A Study Of Nonlinear Hydroelastic Response Of Gravity Fish Cages

With the progress of ocean resource development,the marine cage culture industry has become one of most important sea economic points in our country.However,due to the overfishing and unreasonable development,the marine ecological environment has become worse and worse.All the countries are seeking the better aquacultural environment and higher quality protein.Therefore,the cage farming are developing from the shallow sea to the deep sea.Since the sea environment in deepwater is more harsh compared to near-shore area,the fish cage will suffer severe dynamic responses.In addition,the net cages are considered as high investment,high risk and high productivity equipments,the safety and reliability of fish cage structure would have a great influence on the whole fish farming platform.For the lack of theoretical support of the net cage design in our country,it is still need to import the the similar product from abroad.Besides,although the dimension of the net cage is large,the components such as the floating collar,the net structure,etc.,should be considered as the small scale flexible structures.Thus,it is important to investigate the mechanisms of the interaction between these flexible structures and the waves.In this paper,based on the numerical simulation,the hydrodynamic characters of the fish cages under the wave and current loads are researched.Besides,the hydrodynamics of the segment of the floater and the floater-net system under the full scale KC/Re waves are also studied.Finally,a hybrid method based on the FEM method and experimental results in this study is developed to predict the dynamic response of the full scale fish cages.The main contents of this thesis can be mainly summarized as follows:1.A general introduction about the main types of net cages is conducted.The main characters of each fish cage are summarized.Then,the author gives the recent research status about the hydrodynamic response of gravity fish-farming cages.2.The dynamic behavior and mooring tension of the net cages under the combination of the irregular wave and current are calculated based on the FEM method.It can be found that the presence of the current will have an influence on the amplitude of the surge motion rather than the deformation of the floating collar.The modal superposition method is applied to analyze the weight of the participation of each mode.The results show that the modal weight in both the horizontal and vertical directions becomes larger as the significant wave height increases.Meanwhile,the modal weight of the rigid-body-motion mode in the horizontal direction grows with the current speed,while the modal weight in the vertical direction is only slightly influenced by the variation of the speed.3.The numerical simulation of the multi-net cages in the wave is also conducted.The hydrodynamic response of the net cage and the tension of the mooring line under different direction of wave propagation are investigated.From the analysis,with the increase of pretension in the anchor lines,more decrease can be observed from the average displacement of the surge motion than the amplitude displacement.Meanwhile,the heave motion is not sensitive to the changes in pretension.The results also show that the displacement of the heave motion of the floater is smaller than the wave height,the overtopping phenomenon may occur on the structures when they meet wave crest and wave trough.4.Due to the wave maker could not generate the full scale KC/Re number waves in the laboratory,the forced oscillation will serve to simulate the water particle motions around the floating cylinder in waves.The effect of the Keulegan–Carpenter number(KC),Reynolds number(Re)and reduced velocity on the hydrodynamic coefficients of the collar under different draft is researched.When overtopping phenomenon does not occur,the drag coefficient of floating cylinder is about half of the fully-immerged;yet the added mass coefficient almost kept the same.Meanwhile,overtopping has great influences on the added mass coefficient for the floating cylinder,and the drag coefficients will also increase significantly in the high Re number region.5.The first five orders of the horizontal hydrodynamic forces are also obtained through Fourier transformation.The results show that all the first five harmonics increase with the growth of KC andnumber when the floating collar is under the oscillatory flows.When the current coexists with the oscillatory flows,the first five harmonics would also increase as the KC and Vr increase.6.The hydrodynamic characteristics of a floater-net system in oscillatory and steady flows are investigated through forced oscillation experiments in a towing tank.Meanwhile,based on the forced oscillation experiment of the floater-net system and floater only,a new experimental-numerical hybrid method for the prediction of hydrodynamic response of fullscale fish farming system is proposed.In summary,this thesis is supposed to have a better understanding of the dynamic response of the gravity fish cage in the wave and current,and also focus on the hydrodynamics of the floating collar under the full scale KC/Re number oscillatory flows based on the experimental method.Finally,a hybrid experimental-numerical method is developed to predict the hydrodynamic response of the full scale fish cage and provide a fresh idea to the improvement of the fish cages system design.