Effect of stiffened structure on hydrodynamic performance

 of V-shaped otter board

Abstract: As an important accessory of trawl fishing gears, the hydrodynamic performance of otter board has a very important impact on the operability and catch efficiency of trawl fishing gears. The Vtype otter board is symmetrical in front and back and can be exchanged between left and right, it has the advantages of convenient operation, good stability and easy automatic recovery after falling down. Therefore, Vtype otter board is widely used in offshore trawlers in China. In recent years, to improve the structural strength and stability of Vtype otter board, several largescale stiffeners have been added to the traditional structure in the design and manufacture of Vtype otter board, but there has been no detailed corresponding theoretical research on the influence of these stiffeners on the hydrodynamic performance of Vtype otter board. This study takes the stiffened Vtype otter board configured by Chinese offshore trawlers as the research object, and uses computational fluid dynamics (CFD) software to calculate and analyze its hydrodynamic performance. Through the comparative analysis of the hydrodynamic performance of stiffened Vtype otter board with the traditional unstiffened Vtype otter board, the influence mechanism of the stiffener on the hydrodynamic performance of the Vtype otter board is studied, and the validity of the numerical calculation results is verified by dynamic flume test. On this basis, by changing the aspect ratio, plate folding angle and other design parameters, its influence on the hydrodynamic performance of stiffened Vtype otter board is studied to provide reference for the structural optimization design of new Vtype otter board. Through comparison, the numerical model of stiffened Vtype otter board established in this study is highly effective, with the relative error of lift coefficient of 13.5%27.5%, the average relative error of 24.8%, the relative error of drag coefficient of 4.7%20.5%, and the average relative error of 16.8%. The shape and value of lift drag ratio curves agree well with each other, and the average relative error is 9.0%. The influence of design parameters such as aspect ratio, plate folding angle and stiffener size on the hydrodynamic performance of stiffened Vtype otter board is studied. Results show that the addition of several largescale stiffeners to the Vtype otter board not only increases the structural strength and stability, but also changes the distribution of the surrounding flow field when the otter board works in water. Compared with the traditional unstiffened Vtype otter board, the flow on the back flow surface of stiffened Vtype otter board is more complex and chaotic, and the number of vortices increases. At a small angle of attack (below 40 °), the pressure distribution on the upstream and downstream surfaces of the otter board is not significantly affected by the stiffened structure, which shows that there is little difference between the rise coefficient and resistance coefficient of stiffened Vtype otter board and unstiffened Vtype otter board. However, at large angles of attack (above 40 °), the pressure difference between the upstream surface and the downstream surface of stiffened Vtype otter board decreases significantly, and the rise coefficient and resistance coefficient of the otter board decreases significantly. Since the working angle of attack of Vtype otter board usually does not exceed 40 ° in actual operation, from the perspective of expansion effect, the addition of stiffened structure on the back of Vtype otter board will not have a significant impact on its working performance. The existence of stiffeners reduces the lift drag ratio of Vtype otter board by 1.20 under the optimal working angle of attack (about 10 °). After adding stiffeners, the maximum lift drag ratio of Vtype otter board will still increase gradually with the increase of aspect ratio, but the maximum lift drag ratio of Vtype otter board decreases obviously under each aspect ratio. The changing trend of lift coefficient, drag coefficient and lift drag ratio curve will not be changed by the change of plate folding angle of stiffened Vtype otter board, but the maximum lift drag ratio will increase significantly with the increase of plate folding angle. Moreover, with the increase of the plate angle, the height of the stiffener will gradually decrease due to the change of the structure, and its influence on the flow field distribution on the back flow surface of the otter board will gradually decrease. When the space of stiffeners is small (less than 430 mm), it has little impact on the hydrodynamic performance of the otter board. When the space of stiffeners is large, the maximum lift drag ratio of Vtype otter board will be significantly reduced, and the increase of its size will also reduce the lift drag ratio of otter board to a certain extent. The increase of stiffener size will also reduce the lift drag ratio of otter board to a certain extent, but the reduction range is not obvious. Therefore, when designing the reinforced Vtype otter board structure, the stiffeners should be arranged as close to the middle of the mesh plate as possible, and the space of stiffeners should not be too large. At the same time, it can be considered to appropriately increase the width of the stiffeners, so as to further improve the structural strength and stability of the otter board on the premise of ensuring high hydrodynamic performance. To sum up, reducing the space and size of stiffeners, increasing the aspect ratio and reducing the plate folding angle can improve the hydrodynamic performance of stiffened Vtype otter board. Considering the actual working environment, the Vtype otter board is the bottom trawl otter board located on the seabed when working, it is not convenient to adjust its posture at any time, so it needs to have good stability and sufficient structural strength. The stability of stiffened Vtype otter board is negatively related to its aspect ratio and plate surface folding angle, and the structural strength is also closely related to the space and size of stiffeners. Therefore, when designing the structure of stiffened Vtype otter board, it is necessary to improve and optimize it on the basis of comprehensively considering these factors.