large angles (i.e., AOA=50?-70?). The measured drag coefficient was in the range of for flume tank experiment, and was for flume tank simulation, while the values were for wind tunnel experiment and for wind tunnel simulation.

Fig.3 Drag coefficient of the otter board in relation to the angle of attack.

3.3 Lift-to-Drag Ratio of the Otter Board

Fig.4 shows the lift-to-drag ratio (K) of the otter board in relation to AOA. It showed the same changing tendency between experiment results and simulation results,i.e., the lift-to-drag ratio increased and then decreased with the increasing AOA. When AOA was less than 20?,the K in flume tank experiment (simulation) was larger than that in wind tunnel experiment (simulation). On the contrary, when the AOA exceeded 20?, the K was lower in flume tank compared with that in wind tunnel. The measured maximum lift-to-drag ratio in flume tank experiment was KMAX=3.640 with the AOA of 20?, and the values were KMAX=3.975 (the simulated error was 9.20%) with the same AOA in flume tank simulation. While for the wind tunnel, the values were KMAX=3.753 with the AOA of 20? in experiment results and KMAX=3.769 (the simulated error was 0.43%) with the same AOA in simulation.

Fig.4 Lift-to-drag ratio of the otter board in relation to the angle of attack.

3.4 Center-of-Pressure Coefficients of the Otter Board

The center-of-pressure coefficients (Cpland Cpc) related to AOA is shown in Fig.5. As only the moment of Mzwas measured, we just obtained the curves of center-of-pres-

sure in the chord direction (Cpc) for experiment data. It showed that the center-of-pressure coefficients decreased with the increasing AOA (20?-60?) and then increased(60?-70?). Flume tank experiment (simulation) showed that the center-of pressure point moved to the leading edge with the AOA and then moved to trailing edge with an increasing AOA. The measured Cpcwas in the range of in flume tank experiment, and Cpcwas , Cplwas in flume tank simulation. While for the wind tunnel, the Cpcwas in experiment results and Cpcws , Cplwas in simulation results; the center-of pressure point moved to the upward and leading edge with the increasing AOA and then moved reversely.

Fig.5 The center-of-pressure coefficients (Cpland Cpc) in relation to the angle of attack (top, the flume tank; bottom,the wind tunnel).

3.5 Flow Distribution Around the Otter Board

According to the above results, simulation results showed a good agreement with the experiment results; the flow distribution around the otter board was analyzed based on the simulation results. The flow streamlines from a horizontal plane cutting through the center section of the otter board are shown in Fig.6. The vorticity diagram is illustrated in Fig.7.

The streamlines in the center section of the otter board(Fig.6) were similar between flume tank simulation and wind tunnel simulation. It showed no separation at AOA of 35? and the flow was smooth. At AOA of 45?, a separation was confirmed on the leading edge of the otter board and eddies were observed on the lee side of the otter board. As the AOA increased, more eddies developed and began to shed, and streamline on both sides of the otter board became more concentrated.

The vorticity diagrams (Fig.7) showed big differences between flume tank simulation and wind tunnel simulation, i.e., the vorticity around the otter board of the wind tunnel simulation was much higher than that of flume tank simulation. Flume tank simulation results showed that the region of wing-tip vortex increased at AOA of 35?-45?. The vortex cores moved inward at AOA of 45?-55? and began to interact with each other at AOA of 65?. Whereas wind tunnel simulation just obtained wingtip vortex at AOA of 35?, the vortex cores moved inward and began to combined at AOA of 45?, and the vortex cores almost combined into a larger vortex at AOA of 65?.

4 Discussion

Comparing with conventional rectangular otter board(CL-MAX=1.07, KMAX=1.50) (Xu et al., 2016), V type otter board (CL-MAX=1.08, KMAX=1.86) (Li et al., 2013) and vertical V type cambered otter board (CL-MAX=1.40, KMAX=2.917) (Wang et al., 2004), the double defector rectangular cambered otter board has a better hydrodynamic performance (CL-MAX=2.079, KMAX=3.640). The streamline around the otter board (Fig.6) showed that stall appeared at AOA of 45?, which was higher than that of both biplane-type otter board (Takahashi et al., 2015) and hyper-lift otter board (Shen et al., 2015). It revealed that the structure of this otter board can delay the flow separation appearing. As a result, the critical AOA of this otter board was large. When the AOA was lower than critical AOA,the lift coefficient increased with an increase in the AOA until stall appeared. Therefore the maximum lift coefficient of the double defector rectangular cambered otter board was much larger than that of conventional otter boards. As the parameters of the otter board (AR, camber ratio, etc.) were designed based on experience, therefore,it needs to be optimized to improve the hydrodynamic performance of the otter board in the further study.

文章来源：《水动力学研究与进展》 网址: http://www.sdlxyjyjzzz.cn/qikandaodu/2021/0128/483.html

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