Measurement and simulation of the three-dimensional flow pattern and particle removal efficiencies in a large floating closed sea cage with multiple inlets and drains

This study investigated the three-dimensional flow structures in an ellipsoid, closed sea fish cage. The results are presented using computational fluid dynamics (CFD) simulations and experimental measurements. Experimental residence time distribution (RTD) measurement and CFD simulation are the best methods to study the hydrodynamics of inflow systems. Three-dimensional numerical simulations of the flow and transport characteristics of the system were conducted using a Reynolds-averaged Navier-Stokes equation approach and the results were compared to the measurements performed using acoustic Doppler velocimetry techniques. The objective of the investigation was to characterize the flow field generated in an ellipsoid, closed tank. The flow in the enclosed volume is driven by four inlets pipes integrated into the wall of the cage. The focus is on the turbulent structures and undesirable flow patterns that lead to reduced self-cleaning efficiency and a lower quality habitat for the fish through phenomena, such as recirculation zones or low velocity areas. Correlations between CFD and the experimental data confirm the adequate reproduction of hydrodynamic conditions and reinforce the predictive capabilities of numerical models as tools to simulate field scale closed containment systems or to optimize existing and future aquaculture designs. The simulation of aquaculture-like particles demonstrates that almost 100% of particles with a diameter ranging between 1 μm and 3000 μm are removed during a maximum of two hydraulic retention time (HRT) cycles. Smaller particles are removed via the upper-side outlets and larger particles are removed via the bottom outlet.     

工厂化循环水养殖系统中流场特性与鱼类互作影响的研究进展与展望

随着人口与经济的发展,水产养殖业在世界范围内迅速兴起,集约型工厂化循环水养殖因其高密度、低污染、高效率等独特的优势,契合水产养殖业绿色发展理念,已成为水产养殖转型升级的重要方向之一。水作为循环水养殖系统中重要的环境因子,其流态能够直接影响鱼类的生长及福利,同样,鱼类存在及运动也会影响到系统流态的构建。本文综合分析了循环水养殖系统中流场条件对不同鱼类生长发育及福利的影响,鱼类及其运动行为对养殖池内水动力条件及性能的影响,以及鱼类对养殖池内流场流态、水体混合等的影响。将研究鱼类运动对流场特性的影响方法主要归纳为实测法和数值研究,通过对比分析2种方法的优点和不足之处,并结合当前循环水养殖产业系统构建中的问题提出针对性方法建议,旨在为系统中水动力条件的设计拓展思路,促进循环水养殖产业流态构建向“鱼”与“水”兼顾的方向发展。     

Hydrodynamic implications in and around a caged finfish farm and its implications on the dispersal of farm debris

One of the key factors in determining the accumulation of sediment and soluble nutrients within a lease is water movement. Water masses entering and leaving the farm determine the finfish ecosystem interaction. Understanding the hydrodynamic interaction with the farm is therefore key to understanding the potential ecological effects of individual farms. In addition, finfish farms are now being proposed in exposed offshore environments and have caused concern regarding their potential down stream impacts on currents and wave climate. Seven current meters, oxygen probes and CTD were deployed to examine the hydrodynamic interactions inside and outside a 270 m long Salmon farm in Newfoundland, Canada. Current meter results indicate that the finfish farm cages have a clear shadowing effect on the currents. Currents upstream were found to be considerably faster than those recorded downstream during the sampling period. Current speeds inside the farm were also found to be considerably slower than those found outside of the farm especially during high flow events. In situ observations of currents were found to be similar to those predicted by previous CFD and hydrodynamic modelling studies. Modeling was also undertaken to calculate the energy lost as currents enter and leave a series of fish cages. In comparison to the observed flow the model compares relatively well. Flow recorded downstream of the farm was observed to be in the range predicted by the model as was the flow recorded inside and outside the cages. Current speed downstream of the farm is clearly affected by farm orientation which has important implications for the dispersal of farm debris. Average oxygen saturation within the cages over the 5 day sampling period was 80.2 %± SD 5.7 %, compared to nearly 100 %, 20 m and 50 m from the farm site. Orientation of the farm may play some part in determining the location and amount of oxygen depletion within the farm. The farm also acts to push water from deeper in the water column up into the cages which has implications for farms situated in heavily stratified environments.     

花岗岩崩岗区不同土层的侵蚀水动力学特征

土壤剥蚀率是单位时间单位面积水流剥蚀土壤的质量,定量研究崩岗不同土层土壤剥蚀率对预测土壤剥蚀过程及建立崩岗侵蚀物理模型具有重要的理论和实践意义。针对湖北通城花岗岩崩岗区发育的表土层、红土层、砂土层、碎屑层,采用不同坡度(8.8%、17.6%、26.8%、36.4%、46.6%)和不同流量(0.2 Ls~(-1)、0.4 Ls~(-1)、0.6 Ls~(-1)、0.8 Ls~(-1)、1.0 Ls~(-1))相结合的室内放水冲刷试验,分析表土层、红土层、砂土层、碎屑层土体土壤剥蚀率与水动力学参数之间的关系,初步探讨花岗岩崩岗侵蚀的水动力学机制。结果表明:在一定坡度条件下,土壤剥蚀率随径流流量的增大而增大,且各土层土壤剥蚀率存在很大差异,碎屑层土壤剥蚀率最大,砂土层次之,表土层最小;在相同流量条件下,各土层土壤剥蚀率均随冲刷时间的延长逐渐降低并趋于稳定;径流剪切力、水流功率对崩岗各土层土壤剥蚀率的影响均可采用线性方程很好地描述(R~20.926),相比用单位水流功率拟合的多项式方程的相关性(R0.830)要高,径流剪切力和水流功率均可作为描述崩岗各土层土壤侵蚀的水动力学参数。表土层、红土层、砂土层、碎屑层的临界径流剪切力依次减小,分别为0.28Pa、0.13Pa、0.10Pa、0.07Pa,各土层土壤细沟可蚀性参数差异明显,碎屑层的最大,砂土层次之,表土层最小。因此,在崩岗垂直结构上,随着土层深度的增加,土体抵抗径流剥蚀的能力逐渐减弱。     

基于EFDC的潘家口水库富营养化模拟

基于EFDC建立潘家口水库水动力与富营养化模型,模拟了2006 ～ 2007年潘家口水库内的水动力、营养物质循环以及藻类生长代谢,并对模型进行了校准与验证.结果表明,TN的模拟效果最好,最大相对误差为4.97％;TP的模拟效果次之,最大相对误差为-15.77％;而Chl-a的模拟效果最差,最大相对误差18.29％.模拟与实测结果对比表明,基于EFDC建立的潘家口水库富营养化模型,可以比较真实地反映库区内营养物质的循环以及藻类的生长代谢过程.     

四大家鱼产卵水文水动力特性研究综述

四大家鱼是中国重要的经济鱼类,长江是四大家鱼的天然种质库,长江四大家鱼的自然繁殖数量对四大家鱼的资源保护意义重大.四大家鱼产卵与河流水文水动力关系密切.较为全面的阐述了四大家鱼产卵与水温、涨水过程的关系,并对四大家鱼产卵场的分布和水动力特性进行了简要描述.本工作将为针对四大家鱼产卵的水库生态凋度和四大家鱼产卵场修复提供一定技术支持.     

Modeling hydrodynamics and path/residence time of aquaculture-like particles in a mixed-cell raceway (MCR) using 3D computational fluid dynamics (CFD)

A three-dimensional (3D) simulation of a commercial-size mixed-cell raceway (MCR) was performed using computational fluid dynamics (CFD). Hydrodynamics of the MCR were fully characterized and validated against experimental data. A simulation of path and residence time of aquaculture-like particles was also conducted to understand solids removal mechanisms and potential efficiencies within a mixed-cell. A structured 3D grid model of one mixed-cell was constructed using 250,000 hexahedral elements, and simulations were conducted using the Realizable k–ɛ turbulence model. From the results of the CFD simulations, average cell rotational velocities, radial velocity profiles, and velocity contour and vector plots were generated at three different water depths. As observed in experimental trials conducted by the authors, predicted velocity contours and vector plots revealed the development of a strong rotational flow in each mixed-cell with lower velocity zones at the center and corners of the cells. Also, a linear trend of increasing rotational velocities from the cell's center to its circumferential perimeter was described. Good fluid-flow pattern agreement was observed between the experimental and predicted vector and contour plots. Average rotational velocities at the bottom (95 cm depth), middle (50 cm depth), and top (20 cm depth) planes of the mixed-cell (1-m water depth) were 17.2, 14.9, and 13.7 cm/s, respectively. Comparison of these velocities with the observed values revealed an overall agreement of nearly 96%. Similarly, when the average velocities at radial distances of the MCR were compared at the three depths, the correlation between the experimental and predicted data was on the range of 89–95%.Simulations of aquaculture-like particles trajectories revealed that roughly 100% of particles larger than 500 μm settled and were removed in less than 15 min through the mixed-cell’ central drain. Also, 100% of the 100-μm particles were removed within one mixed-cell hydraulic retention time (HRT) cycle, with approximately half being removed by the bottom-center drain and half being removed via the upper-side drains. Only 50% of the 10-μm particles was removed; and this mainly occurred through the upper-side drains of the mixed-cell after 17 HRT cycles. Smaller particles appeared to stay in the tank for an indefinite period of time.     

生态调度专刊：中华倒刺鲃在人工塑造生境条件下的栖息偏好研究

为了促进受水电工程影响河段鱼类资源的恢复,以中华倒刺鲃为例,依托大型生态试验场塑造多种近自然微地形生境,研究鱼类的栖息行为偏好特性,营造适宜鱼类栖息的水文水动力环境。采用PIT射频识别系统实时监测鱼类栖息行为,选取流速、涡量、湍流动能和床体切应力四个水动力指标计算了对应流场条件下的中华倒刺鲃栖息适宜度,并基于随机森林和CART算法提出了影响中华倒刺鲃栖息选择的主要水动力指标,分析揭示了其栖息行为的水动力选择机制。结果表明：深潭浅滩及沙洲河段的进出口是中华倒刺鲃的喜好栖息场所,涡量和流速是影响中华倒刺鲃栖息的主要水动力指标。流速高于0.545m/s低于2.3m/s,涡量高于0.72m_-1^低于15.7m_-1^时的流场条件为试验工况下中华倒刺鲃的最适宜栖息环境。研究为鱼类栖息地生态修复及生态调度工作提供了一定的理论依据和技术支撑。     

Performance and impact on the seabed of an existing- and an experimental-otterboard: Comparison between model testing and full-scale sea trials

A new door has been designed to reduce hydrodynamic drag coefficient and increase spread of door commonly used in the Mediterranean commercial demersal trawl fisheries. Flume tank testing and engineering sea trials provide data which allow us to illustrate the performance and impact on the seabed of an existing door and a new door design. In the flume tank, each model was tested over a range of attack angles and for a limited range of otterboard heels. Curves of spreading-, drag- and down-force coefficients have been calculated. In the case of sea trials in order to extract the hydrodynamic coefficients an analysis has been applied and a mathematical model was used to calculate attack angle functions. From analysing the differences between engineering sea trials and flume tank tests we have deduced some conclusions about additional ground contact forces on sea trials that affect the performance of the doors. Moreover, a comparison between reaction forces of the flume tank and the estimation of reaction forces at sea has been given. Finally, this study allowed us to notice important differences between traditional and experimental otterboards.     

采用SPH方法的黏土切削特性分析

黏土在切削破坏过程中力学特性复杂。为较精确地描述黏土切削破坏过程与预估切削阻力，该研究基于弹塑性力学本构，建立了光滑粒子流体动力学（SPH）框架下的黏土剪切破坏模型和Grady-Kipp张拉损伤破坏模型，并进一步将其耦合得到混合破坏模型。为验证该混合破坏模型开展了简单的黏土切削破坏试验，切削破坏变形的试验结果与模拟结果展示了良好的一致性，裂纹发展方向和裂纹形状有着良好的对应关系。在土-切削部件互作模型的基础上，进一步预测了黏土切削破坏过程中切削部件的切削阻力演化过程，并分析了切削深度及切削部件摩擦系数对黏土切削破坏形态和切削阻力的影响。结果表明，切削深度为0.01、0.02和0.03 m时对应的切削阻力峰值分别为7.89、9.90和11.07 N，切削阻力随切削深度的增加呈增大趋势。切削部件摩擦系数为0.1时对应的波动平缓后的切削阻力最小，为6.1 N。研究可为优化农业机械刀具的结构参数、运动参数，降低作业功耗，减少切削阻力，实现更好的黏土耕作效果提供参考。