海洋养殖柔性网衣结构水动力特性研究进展

Research process for the hydrodynamic characteristics of flexible net structures for offshore aquaculture

中文关键词: 深水网箱养殖  网衣  水动力特性  有限元方法  计算流体力学

英文关键词: Offshore cage aquaculture  Net  Hydrodynamic characteristics  Finite element method  Computational fluid dynamics

中文摘要:

      发展深水网箱养殖是保障我国粮食及食品安全的长远战略，也是缓解近海网箱养殖环境胁迫力、拓展食物生产空间的必然选择。发展深水网箱养殖首先需要面对的突出挑战是养殖设施在外海恶劣海况下的安全性问题。网衣是深水网箱的主体结构，由于其自身具有高柔性、小尺度的特点，在波浪和水流作用下易出现大位移和大变形的极端响应。当前，网衣结构分析技术已成为我国深水网箱养殖工程技术的薄弱环节，一定程度上制约当前海上养殖网箱向大型化和深水化发展。因此，网衣水动力特性研究对于深远海网箱养殖的发展具有重要意义。本研究系统介绍了计算网衣水动力荷载的主要方法及其适用范围。同时，对网衣动态响应数值计算中的主流建模技术进行了总结和分析。最后，根据目前网衣水动力特性研究中存在的热点问题，提出了网衣流固耦合分析、生物污损分析、数字孪生技术等前沿发展方向，为网衣水动力学分析向数字化、精准化发展提供参考。

英文摘要:

      With the increase in global population, the demand for aquatic products rises annually. As a sustainable industry, aquaculture offers a promising solution to address the growing demand for fish products. Meanwhile, intensifying and expanding sustainable aquaculture is essential for achieving the United Nations’ global goal of the Decade of Action. Cage culture is a typical aquaculture method used globally. This culture method has the following advantages: (Ⅰ) Highly researched with a long history: cage culture has been in use since the 1970s. Cultivating high-value aquatic products through cage culture has become indispensable in aquaculture. (Ⅱ) The development prospects of cage culture are broad and include realization of the comprehensive breeding of different species. In the integrated multi-nutritive aquaculture (IMTA) system, cage culture enables a multi-species combination in which uneaten feed and nutrients from the excreted waste of one breeding species are used as food for other breeding species, thereby reducing nutrients released into the environment and increasing overall productivity. Net cage culture in shallow waters has grown exponentially in the past few decades. However, due to the limited space in nearshore areas and an increasing number of countries paying attention to the environmental problems caused by cage culture, the method has gradually shifted to deeper waters to ensure food security and safety. At the same time, alleviating environmental stress caused by nearshore cage culture and the expansion of the food production space are inevitable. Compared with nearshore aquaculture, the environmental conditions of offshore aquaculture are more complex, and the net cages are inevitably subjected to harsh sea conditions. To achieve the safe production of offshore aquaculture, it is crucial to study wind and wave resistance technology for use in culture net cages. As an essential part of net cages, the netting system is mainly used to maintain breeding space, protect the breeding species, prevent them from escaping, and protect them from predators. In the flow and wave field, the netting system bears most of the load on the farming facilities. Compared with other traditional marine structures, the nets have the mechanical characteristics of small scale and high flexibility. Under external force, the nets show large displacement and massive deformation, reducing breeding space and increasing the likelihood of damage to breeding species due to crowding. At the same time, the nets change the flow and wave field around the facility, affecting the distribution of the remaining bait, breeding species’ excrement, and dissolved oxygen in the water. Currently, the lack of analysis technology for nets is a limitation in offshore cage culture engineering, which restricts the large-scale development of offshore cages. Therefore, research on the hydrodynamic characteristics of nets is vital for developing offshore cage culture. This study introduced primary methods for calculating the netting hydrodynamic loads and their applicability. Simultaneously, the predominant modeling techniques in the numerical calculation of the netting dynamic response were summarized and analyzed. Furthermore, a systematic review of studies relating to the wave flow field around net mesh was conducted. Finally, the current hot topics in the research of netting hydrodynamic characteristics were reviewed to provide a reference for designing and optimizing cages. Considering the influence of various parameters on the hydrodynamic loads on the nets is an effective way to improve the prediction accuracy for the loads, which is also a critical area of research that requires further investment. In order to restore the real force characteristics of nets, a database of the netting hydrodynamic coefficients should be established through experiments. Intelligent algorithms, such as digital twin technology, are used to construct prediction models and generate mapping relationships between multiple factors and the hydrodynamic coefficients of the nets. Moreover, databases and algorithms should be updated regularly to improve the accuracy of calculating the netting hydrodynamic load. For the dynamic response of the nets, the fluid-structure coupling of the flexible nets should be studied intensively in the future, focusing on the two-way coupling between the nets and the fluid and exploring direct numerical simulation methods. Meanwhile, considering computational accuracy, suitable algorithms, such as the submerged boundary method, should be selected, and parallel computational methods should be developed to improve computational efficiency. In terms of the flow and wave field around the nets, further research should be conducted to determine the effects of extreme waves, biofouling, fouling density, cultured fish species, fish size, fish number, fish swimming speed, and fish swimming status on the flow field in the net cages, which will be conducive to monitoring the health of fish, reducing the risk of fish diseases, and ensuring the sustainable development of cage culture. This study provides a reference for developing net hydrodynamic analysis for digitalization and precision. It also provides more information for the sustainable development of aquaculture.