Effect of bevel angle of three-layer parallel plate on hydrodynamic performances of otter-board

The effect of bevel angle of three-layer parallel plate of otter-board on hydrodynamic performances is investigated by model wind tunnel test. Four otter-board models are designed with four different bevel angles (8°, 10°, 12° and 14°) , and tested under the wind speed 28 m / s in wind tunnel. Model experiment is conducted to obtain the drag coefficients C x , the lift coefficient C y , the pitch moment coefficient C m , the center of pressure coefficient C p and calculated the lift-drag ratio C y /C x . The result shows that the maximum lift coefficient and the maximum lift-drag ratio of the otter-board model with a bevel angle 12 degree is higher, there is 2.598(a=65°) and 2.607(a=37.5°); For comparison in stability of otter-board, the stability of the otter-board model with the bevel angle 8 degree is better by the comparative analysis of C m and C p , the absolute value of C m is 0.174 and the minimum variation coefficient of C p is 4.37%. The results can offer reference for the structural optimization design of trawl otter-board.


Introduction
Trawl doors is an important member of fishing gear for spread of trawl. The merits of otter-board hydrodynamic performance can be measured by the lift coefficient of the trawl door, the drag coefficient of the trawl door and pitching moment coefficient of the trawl door [1]. By optimizing the structure of otter-board may improve hydrodynamic performance of the otter-board, and reducing the energy consumption of fishing vessels [2][3]. Currently, extensive studies on otter-board hydrodynamic performance had been conducted in the US, Japan, Norway and other countries [4][5][6][7][8][9]; In China, researchers have studied the relevant hydrodynamic performance of otterboard early since 1980s, and the series of optimization studies on otter-board structure is still necessary [10][11][12][13][14][15]. With the development of offshore fishing, higher requirements on the otter-board design is put forward, hydrodynamic performance improvement of otter-board will be the focus of future research. This article is part of the series of tests, which analyze the hydrodynamic performance of otter-board with the bevel angle of threelayer parallel plate changing.

Design and manufacture of otter-board model
Test model of otter-board with three parallel plates is designed in different bevel angle of the three parallel plates.
Ensuring the premise of other parameters in common, the design of model structure has been simplified based on the test objectives and requirements, while, only the bevel angle of three parallel plates changes. The structure and parameters of the model are illustrated in figure 1.

Test facility
Wind tunnel for this test is Nanjing University of Aeronautics and Astronautics NH-2 wind tunnel, the tunnel is closed reflux wind tunnel with double string test section, this experiment is carried out in a small test section, the main technical performance of the small test section: 6 m (length) 3 m (width) 2.5 m (height), import crosssectional area 7.18 m 2 , the maximum wind speed is 90m / s, the minimum steady wind speed is 5 m / s.

The dynamometer test uses six-component mechanical tower -Balances to measure, test model installation is shown in figure 3.
Test data acquisition and processing system used is made up by the pre-amplifier and four networked computer system.   Three components: lift Y, drag X, pitching moment M (around the fulcrum), while the distance from the center of pressure to the front-end otter-board d=e-(M/N) [12], (N is the normal force).

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. Cy / Cx is the lift-drag ratio, which is an important factor to determine the merits of otter-board hydrodynamic performance. Otter-board with excellent hydrodynamic properties will get higher liftdrag ratio and better stability, what can be analyzed by comparing the pitching moment coefficient Cm stencil and the center of pressure coefficient Cp [1][2][3]. The test data is divided and made Cx-α, Cy-α and Cy / Cx-α graph shown in figure 5 for analyzing the differences of hydrodynamic properties of four otter-board models clearly.

Lift-drag ratio
The lift-drag ratio can be more intuitive to analysis the hydrodynamic performance of otter-board, and the otterboard with higher lift-drag ratio gets the better operational efficiency. In figure 5   The stability of otter-board can be measured by the center of pressure coefficient Cp, and it is generally compared by analyzing the coefficient of variation of Cp within the range of angle around 5 ° of the angle of attack corresponding to the maximum lift-drag ratio, the smaller the coefficient, the better the stencil stability [17]. Data Calculated is shown in table 2, the minimum variation coefficient of Cp is 4.37%, and it also means the stability of No.1 otter-board model is better.

Conclusion
The test study concludes that designing the bevel angle of otter-board plates appropriately can improve the lift coefficient of the otter-board. When the bevel angel of three parallel plates of otter-board is designed in 8°or 12°, the otter-board has the higher maximum lift coefficient. It is also needed to pay attention to adjust the lift -drag ratio and stability of the otter-board, test shows that the maximum lift-drag ratio of otter-board model with the bevel angle of 12°is higher than the other three models, and the stability of the otter-board model with the bevel angle of 8°is better. Data and conclusions of this study can provide a reference for the design of otter-board.