Research On Hydrodynamic Performance Of Combined Rigid Frame Cage

With the development of the world economy and the growth of population,the market demand for fish is growing,the prospect of mariculture is bright,and the scale of net cage is becoming larger and larger.For the combined rigid frame cage,the change of cage volume and the stress of mooring line are important indicators to measure the wind and wave resistance of the cage.The discussion of the connection arrangement of the combined cage is also of great practical significance.In the numerical simulation of the cage,the force and motion of the connection point of the cage,the force of the anchor rope and the volume residual rate of the cage are mainly studied.In this thesis,based on Morison equation and force similarity conditions,considering the shielding effect of mesh,the effects of different environmental conditions on the force and motion of rigid frame cage and combined rigid frame cage are analyzed.The specific research contents are as follows:(1)Based on Morison equation and force similarity condition,considering the scale problem of transforming prototype mesh into experimental model mesh,a new improved method is proposed.On the premise of maintaining the geometric similarity of the total scale of the mesh,the improved method ensures the geometric similarity of the total force area of the mesh,increases the mesh diameter and mesh size,and coordinates the gravity and buoyancy by adjusting the hydrodynamic coefficient,so as to achieve the same hydrodynamic force of the mesh before and after the improvement.The accuracy and feasibility of the improved method are verified by the numerical simulation of the plane mesh and the comparison with the previous experimental research.(2)The shielding effect of mesh clothing.STAR-CCM + software is used to simulate the double plane mesh clothing and circular mesh clothing.It is found that the velocity attenuation coefficient,force magnitude and resistance coefficient of the two kinds of mesh clothing are similar.Due to the shielding effect of net clothing,there is obvious velocity attenuation in the net and behind the net.A very short vacuum belt can be clearly seen in the area near the net clothing,and then the flow rate increases and gradually reaches the steady state.As can be seen from the vertical flow field,with the increase of distance from the net coat,the width of the velocity attenuation area has a trend of gradually narrowing,and the flow velocity at the bottom of the attenuation area increases accordingly.It can be seen from the horizontal flow field that the flow velocity on both sides of the attenuation area increases with the increase of the distance from the net coat.When the flow rate increases,the horizontal drag force increases,and the drag coefficient decreases.(3)Study on hydrodynamic performance of rigid frame cage.A complete rigid frame cage is numerically simulated.Under pure water flow,the mooring line anchor force,the effective tension between the floating frame and the net clothing connection point and the volume surplus rate of the net cage after reaching the steady state are studied;Under pure waves,the effects of regular and irregular waves on the cage are studied;Under the combined action of wave and current,the influence of the angle between the current direction and the wave direction on the mooring,motion and deformation of the cage is studied;The influence of diving depth on net cage in storm self storage state is studied.(4)Study on hydrodynamic performance of combined rigid frame cage.The combined rigid frame cage model is established,and the hinges are arranged to connect the inner and outer cage.Under the wave and current environment,the stress of the connection point between mooring line,floating frame and net clothing,the vertical motion response of hinge system and the change of volume residual ratio of inner and outer net boxes are analyzed.By comparing and analyzing the results of rigid frame cage and combined rigid frame cage,it is found that the layout of outer cage has a good protective effect on the living space of inner cage;Analyze the influence of diving depth on the cage under the condition of storm self storage.