深水养殖网箱试验中网衣相似准则的应用

Simulation criteria of fishing nets in aquiculture sea cage experiments

通过分析与试验相结合的方法对重力相似准则在网衣试验中的适用性进行研究。试验以在国内应用较广的田内相似准则作为参照,分析讨论了两种相似准则的异同,推导得出了重力相似准则结合大小尺度比在网衣模型设计中的质量校正公式及刚度校正公式。并据此按照两种相似准则,设计了包括网衣、底杆和沉子系统的两套模型,模型上端固定于设置了受力传感器的刚性直杆上。试验在纯流条件下进行,分别设置5组流速,试验水深0 8m。各组流速重复试验3次,受力由固定于刚性直杆上的传感器测量获得,网衣的变形则通过系缚在网衣上的二极管的坐标确定。在进行受力分析时,由于两相似准则的受力比尺不同,因此必须将模型受力按照对应比尺统一转换为原型值,同时利用受力与流速的回归关系式将模型受力校正为同一流速下的数值然后再进行比较。试验结果表明,相同流速条件下,两种相似准则的模型受力及变形总体上吻合较好,说明重力相似准则在网衣试验中采用质量校正及刚度修正后同样能够得到令人满意的结果,而且试验中模型的大尺度比达到1∶20,这无疑为涉及波浪问题的深水网箱模型试验的设计提供了保障,从而避免了田内相似准则在深水网箱试验研究中应用的局限性。

The deep water sea cage engineering developed very fast,but the research on its hydrodynamic characteristics is relatively weak.Model experiments are important methods for such study, but the main difficulty is how to give a reasonable simulation criteria for the model test of fishing net.Many researchers proposed different methods to solve this problem when dealing with sea cage experiments.Based on Morison Equation together with consideration on the change of drag force coefficient, Fredriksson (2001) in his study proposed a method to model the fishing net by compensating the error of twine projected areas.But the relationship between drag force coefficient and Reynold number has to be set up firstly through experiments.While the experiment results,in great sense,depend on the conditions of the model configuration,the components and material used,Phe sea cage model without any modification on net mass and twine projected area for the difficulties in modelling the net system reported in one of his research papers.In trawl net experiments, several simulation criteria of net models were proposed by various scholars.The familiar criteria are Dickson criteria,Christensen criteria and Tauti criteria.Dickson criteria,which is used popularly in west Europe and Christensen criteria,is often used in America,and Tauti criteria was worked out by Professor Tauti and is used frequently in Japan and China.But for sea cage experiments,all these simulation criteria encounter different problems and have different restricts.The main difficulty of Tauti criteria applied in sea cage experiments is the large scale of particle velocities which will make the wave conditions difficult or even impossible to implement.So once more we paid our attention to the Froude law.This paper focused on the feasibility of the application of Froude law for fishing nets model design and during the experiments the Tauti law was selected for comparison.Through this analysis,the differences were found out between Froude law and Tauti law and the mass and rigidity modification formula of fishing nets was proposed.Based on this consideration two models including net,low-bar and weighting system were designed according to Froude law and Tauti law respectively.The model scale of mesh was 1∶2 while that of other parts of the model was kept as 1∶20.The models were fixed on the rigid bar on which a force transducer was attached.And then the total model system were assembled in a wave flume with 69m long, 2m wide and 1.8m high in the State Key Laboratory of Coastal and Offshore Engineering in Dalian University of Technology.The water depth was kept at 0.8m constantly in the whole experiment.And five values of flow velocities under each simulation criteria were selected respectively.Force data were measured by the transducer fixed on the rigid bar and transferred to the computer data collecting center through data lines.During the experiment, total three diodes were attached to the net model along the water depth direction and a Cartesian coordinate was set up on the glass plate where the model was assembled.Net deflection then was measured by tracing the diodes on the net model.Analyzed results of forces and net deflections showed the consistency between Tauti simulation criteria and Gravity simulation criteria.This achievement ensures the use of smaller model scale as 1:20 for deep water sea cage experiments which is smaller than the scale usually used for trawl model test and avoids the restrict of Tauti simulation criteria.