Research Process for the Hydrodynamic Characteristics of Flexible Net Structures for Offshore Aquaculture

Abstract

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.

Keywords

• offshore cage aquaculture
• net
• hydrodynamic characteristics
• finite element method
• computational fluid dynamics