Study On The Fluid Forces On Raft-culture Structures And Flow Field In Marine Ranching

The response of offshore systems to natural variations,human activities and global changes is one of the hot issues in marine scientific research.Mariculture is an important part of the research on the impact of human activities on the coastal environment.The aquaculture canopies produce resistance to water flow,which alter local hydrodynamic and ecological environment,thereby changing the transportation route of nutrients.In return,these ecological implications will have a feedback to the growth of aquaculture products.On the other hand,the aquacultural structures are subject to complex flow conditions including waves and currents.With the expansion to more open areas with severe flow field conditions,marine aquacultural structures face greater challenges and risks of damage.The safety and reliability of the structure directly affect the production efficiency of marinculture.Therefore,it is very important to study the dynamic response of the aquacultural structure under the action of oscillatory flows such as tides,circulation and waves.And the accurate prediction of the interaction between aquaculture canopies and ambient water field is quite necessary to study the hydrodynamic impacts and biological process.Therefore,this paper applies the combination of physical model experiments and numerical model calculations to analyze the dynamic response of aquacultural structures under the action of waves and currents,and study the hydrodynamic environment inside and outside the culture area.The main research contents of this paper are as follows:(1)Laboratory experiments are carried out to investigate the mooring line force and the wave propagation through the canopy area formed by culture raft of different forms under the action of regular and irregular waves.Based on the experiment results,the relationship between the characteristics of the mooring line force and the wave conditions and the structural parameters are discussed,and the impact factors of the wave transmission,reflection and energy dissipation in the canopy area are analyzed.It provides data support for revealing the characteristics in the process of canopy and fluid interaction,and provides reference for the improvement and verification of numerical model.(2)A numerical model is established to simulate the dynamic response of aquacultural structures under the action of waves and currents.As the flexibility of the culture units is barely considered in existing researches,a numerical model is developed to study the coupled dynamics of structural elements,including simulations of the flexibility of culture droppers and attached algae.The experimental results are used to verify the flexible culture unit model and the structure dynamic model.And the sheltering effect between culture units and the impact factors of the mooring line force under the action of waves are discussed and analyzed.The results show that the maximum mooring line force generally appears near the relative raft length B/L=1.15?1.35,and the maximum mooring line force presents an approximately linear growth trend with the increase of the relative wave height.The numerical model and discussion results in this paper can be used to verify the safety and reliability of the structure,as well as provide references for the optimization and arrangement of the aquaculture structure in marine ranching.(3)Research on the interaction between water flow and flexible canopy.A hydrodynamic numerical model is established to simulate flow through suspended and submerged flexible canopies,aiming to provide basis for the investigation of the interaction between waves and aquaculture canopy.The numerical model is established based on the Navier-Stokes equations and two-equation k-? turbulence closure model.First,the empirical relationship between the bulk drag coefficient and the canopy characteristics and flow field conditions is given by a combination of experiment model test and numerical simulation studies.The bulk drag coefficient is related to the characteristics of the culture dropper(plant),and increases with the increase of the culture density and the ratio of the canopy height to the water column.Second,the relationship between the effective length of the culture unit and its physical parameters and flow conditions are analyzed based on the numerical calculation and the concept of effective length theory.And then an empirical formula is given and introduced into the hydrodynamic model to determine the effective canopy height range that responds to the flow field condition variation.Finally,the changes in the effective canopy height along the direction of water flow and the distribution of flow velocity inside and outside the canopy area are obtained through numerical simulation.The numerical results are consistent with the experimental phenomenon and measurement,and are more consistent with the actual situation compared with rigid assumption.(4)Research on the interaction between waves and aquaculture canopy.The interaction between waves and aquaculture canopy is investigated incorporating the canopy height variation and the relative motion between the structure and the flow field.The numerical model is validated by laboratory experiment conducted in our work.Then the wave height variation along the canopy region and the relationship between related parameters and the transmission,the reflection and the energy loss coefficients are studied based on the validated numerical model.The results show that the wave height attenuation generally decreases with the increase of the wave period,indicating that the suspended culture canopy has a stronger effect on the reduction of high-frequency waves.Due to the different characteristics of the culture units,the wave height attenuation in the vertical raise culture canopy is relatively uniform.While the wave height attenuation in the flat raise canopy is mainly concentrated in the rear part of the canopy area,and this phenomenon is more obvious in high-frequency wave cases.In addition,the effect of the relative canopy width B/L on the wave transmission,the reflection and the energy loss coefficients are similar in the vertical and flat raise culture canopies.The wave transmission coefficient Kt has a maximum value near the relative canopy width B/L=0.65,while the energy loss coefficient Kd has an opposite trend,with a minimum value near B/L=0.65.Finally,formulas for calculating the transmission coefficient of the vertical and the flat raise culture canopies are obtained based on the results of experiments and numerical calculations.