Study On Characteristics Of Flow Distribution Around Marine Cage Culture Area

As one of the main aquaculture modes in China,sea cage aquaculture has the characteristics of high efficiency,intensive,low investment and high efficiency,and has become a new growth point of marine economy in China.However,a large number of aquaculture facilities and aquaculture fish gathering in the coastal waters or semi closed bays will hinder the flow of aquaculture waters,reduce the flow velocity,and to a certain extent,aggravate the enrichment of pollutants and the consumption of dissolved oxygen,resulting in the deterioration of the water environment and frequent occurrence of diseases,and a large number of cultured fish died.Therefore,quantifying the blocking effect of cage and cultured fish on the water body,and analyzing the influence of cage culture on the hydrodynamic structure characteristics of typical sea areas are conducive to evaluate the aquaculture capacity and optimize the aquaculture layout.Based on Reynolds time averaged equation and Realizable k-εturbulence model,coupled with porous media model,a three-dimensional hydrodynamic mathematical model was established to study the flow field around a single fish cage.The resistance coefficient of porous media was calculated by empirical formula.The model was verified by the experimental results of single mesh and emergent vegetation test.The simulation results were in good agreement with the experimental results,indicating that the porous media model can accurately simulate the flow field structure and wake characteristics around the porous media.Based on the numerical model of single fish cage,the flow field around the cage was studied under the action of net cover only,fish only,and the interaction of cultured fish and net.The effects of net density,fish volume fraction and vertical distribution of cultured fish on hydrodynamic characteristics around the cage were considered.The attenuation of velocity in the downstream of the cage was significantly enhanced when considering the effects of fish culture and net.According to the field monitoring data of a typical farm in Chengmai Bay of Hainan Province,considering the growth of fish and the influence of changing nets,it is found that the water blocking effect of cage is stronger and stronger in the process of cultivation.Based on the Delft3D software,a three-dimensional hydrodynamic model of Chengmai Bay is established.Based on the good verification of the flow field in the natural sea area,the aquaculture resistance coefficient is estimated according to the numerical calculation results of single fish cage,and it is added to the hydrodynamic model of Chengmai Bay.By comparing the difference of tidal current field in Chengmai bay with and without cages,it is found that cages will not only reduce the flow velocity in the cage culture area and downstream,he maximum velocity attenuation can reach0.3 m·s^-1 at flood time,but also change the structure of horizontal flow field and the vertical distribution characteristics of tidal current in the bay.In addition,compared with the influence of cage culture on velocity,it has less impact on water level.The results show that the cage can reduce the velocity inside the cage culture area and the backflow area,and the maximum velocity attenuation can reach 0.3 m/s at the time of rapid rise,and also change the horizontal and vertical flow field structure of the bay.