Investigations On Critical Issues In Hydrodynamic Performance Of The Offshore Gravity Cage

The investigations on the hydrodynamic behavior and dynamic response of offshore aquaculture structures are the fundamentals for cage design,deployment and maintenance.The existing methods mainly divide the aquaculture cage into components such as the floating collar and the net system,then the hydrodynamic behaviors of these components can be studied separately.So far there is no such method that considers the net deformation and its flow field.Moreover,experimental investigations are not sufficient for floating collar under high sea states and the floating cage under combined waves and current.To resolve these issues mentioned above,this thesis proposes a coupled fluid-structure model based on mechanism of flexible body;this thesis also examines hydrodynamic coefficients of the segments of floating collar via the forced oscillation experiments;Alongside this,this thesis provides an experimental approach according to the theories of wave-floating structure interactions.Works of this dissertation consist of five chapters:chapter one is the introduction,which gives the motivations and drivers for the first place,and this is followed by classifications of commercial aquaculture cages.Aquaculture cage and its components such as floating collar and netting are also reviewed in this chapter.Chapter two presents fundamental theories and mathematical formulations.In particular,this chapter provides the numerical formulations of the coupled fluid-structrue model.These fundamentals facilitate a solid support for physical the implementation of the coupled fluid-structure model.Chapter three studies the hydrodynamic forces and flow characteristics of the flexible net panels under steady flow using a coupled fluid-structure model.Especially considering the large deformation of the flexible net panels.Chapter four focuses on the water flume experiments of the floating collar,which is divided into several segments and their hydrodynamic coefficients have been investigated under oscillating flow.The experimental results are verified by numerical simulations.Chapter five experimentally studies the vertical movement of the floating collar under waves and the vertical movement of the entire cage under combined waves and current.The innovations and results are as follows:(1)To study the hydrodynamic forces and flow characteristics of the flexible net panels under steady flow,this thesis proposes a coupled fluid-structure algorithm.This algorithm combines the fluid and solid solvers to directly model the fluid-structure problems of the flexible net panel instead of using porous media model.The flow field and wake cahracteristics of the deformed nets are assessed.(2)The water flume experiments of the floating collar have been conducted to obtain hydrodynamic coefficients under oscillating flow.The distance between the floating collars has been considered to analyze the effects of the distance on the hydrodynamic coefficients.The dependence between hydrodynamic coefficients and KC number has also been established.(3)This thesis experimentally studies the vertical movement of the floating collar under waves and the vertical movement of the entire cage under combined waves and current.The dependence between vertical acceleration and wave amplitude is established.The effect of high-order terms on the first-order vertical acceleration are analyzed.