Wenhu Chu, Chunqing Yin, Gng Chen, Horn Li, Xuchng Ye

aCollege of Marine Sciences, Shanghai Ocean University, Shanghai, 201306, China

bNational Engineering Research Center for Oceanic Fisheries, Shanghai, 201306, China

cLaboratory of Sustainable Exploitation of Oceanic Fisheries Resources, Ministry of Education, Shanghai, 201306, China

dCollege of Engineering Science and Technology, Shanghai Ocean University, Shanghai, 201306, China

Keywords:

ABSTRACT As the key structure of buoyancy and dynamic lift generation, the float is one of the most important attachments of the fishing gear in ocean fishery. For the special work environment, the float may deform and move under the hydrodynamic force when it works. At the same time, the motion may cause the impacting action from fluid and structures around. Then the investigation of the mechanical behavior of the float is of important significance for the structural design and performance optimization of the fishing gear, and even for the development of the ocean fishery. The three-dimensional numerical model of marine float working in the flow field was established based on the Computational Fluid Dynamics (CFD) method and the finite element (FEM) method according to the structural form and the mechanical characteristics in operation. The hydrodynamic performance and impact resistance were analyzed, and the response mechanism of float structure under the action of hydrodynamic load and impact load was summarized. It aims at providing basic theory support for the structure design and optimization of marine float and the related fishing gear.

1.Introduction

The float is a vertical expansion device of fishing net. In the process of marine fishery, it completes the vertical expansion of the net by cooperating with the sinker, so as to keep the fishing gear in a certain working shape and reach the desired fishing water layer. The float is widely used in various fishing gear, its type, hydrodynamic performance and adaptability to the marine environment will directly affect the fishing efficiency and economy of fishing gear. The hydrodynamic performance and structural strength are the key to the design and optimization of the float structure.

For a long time, theoretical analysis (Ye, 1997) and experimental research (Chen & Chen, 1994) have been mainly used in the study of mechanical properties of float, while relevant numerical simulation techniques are rarely published. There has been a relatively complete system for theoretical model on hydrodynamic performance of the float(Liu, 2012; Sun, 2009, pp. 203–214; Xu, 2004, pp. 23–27; Zhou & Xu,2018, pp. 24–27) after long-term development and constant modification, which can be preliminarily used as a reference for empirical formulas in engineering applications. However, the float structure is often simplified into a simple spherical or cylindrical structure in the process of theoretical analysis, and the force is also involved in varying degrees of simplification. It is difficult to include complex working conditions such as float structure shape and boundary conditions into the calculation process for effective analysis. Therefore, there are still some limitations in the application of the theoretical model in the performance analysis of float. The test method (Wang et al., 1994; XiaoWang et al.,2010) is widely used for the research on its structural strength, such as the free-fall test on the impact resistance of float structure. While there are also some unsatisfactory points due to the limitation of test equipment and technology.

By contrast, the numerical simulation for the performance analysis of float is economical and efficient, and it can easily change the parameters such as initial conditions, boundary conditions and material properties and even complex conditions that are difficult to achieve by some experimental techniques. The numerical simulation of the float mechanical properties has been rarely published so far, while some scholars have carried out relevant numerical research on the flow around the sphere (Azmahani et al., 2014; Cai et al., 2011; Han et al., 2008;Hanazaki, 1998; Hu et al., 2016; LuChen et al., 2014; REN and Guang-wangZOU, 2004; Sungsu, 2000; Zou et al., 2004; Zou & Ren,2006), which has certain enlightenment for the work in this paper.Based on the computational fluid dynamics (CFD) method and finite element (FEM) method, a three-dimensional numerical model of float in the flow field was established to analyze its hydrodynamic performance and structural strength. The response mechanism of float structure under hydrodynamic load and impact load is summarized in order to provide basic theoretical support for float structure design and optimization.

Fig. 1.Float structure and numerical model.

Fig. 2.Numerical model for the hydrodynamic performance calculation of float.

Fig. 3.Numerical model of the float structural strength.

Fig. 4.Comparison of the hydrodynamic performance between theoretical value and numerical simulation value.

2.Numerical model

2.1.Governing equation

It is assumed that the fluid is incompressible and isotropic, and the governing equations include the continuity equation and the Navier-Stokes equation:

2.2.Parameter definition

The main parameters describing the hydrodynamic performance of the spherical float are drag forceRx, static buoyancyRyand buoyancydrag ratio K, and their calculation formula is as follows:

2.3.Numerical model of the float

The calculation model in this paper is the rigid plastic spherical float with ABS material, as shown in Fig. 1 (a). This type of float is used for trawling or other nets and is also used on cages in a small amounts. The finite element model of it is established as shown in Fig. 1(b), which is a hollow sphere with outside diameterD=280 mm and shell thicknessB=5 mm. Based on it, the Computational Fluid Dynamics software(Fluent) and Finite element analysis software (ABAQUS) were used to study the hydrodynamic performance and structural strength of the float.

2.4.Parameters for the hydrodynamic performance calculation of float

The flow field around the float is meshed by the Computational Fluid Dynamics software (Fluent) as shown in Fig. 2 in order to study the hydrodynamic performance of the float, in which the tetrahedral element was adopted. The fluid densityρ=998.2kg⋅m−3and the coefficient of kinetic viscosityμ=1.003×10−3kg⋅m−1⋅s−1. The upstream inlet is set as the velocity inlet boundary with the velocity ofV=2m/s in the X direction, and the downstream outlet is set as fully develop free outflow boundaries. The surface of float and the boundary of calculation field are set as no-slip wall. The turbulence model was selected as the standardk−εmodel.

2.5.Parameters for the structural strength calculation of float

The numerical model for the float structural strength calculation was built by the finite element software ABAQUS, as shown in Fig. 3. The float structure size is consistent with the hydrodynamic calculation model. The material is PC-ABS, whose density is 1.14 ×103kg/m3,Young’s modulus is 2.6 ×109Pa and Poisson’s ratio is 0.38. The steel plate thickness is 5 mm and its material is ordinary marine steel, whose density is 7.8 ×103kg/m3, Young’s modulus is 2.1 ×1011Pa and Poisson’s ratio is 0.30.

3.Results and analysis

3.1.Verification of calculation results

Fig. 5.Velocity distribution of the flow field around the float with different sizes.

Fig. 6.Pressure distribution of the flow field around float.

Fig. 7.Drag force and ratio of buoyancy and drag force changing with diameter of float.

The hydrodynamic performance of the float was calculated by the FLUENT software in this paper, at the same time the drag force and buoyancy-drag ratio at the velocity of 1.0 m/s is calculated respectively by the spherical float hydrodynamic calculation formula (3) and (5). The results were compared to verify the validity of the numerical results.Fig. 4 shows the drag and the buoyancy-drag ratio of the float changing with its diameter.

It can be seen from the comparison in Fig. 4 that the hydrodynamic numerical model of float established in this paper can well simulate the variation law of the drag force and buoyancy-drag ratio, and the numerical results agree well with the theoretical ones. While the numerical drag force is larger than that calculated by the theoretical formula. The reason may be that the float structure established in this paper is a sphere with two ears, which causes the bigger incident flow area than the projected area of the sphere. Then the validity of the numerical model is verified on the other hand.

3.2.Hydrodynamic performance of the float

Based on the above verified effective numerical model, the hydrodynamic performance of the float and its influence parameters were further calculated and analyzed. The velocity distribution of the flow field around float with different sizes is shown in Fig. 5.

As can be seen in Fig. 5, the area behind the float shows obvious reflux and vortex under the action of flow, and the area of reflux and vortex gradually shrinks with the increase of float size (diameter). In order to further observe the influence of the reflux region on the flow field around float, Fig. 6 shows the flow field pressure distribution of the horizontal section (perpendicular to the incident flow surface of two ears) and vertical section (parallel to the incident flow surface of two ears).

It can be seen from the comparison between Figs. 6 (a) and Fig. 5 (c)that the negative pressure area with a certain width appears at the reflux region behind the float on the same section. Due to the symmetry of the structure, the fluid pressure on the float in the vertical direction of the incoming flow is almost symmetrically distributed. While in Fig. 6 (b)the vertical distribution of fluid pressure is asymmetric because of the lower part of the float with two ears causing the asymmetry of the structure. It can be observed that the existence of the two ears causes an additional flat plate flow under the float. Therefore, the negative pressure region in this position is elongated, and pressure difference appears on the upper and lower surface of the float, which may affect the buoyancy-drag ratio to a certain extent.

The variation of drag force and buoyancy-drag ratio changing with the diameter of float are shown in Fig. 7. As can be seen, the drag force and buoyancy-drag ratio of float both show an increasing trend with the increase of diameter. While the increasing trend of buoyancy-drag ratio is slightly gentler than that of drag force, because the larger diameter will lead to the increase of buoyancy.

In order to analyze the influence of flow field velocity on the hydrodynamic performance of float, the flow field pressure distribution in the vertical section of the float with diameter of 280 mm at different flow velocities is shown in Fig. 8.

It can be observed that a fan-shaped positive pressure region diverges outward at the front of the float, and two negative pressure sectors form in the middle edge of the float, which shrink sharply in the wake field.Besides, a positive-negative pressure region appears in this region.

Fig. 9 shows the variation of drag force and buoyancy-drag ratio of the float with different size changing with flow velocity, for the further study on the influence of flow velocity on the hydrodynamic performance of the float.

Fig. 8.Pressure distribution of the flow field under different velocity.

Fig. 9.Drag force and ratio of buoyancy and drag force of floats with different size changing with flow velocity.

Table 1 Material parameters of different materials.

It can be seen that the drag force increases gradually with the increase of incoming flow velocity, and this trend will not be influenced by the float size. While the buoyancy-drag ratio decreases with the increase of the flow velocity, and the decreasing trend gradually becomes gentle under the combined influence of the inflow velocity and float size.

3.3.Structure response characteristics of the float

The float bears a certain impact load in the lifting, transportation and operation process. For the float structure, the time of impact is short while the pressure is large, which may cause structural damage to the float and then affect its work performance. In order to accurately analyze the structural response characteristics of the float, the structural dynamic response of the float under impact load is calculated based on the numerical model shown in Fig. 3.(see Table 1).

Fig. 10 shows the stress distribution at typical moments of the float impacting on the steel plate with the velocity of 5 m/s. The structural responds of the float under impact can be clearly observed.

As can be seen in the Fig. 10, the structural response first appear in the contact region, and the stress reaches its maximum value in a very short time, which is about 4.7 ×107Pa. With the increase of time and the transfer of load, the structural response is rapidly transferred. The stress gradually decreases and the distribution tends to be uniform. Since the impact velocity is not very large, the structural stress is relatively small compared with the yield strength of the material, and only slight elastic deformation occurs in the impact contact region.

The impact resistance of float structure is closely related to its material. At present, most of the fishing floats are made of plastic, which has the advantages of low density, high strength, chemical stability and good wear resistance, including ABS, PVC, PP, PE, PA and so on. While the plastic floats made of different plastic materials has different structural strength. In order to analyze the influence of material characteristics on the mechanical properties of float, the time-history curves of elastic strain under impact of floats with different materials are shown in Fig. 11.

It can be seen that at the same impact velocity, the elastic strain peak of PA material is the smallest, while that of PVC material is the largest.Therefore, in the process of float structure design and manufacture, in order to increase the service life of the float, PA material has better impact resistance and deformation resistance from the perspective of structural strength. While fragility, pressure resistance, corrosion resistance and other comprehensive factors also need to be taken into account. Then a reasonable selection of plastics with different properties is made according to the requirements of float operation.

4.Conclusion

The hydrodynamic performance and structural strength of the float are the key to the structural design and optimization of the float. Based on the computational fluid dynamics (CFD) method and finite element(FEM) method, a three-dimensional numerical model of float in the flow field was established to study its hydrodynamic performance and structural strength in this paper. The main conclusions are as follows:

1) The area behind the float shows obvious reflux under the action of flow, and the reflux area significantly shrinks with the increase of float size.

2) The vertical distribution of fluid pressure is asymmetric because of the lower part of the float with two ears causing the asymmetry of the structure. The negative pressure region under the float is elongated,and pressure difference appears on the upper and lower surface of the float. The drag force and buoyancy-drag ratio of float both show an increasing trend with the increase of diameter.

3) A fan-shaped positive pressure region diverges outward at the front of the float, and two negative pressure sectors form in the middle edge of the float, which shrink sharply in the wake field. Besides, a positive-negative pressure region appears in this region. The drag force increases gradually with the increase of incoming flow velocity,while the buoyancy-drag ratio decreases with it, and its decreasing trend gradually becomes gentle under the combined influence of the inflow velocity and float size.

4) At the same impact velocity, compared with other plastic materials the elastic strain peak of PA material is the smallest, while that of PVC material is the largest. Therefore, PA material has better impact resistance and deformation resistance for increasing the service life of the float from the perspective of structural strength.

Fig. 10.The stress distribution at the typical moment of the float structure impacting on the steel plate.

Fig. 11.Structural response characteristics of the floats with different material.

Declaration of competing interest

The authors declare that there is no conflicts of interest.

Acknowledgements

This research was supported by the National Natural Science Foundation of China (Grant No. 31972845; Grant No. 11402143).