Journal of Marine Science and Engineering Article Numerical Investigation of a Two-Element Wingsail for Ship Auxiliary Propulsion Chen Li 1,2,3,*, Hongming Wang 2,3 and Peiting Sun 1 1 Marine Engineering College, Dalian Maritime University, Dalian 116026, China; [email protected] 2 College of Marine, Electrical and Intelligent Engineering, Jiangsu Maritime Institute, Nanjing 211170, China; [email protected] 3 Jiangsu Ship Energy-Saving Engineering Technology Center, Nanjing 211170, China * Correspondence: [email protected] Received: 16 March 2020; Accepted: 1 May 2020; Published: 9 May 2020 Abstract: The rigid wingsail is a new type of propulsion equipment which greatly improves the performance of the sailboat under the conditions of upwind and downwind. However, such sail-assisted devices are not common in large ships because the multi-element wingsail is sensitive to changes in upstream flow, making them difficult to operate. This problem shows the need for aerodynamic study of wingsails. A model of two-element wingsail is established and simulated by the steady and unsteady RANS approach with the k-! SST turbulence model and compared with the known experimental data to ensure the accuracy of the numerical simulation. Then, some key design and structural parameters (camber, the rotating axis position of the flap, angle of attack, flap thickness) are used to characterize the aerodynamic characteristics of the wingsail. The results show that the position of the rotating shaft of the flap has little influence on the lift coefficient at low camber. When stall occurs, the lift coefficient first increases and then decreases as the flap axis moves backward, which also delays the stall angle at a low camber. At the high camber of AOA = 6◦, the lift coefficient always increases with the increase of the rotating axis position of the flap; especially between 85% and 95%, the lift coefficient increases suddenly, which is caused by the disappearance of large-scale flow separation on the suction surface of the flap. It reflects the nonlinear coupling effect between camber of wingsail and the rotating axis position of the flap Keywords: wingsail; aerodynamics; numerical simulation 1. Introduction The rigid wingsail is a common auxiliary propulsion device. Due to its characteristics of environmental protection and energy saving, the rigid wingsail has been applied to various types of ships. In 1980, two rectangular rigid wingsails with a total sail area of 194.4 m2 on “Shin Aitoku Maru” were installed in Japan, which was the world’s first modern sail-assisted commercial tanker. After four years of actual sailing, oil tankers can save 8.5% a year compared with conventional ships [1]. Although the international oil price fluctuates greatly, the research on sail-assisted vessels varies from country to country. In 2018, China Shipbuilding Group delivered the world’s first Very Large Crude Carrier (VLCC), “Kai Li”, with wingsails (Figure1). The trial results show that the energy-saving efficiency of the VLCC is obvious [2]. At the same time, the wingsails have also been rapidly developed in the field of unpowered navigation. Particularly in 2010, BMW Oracle was powered by a 60-meterhigh, multi-element wingsail, which won the 33rd America’s Cup. The flexible size and excellent performance of the wingsail has rekindled the interest of the shipping industry [3]. However, due to the phenomenon of flow separation or stall on the surface of the wingsail, the propulsion performance of the wingsail will be deteriorated, which will seriously affect the stability J. Mar. Sci. Eng. 2020, 8, 333; doi:10.3390/jmse8050333 www.mdpi.com/journal/jmse J. Mar. Sci. Eng. 2019, 7, x FOR PEER REVIEW 2 of 16 J. Mar. Sci. Eng. 2019, 7, x FOR PEER REVIEW 2 of 16 ρ The density of the air [kg/m3] ρz The densityheight of of wingsail the air [kg/m in the3] vertical direction[m] hz TheWingsail height height of wingsail [m] in the vertical direction[m] Lh WingsailLift force[N] height [m] DL LiftDrag force[N] force [N] Dv DragThe velocity force [N] of inflow [m/s] Xvr The positionvelocity of inflowflap rotation [m/s] axis in the direction of Xr Thethe wing position chord of flap[-] rotation axis in the direction of the wing chord [-] 1. Introduction 1. IntroductionThe rigid wingsail is a common auxiliary propulsion device. Due to its characteristics of environmentalThe rigid protectionwingsail is and a commonenergy saving, auxiliary the rigipropulsiond wingsail device. has been Due applied to its to characteristics various types of 2 environmentalships. 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[14] and Thisleading-edge idea has tubercles been applied [15], inetc. the American Cup Sailing Competition with great success. The rigid wingsail consists of two or three symmetrical wings. There is a gap between them to control the wingsail camber on starboard tack and port tack (Figure3), so as to improve the propulsion performance and delay stall. Many scholars have also carried out research on theJ. Mar. aerodynamic Sci. Eng. 2019, characteristics.7, x FOR PEER REVIEW 4 of 16 Figure 3. The geometry and simplified simplified configurationconfiguration of the two-element wingsail. In 1996, Daniel [11] designed a high-performance, three-element wingsail. The experimental Table 1. Parameterization of wingsail. results show that the maximum thrust coefficient of the three-element wingsail is increased by 68%, the stall angle is delayed between 4◦ and 6◦, andc the0.35m thrust in the whole operation area is also improved.