). The best curvy twin sail design with appropriate camber and spacing is identified.
Furthermore, to serve time experiment and optimize the curvy twin sail design, an in-house velocity prediction is developed to suitable with curvy twin sail and to evaluate the performance characteristics. Relying on that, the best curvy twin sail design is identified with optimal camber and spacing values for autonomous sailing drone. Moreover, a velocity prediction procedure is established. These results are expected to contribute to the current knowledge on developing the sea autonomous vehicles.
The camber optimization results have shown that the lifts generated by port sail and starboard sail are counter. While the port sail generated a positive lift, the starboard sail made a negative lift. The flow pattern formation around the curvy twin sail is takes into consideration. The full flow separation is inevitable at the starboard sail's upper area, so the adverse lift mainly depends on the flow velocity at the area between two sails. For the port sail, the flow separation can occur at the leading edge and trailing edge. It leads to the favorable lift decreases dramatically and unstable. The optimum driving force coefficient of curvy twin sail is found at the angle of attack 15˚ in case of camber 7%.
Contemporarily, the spacing effect is considered and investigated. The augmentation of the spacing creates the local low pressure at the area between port and starboard sails. This increase is not mean that the pressure along the upper area of the port sail grows. In case of spacing S = 0.25C, at angle of attack above 25˚, the harmful flow separation appears at the port sail's upper area. However, the curvy twin sail is recommended to work at angle of attack under 15˚. The best spacing still is selected with S = 0.25C.
In the velocity prediction program, we are interested in the dynamical behavior, the effect of sail designs, the precise prediction of the actual reachable speeds is of significant interest. The algorithm and process for the prediction of sailing drone speed were established. The sailing drone velocity prediction model was validated by comparison with previous experimental results and achieved an acceptable level. It is also able to help evaluate the sail and hull configuration.
The hydrodynamic results indicated that the designed sailing drone's speed should be limited to 2.2 m/s. Although the maximum lift coefficient of curvy twin sail is recorded at the angle of attack 25˚, the optimization angle of attack for curvy twin sail is found at 15˚. The curvy twin sail can improve the sailing drone speed in downwind condition at true wind angle above 40˚. In contrast, it does not work effectively compared to wing sail in upwind condition at true wind angle from 20˚ to 40˚.