Effect of deflector angle on hydrodynamic performances of a double-slotted cambered otter-board

. The effect of deflector angle on hydrodynamic performances of a double-slotted cambered otter-board is investigated using engineering models in a wind tunnel. Three different angle combinations (35°-30°, 30°-25°and 25°-20°) are evaluated at a wind speed of 28 m/s. Parameters measured included: drag coefficient C x , lift coefficient C y , pitch moment coefficient C m , center of pressure coefficient C p , over a range of angle of attack (0° to 70°). These coefficients are used in analyzing the differences in the performance among the three otter-board models. Results shows that the maximum lift coefficient C y of the otter-board model with the angle combination (35°-30°) of two deflectors is the highest (2.160 at  =55°). The maximum C y /C x of the otter-board with the angle combination (25°-20°) is the highest (3.842 at  =2.5°). A comparative analysis of C m and C p shows that the stability of otter-board model with the angle combination (25°-20°) is better in pitch, and the stability of otter-board model with the angle combination (30°-25°) is better in roll. The findings of this study can offer useful reference data for the structural optimization of otter-boards


Introduction
Trawl doors are an important part of fishing gear for the spreading of a trawl. The merits of the hydrodynamic performance of otter-boards can be measured on the basis of the lift coefficient of the trawl door, the drag coefficient of the trawl door, and the pitching moment coefficient of the trawl door [1]. Optimizing the structure of otter-boards may improve the hydrodynamic performance of the otterboard and reduce the energy consumption of fishing vessels [2][3]. Extensive studies on the hydrodynamic performance of otter-boards have been conducted in the United States, Japan, Norway, and other countries [4][5][6][7][8][9]. In China, researchers have studied the relevant hydrodynamic performance of otter-boards since the early 1980s, including the hydrodynamic performance and optimization of different otter-boards with various structure types. The development of offshore trawler fleets has increased globally in recent decades, raising the demand for improved otter-board designs. Accordingly, improvements in the hydrodynamic performance of otter-boards has become a major research interest. Some studies have shown that the slit in otter-boards can reduce the resistance and improve the stability of otter-boards [10][11][12][13][14][15]. The following study investigates the importance of the angle of the deflector within the otter-board. We describe an experiment using model otter-boards (n=3 designs) in a wind tunnel in which we measured various hydrodynamic coefficients for a range of angles of attack. The results are relevant as a reference for the study of the structural parameters of the main-panel of otter-boards.

Design and manufacture of otter-board model
The otter-boards evaluated in this study are double-slit curved structures comprising two deflectors and one mainpanel ( figure 1). This simplified design is selected in order to meet objectives and requirements of the study. Only the angle of two deflectors are modified.

Figure 2
Three otter-board models evaluated in this study.

Test facility
The wind tunnel used for this experiment is the NH-2 wind tunnel located at Nanjing

Lift coefficient
Air density =1.225 kg/m 3 in above formula; S is otterboard area (m 2 ); L is the otter-board chord length (m).
All the experimental data have been carried out the stent disturbance correction which is completed by the method of taking out light pole directly.

Drag coefficient and lift coefficient
Data from the experiment includes the drag coefficient Cx, the lift coefficient Cy, the pitch moment coefficient Cm, and the center of pressure coefficient Cp. The lift-drag ratio is computed (Cy / Cx), which is an important factor for determining the merits of the hydrodynamic performance of otter-boards. An otter-board with excellent hydrodynamic properties can achieve higher lift-drag ratio and improved stability; such performance can be analyzed by comparing the pitching moment coefficient Cm stencil and the center of pressure coefficient Cp . The test data are divided into groups, yielding Cx-α, Cy-α and Cy / Cx-α graphs shown in figure 5. These graphs are used for analyzing the differences in the hydrodynamic properties of the three otter-board models.     The stability in roll of otter-board can be measured according to the center of pressure coefficient Cp; and the way of comparison is analyzing the coefficient of variation in Cp within the range of angle approximately 5° of the angle of attack corresponding to the maximum lift-drag ratio; a smaller coefficient results in the improved stability [17]. The calculated data is shown in table 2. The minimum variation coefficient of Cp is 5.49%; this value also means that the stability of No. 2 otter-board model is better in roll of otter-board.

Conclusion
Test analysis shows that the angle combination of two deflectors has a point for equilibrating the hydrodynamic performances of otter-board, results from this experiment suggest that a better angle combination of two deflectors should be 30° and 25°, thereby yielding has a higher lift coefficient and higher lift-drag ratio on average, and the stability in roll and pitch of otter-board is also better comparatively. The data and conclusions of this study can provide a reference for the design of otter-board.