水产养殖网箱尾流特性及近区传质机制研究

Cage-based aquaculture has been growing rapidly in recent years, which results in clustering of large quantities of cages in fish farms located in inland lakes and reservoirs and coastal embayment, and hence significantly affecting flow and mass transport processes in the surrounding waters. However, existing studies focus primarily on the macro-scale flow blockage effects of the fish cages, and the complex wake flow pattern and the resultant mass transport processes induced by the cage are yet to be studied. To address this issue, the proposed study employs the combined particle image velocimetry-planar laser induced fluorescence (PIV-PLIF) technique and proper orthogonal decomposition (POD) based flow reconstruction technique, to elucidate the dependence of the complex flow pattern consisting of multiscale wake-vortex structures induced by net panel, which is the key component of the fish cage, and fish cage per se, as well as wake characteristics such as the extent of the wake area, flow pattern transition and vortex shedding, etc, on the major influencing factors including the properties of the net panel and the flow conditions, and analyze the mechanism of the highly heterogeneous wake field on the mass transport processes through theoretical analysis based on turbulent kinetic energy production and dissipation equilibrium model and diffusion scaling model. Numerical models will be further developed using OpenFOAM and extended to full scale to examine and rectify the scaling effects associated with the physical modeling results. The research output is expected to assist in the assessment of the environmental impacts and environmental carrying capacity as well as the optimization of the design and deployment of cages in fish farms.

伴随水产养殖近年来的飞速发展，大量网箱密集分布于湖泊水库和沿海内湾的养殖区，对周边流动和传质过程造成了显著影响。但目前相关研究多侧重于网箱宏观的阻流作用，对水流流经网箱形成的复杂尾流特性及其主导的近区传质机制还有待进一步研究。有鉴于此，本项目从经典的恒定均匀流入手，通过PIV-PLIF耦合同步观测和POD流场重构考察作为网箱关键组成部分的网衣及网箱整体形成的不同尺度尾流-涡旋结构叠加的复杂流态及尾流区范围、流态转捩、涡旋脱落等尾流特性随网衣（箱）属性和水流条件等主要影响因素的变化规律，通过湍动能生消平衡模型和扩散比尺模型理论分析揭示网衣（箱）通过高度空间异质性的尾流流场对传质过程的影响机制，并进一步基于OpenFOAM平台构建网衣（箱）尾流和传质过程数值模型，通过原型尺度模拟检验并修正模型实验的尺度效应，为网箱养殖场近区水环境影响评价和环境容量评估以及网箱的优化设计和布设提供支撑。

随着水产养殖近年来的飞速发展，大量网箱密集分布于湖泊水库和沿海内湾的养殖区，对周边流动和传质过程造成了显著影响。但目前相关研究多侧重于网箱宏观的阻流作用，对水流流经网箱形成的复杂尾流特性及其主导的近区传质机制还有待进一步研究。有鉴于此，本项目从经典的恒定均匀流入手，使用粒子图像测速 (PIV) 和平面激光诱导荧光 (PLIF) 技术考察作为网箱关键组成部分的网衣及网箱整体在近尾流区的流动特性和传质过程随网衣（箱）属性和水流条件等主要影响因素的变化规律。结果表明网衣存在明显的阻流效应，近尾流区的紊流呈现高度空间异质性和各向异性，紊流强度遵循幂律衰减。网衣的存在略微增强了标量物质羽流扩散，时均浓度、脉动浓度和紊流标量通量的横向分布表现出自相似的高斯分布。近尾流区羽流扩散处于从紊动对流主导向紊动扩散主导的过渡阶段，但从羽流扩散推算的表观紊动扩散系数与从紊流场确定的泰勒扩散系数（横向脉动速度与表征大涡尺度的积分长度尺度的乘积）仍吻合较好。与柔性网衣相比，刚性网衣在相同密实度下引起的时均流速衰减和紊流强度较小，但倾向于产生更大尺度的大涡从而促进羽流扩散及浓度衰减。与柔性网箱相比，刚性网箱更有可能为鱼类生长创造有利环境并减少对周围环境的负面影响。在实验观测和理论分析的基础上，进一步基于OpenFOAM平台构建网衣尾流和传质过程数值模型。模型将网衣概化为多孔介质，并应用有限体积法和标准κ-ε模型求解控制方程，通过实验结果率定和验证模型，并设置不同密实度和来流速度开展情景模拟。结果表明基于多孔介质概化处理的数值模型能够较为精确的模拟网衣的阻流效应和传质过程。网衣引起的流速衰减与网衣密实度和来流速度分别呈现正、负相关关系。当来流速度增大时，羽流扩散宽度和浓度衰减减小。忽略紊流强度变化时，网衣距离源的位置变化对羽流扩散的影响较小。研究成果可为网箱的优化设计和布设提供参考，同时也为网箱养殖场近区水环境影响评价和环境容量评估提供支撑。