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1 Introduction

The otter board, which is designed to maintain the horizontal opening of the trawl net, is a vital component for a trawl system (Niedzwiedz and Hopp, 1998). It requires high lift-to-drag ratio, which is directly related to the fishing efficiency of single trawling. In order to improve the hydrodynamic efficiency of the otter board,many researchers paid attentions to the physical design of the otter board in the past decades. Yamasaki et al. (2007)designed a high-lift V type otter board to improve the otter board used in a semi-pelagic trawl net in Ise-wan Bay; its lift-to-drag ratio was 1.41 times higher than that of the conventional rectangular otter board. Sala et al.(2009) designed a new otter board, the Clarck-Y door, to improve the water flow on the upper part of the otter board to avoid vortices. The flume test results showed a higher efficiency (1.33 times) than the cambered V type otter board. A fundamentally different design of otter board, namely ‘Batwing’, had been proposed by Sterling(2008, 2010). In this design, a flexible sail was utilized to operate at a low AOA, and a seabed-contact shoe aligned with tow direction. The experiment results showed a greater (3 times) efficiency than the flat rectangular otter board (Balash and Sterling, 2014). Hu et al. (2015) designed a ‘High-lift’ otter board, which used the airfoil structures to improve the lift coefficient and the lift-todrag ratio. The double defector rectangular cambered otter board used in the present work was a new design combining the advantages of the above otter boards.

The resistance of otter board accounts for up to 30% of the total-system drag according to Sterling (2000). Therefore, research on the hydrodynamic performance of otter boards has a great significance for energy saving. Kawakami et al. (1953) constructed theoretical equations of otter board and verified them by model tests. Matuda et al.(1990) measured the maximum lift coefficient (1.27) and lift-to-drag ratio (4.03) of vertical V-type otter board by conducting flume tank experiment. Shen et al. (2015)studied the hydrodynamic performance of a hyper-lift otter board by flume experiment. Zhang et al. (2004)studied the relationship between cambered ratio and hydrodynamic characteristics of the cambered V type otter board through wind tunnel experiments. Wang et al.(2016) studied the effect of vane length on hydrodynamic performance of single slotted cambered otter board by wind tunnel experiment. Takahashi et al. (2015) studied the hydrodynamic performance of a biplane-type otter board using computational fluid dynamics (CFD). CFD analysis was in agreement with the results of flume tank experiment. Xu et al. (2016, 2017a, b) optimized the rectangular and V type otter board using numerical simulation, and obtained the optimal parameters of the otter board.

Generally, theoretical analysis, flume tank tests, wind tunnel experiment, and numerical simulation were the main methods of studying the hydrodynamic performance of otter board. The present work studied the hydrodynamic performances of a double defector rectangular cambered otter board using flume tank experiment, wind tunnel experiment and numerical simulation, and analyzed the suitable methods for studying otter boards by comparing the results of three methods.

2 Materials and Methods

2.1 Model Otter Board

The double defector rectangular cambered otter board was made of steel and worked at a speed of 3 -4 kn. Its main dimensions were given as: the wing span l = 2.2 m,the chord c = 2.2 m, the plane area S = 4.84 m2and the aspect ratio AR = 1.

Fig.1 The structure of the model otter board (the coordinate frame on it was used to calculate the center-of-pressure).

Fig.1 shows the sketch of the model otter board. D1and D2are the defectors, P is the main panel, d1and d2are the horizontal intervals between defectors and the main model otter board was designed based on the similarity criterion of Froude’s law. The relationships between the plane area of otter board and the flow velocities are given in the following equations:

where the subscript ‘1’ represents the prototype while the subscript ‘2’ designates the model, v is the velocity, s is the scale ratio and s = 4.4 in the present work. The main parameters of the model otter board are shown in Table 1.

Table 1 Parameters of the model otter boardParameter l(m)Cambered ratio Angle (?)(m)AR s (m2) D1D2P D1D2P c Model 0.5 0.5 1 0.25 12% 12% 12%

2.2 Wind Tunnel Experiment

Wind tunnel experiments were conducted in a wind tunnel of Aeronautics and Astronautics at Nanjing University (testing section: length 6.0 m, width 2.5 m and depth 3.0 m). The stable wind velocity of the tunnel ranged from 5 to 90 m s-1. The tunnel was equipped with a six-component force sensor (range: 0-60 kg; accuracy:0.1%). During the experiments, the wind velocity was set to be 28 m s-1(Re= 0.93×106), and the AOA varied between 0? and 70? at a increment of 5?. The velocity was designed based on the self-modeling region of Reynolds number (i.e., Re= 104-107, Xia et al., 2014), in which the hydrodynamic coefficients are independent of Reynolds number and show almost constant values.

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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