Study On Hydrodynamic Characteristics Of Pile-column Type Net Enclosure Aquaculture Facility

The Net Enclosure Aquaculture(NEA)in shallow water is a new developing ecological aquaculture model.But,this kind of aquaculture mode is still in its infancy.There have been many key technical problems to be solved such as the potential risk of infrastructure caused by the immature structure design and construction technology in the adverse environment including heavy wave,rapids,typhoon and so on.Thus the hydrodynamic characteristics of pile-column type net enclosure aquaculture facility were studied in waves and currents.In this paper,the net was simplified reasonably based on the lumped mass point method,and the mathematical model which was used to calculate the motion response of net was established after force analysis.In addition,the mathematical model was verified through physical verification test and the simulation results and experimental results were in good agreement with compared each other.The hydraulic characteristics of unit net of the Pile-column type Net Enclosure were studied by the established numerical simulation method in wave.And the impact on line tension distribution,node offset and force on anchor points of unit net was analyzed for wave height,wave direction,net size,fixed ways,the diameter of rope and rope gridding size.The results showed that the change of wave height had a more impact on force on anchor points at the beginning and the end of net than at the middle of net.It was 30°~70° that the main increasing segment for maximum tension of line and force on anchor points was.The area of maximum tension force twines could be determined through the ratio between anchor points spacing and width of net.Increasing the quantity of anchor points on both sides of unit net contributed to decreasing force on all anchor points.While more than the critical number of anchor points about 4 ~ 5,maximum tension and maximum offset no longer dropped significantly.When the diameter of rope is less than 4 mm,inside tension of the net mainly is borne by line;when more than 4 mm,rope turns into the main undertaker of tension and the deformation of rope and line no longer reduce significantly.The maximum tension of line on net with diamond rope stiffening is somewhat less than with equivalent rectangular rope stiffening,which reflects diamond rope can conduct force while rectangular rope cuts off the transmission of internal tension.The force on anchor points at the beginning and the end of net with rectangular rope stiffening is less than the one with diamond rope,but the force on anchor points at the middle of net with rectangular rope stiffening is huger than the one with diamond rope.In addition to considering the effect of wave,this paper also studied and analyzed the hydrodynamic characteristics of unit net of the Pile-column type Net Enclosure by numerical simulation in current.It was found through analysis that the force and deformation on various parts of net in current is less than in wave.A lot of hydrodynamic characteristic forms in current almost were symmetric compared with those in wave,but there were also many unique hydrodynamic characteristics in current.For example,the maximum tension force twines and the maximum force on the fixing points increased with the flow direction increasing.The increase of the rope gridding density will not affect the force of anchor points at the beginning and the end for the net with diamond rope stiffening.And the force of the anchor point which rope just is connected to will increase,at the meanwhile,the force of nearby anchor points will reduce.Rectangular rope can cut off the transmission of internal tension,which will reduce the force of anchor points at the beginning and the end of net,but will increase the force at the middle.Especially in current,the force of anchor points at the middle of net even is huger than the one at the beginning and the end.The distribution of maximum tension on diamond rope and rectangular rope was different from in wave,etc.Based on the research experience of local net,the hydrodynamic characteristics of pile-column type net enclosure aquaculture facility on the whole were studied by numerical simulation.Through analyzing the load on net,rope tension,deformation of net and resultant force of pile on the representative parts of the system,research found that: the middle part of the system is the main affected parts.Generally,the forces on piles at the corner of system were larger than on the sides.The change of angle had a certain influence on the internal system,but less impact on the system as a whole,and no obvious rules to follow.The results of this study will provide important reference value for the design,production,offshore installation,safety performance evaluation and anti-wave technology research of the Pile-column type Net Enclosure Aquaculture Engineering.