The Role of Mangroves in Retaining Penaeid Prawn Larvae in Klang Strait,Malaysia

The mangrove-fringed Klang Strait, Malaysia, retains approximately 65 billion penaeid prawn larvae annually prior to their settlement in coastal nursery grounds. This phenomenon appears to be due principally to tidal currents and lateral trapping in mangrove-fringed channels, the wind playing an insignificant role.     

黄土坡面细沟侵蚀能力的水动力学试验研究

建立以水动力学为基础的侵蚀预报模型,必须解决影响土壤侵蚀发生过程及其强度的各水力要素的合理计算问题。本文通过室内径流冲刷试验,研究了黄土坡国沟侵蚀发生的水动力学机理及其输沙特征,探讨了细沟侵蚀预报模型中,细沟侵蚀民涉衣的土壤及有关径流的水力参数;土壤侵蚀性系数,细沟侵蚀发生的临界切应力,细沟径流的平均流速,Ｍａｎｎｎｉｎｇ糙率系数,以及反映细沟断面形态特征的系数的估算问题。     

A prediction on structural stress and deformation of fish cage in waves using machine-learning method

Generally, the hydrodynamics of a fish cage are investigated using numerical simulation, physical model experiments, and field measurements. However, these traditional research methods are time consuming and low in efficiency. In this study, an artificial neural network (ANN) model is built such that the hydrodynamic characteristics of a fish cage in waves can be predicted rapidly. The training data of the ANN model are generated by a well-developed numerical model from our previous studies. The parameters in the hidden layer of the ANN model are determined considering the prediction accuracy of the hydrodynamic results of the fish cage. The ANN model is validated against a well-developed numerical model with satisfactory accuracy. Using the proposed ANN model, the hydrodynamic results of the fish cage including the maximum tension in mooring lines, minimum effective-volume ratio, and maximum stress of the floating collar are predicted for various waves. Overall, the predicted results indicate a trend consistent with that of the previous studies. The present model can potentially forecast disasters for an oncoming typhoon, which is important for the hazard prevention of a fish farm.     

A method for the quantification and optimization of hydrodynamics in culture tanks

A method was developed to quantify hydrodynamic mixing parameters, and to optimize the physical environmental conditions, in culture tanks. Improved mixing will result in better tank water quality, more efficient use of available volume by the culture animals (leading to optimal stocking densities and better feed management) and possibly reduced water pumping requirements. Experiments were conducted to determine the influence of a range of flow rates, residence times, water depths and stocking densities on hydrodynamics in juvenile turbot (Scophthalmus maximus (L.)) tanks. Decreases in water depth resulted in significant improvements in mixing and the efficiency with which the water was used, as indicated by reductions in dead volumes. A depth of less than 9.4 cm at a flow rate of 2 l min^–1 was expected to minimize dead volumes in the tank. This indicated that mixing was better in shallower tanks. Within the range 0–13 l min^–1, increased flow rate improved mixing at a constant depth of 9 cm (and water volume of 18.54 l) though increased flow rates greater than about 2.5 l min^–1 produced only small improvements in mixing. Within the range 0–50 fish per tank (equivalent to a mean stocking density of 0–1.84 kg m^–2), stocking density did not significantly influence mixing in tanks with a depth of 9 cm and flow rate of 2 l min^–1. Such depth reductions, for demersal species, may be a useful means to either decrease water use without reducing residence time, or alternatively to increase the flushing rate without increasing water use, at a given stocking density. The large changes in the efficiency with which the tanks were used, which were achieved with ease, indicates that attention to water mixing can give positive benefits to a wide range of land-based farm operators. Care must be taken when adjusting tank hydrodynamics, that water quality is maintained and that biological parameters such as stress levels, sunlight effects and feed management are optimal.     

导流盘对循环水养殖池水动力特性的影响

为提高循环水养殖系统的集污自净化效能,设计了一种带导流盘的正八边形养殖池结构。建立了三维非定常流场数值计算模型,在验证计算方法有效性的基础上,研究了导流盘的几何参数和位置对流速分布、涡量强度、壁面剪切应力和水体混合均匀性等水动力特性影响。结果表明：在导流盘直径与养殖池宽度之比一定时,随着导流盘高度增加,流场中心涡柱呈先减小后增大趋势, 涡环数呈先减小再增加趋势,其形状从不规则逐渐发展成环状结构;在同一高度下,随着导流盘直径与养殖池宽度之比增大,涡柱呈先增大后减小趋势, 涡环的数量先增加后基本保持不变;导流盘安装在距池底20~40 mm,直径与养殖池宽度之比为0.05~0.08时,有助于增大养殖池底流口附近的速度梯度,减小养殖池壁面与水体的碰撞,提高涡流强度和水力混合均匀性;有利于形成颗粒物聚集和排出的水动力条件。     

Engineering procedures for design and analysis of submersible fish cages with copper netting for exposed marine environment

Novel design and analysis procedures are needed for engineering of the offshore fish farms utilizing copper alloy netting. Existing technologies developed for fish cages with polymer nets are not directly transferrable to the fish cages with copper netting. In particular, the structural integrity of fish cage/mooring systems, and initial high costs of fish cages with copper netting are two major concerns in the engineering for open ocean aquaculture. We propose the modified engineering procedures, which address these concerns and allow for retrofitting of existing fish farming systems with polymer nets. These procedures are illustrated by considering two case studies: design of a rigid-frame and flexible gravity-type fish cages. Performance of both designs is analyzed after the field trials in the North Atlantic and South Pacific oceans, correspondingly.     

花道泡水质模拟预测研究

Delft-3D是由荷兰水力研究院（Delft Hydraulics）开发设计的三维或二维可视水动力一水质联合模拟软件系统,可全面仿真水域的三维或二维循环水流水质情况。该模型不仅考虑了复杂的流体力学问题和输送过程,还考虑到众多与有关水体相关的物理、化学和生物的相互作用,使仿真结果较先前使用的水质及水力模型仿真套件所作的预测更为可靠和准确。该研究以Dent-3D软件为技术平台,对饮用水水源地花道泡引水初期的水质进行了模拟预测,初步确定了水体适合农业灌溉和生活饮用的时间,并且给当地的决策部门提出了一些建议。     

Applications of computational fluid dynamics to modeling hydrodynamics in tilapia rearing tank of Recirculating Biofloc Technology system

Biofloc Technology (BFT) has been widely used in tilapia rearing. However, the Total Suspended Solids (TSS) at the bottom of rearing tank will increase to high concentration due to biofloc sedimentation and accumulation. Therefore, it will be have negative effects on the feeding enthusiasm, gill and even survival of cultured species, and especially in the initial stages of larvae culture. The Recirculating Biofloc Technology (RBFT) would be one of the potential solution for the above concerns. To achieve an accurate TSS regulation, the biofloc distribution should be as homogenous as possible. The aim of this work is to specify the optimal bubble size and Hydraulic Retention Time (HRT) to the uniformity of biofloc distribution in a RBFT system through Computational Fluid Dynamics (CFD) method. For this, a three-dimensional and three-phase unsteady transient model was developed to simulate the hydrodynamics in a gas-liquid-solid tilapia rearing tank based on the commercial software Workbenching 15.0. Firstly, to achieve a more reliable prediction, the influence of the grid types and the computation turbulence model (Standard, Renormalization-Group (RNG), Realizable) to the simulation result were discussed by contrasting the simulations and experiments results of solid holdup at dimensionless radial position. Secondly, appropriate mesh size (181395 elements) and mixture standard k-e model were implemented to study the effect of bubble size and HRT on solid distribution uniformity. Simulations were performed by using three different mean bubble sizes (diameter = 1, 2, 3 mm) and three different HRTs (0.56, 1.13, and 2.25 h) to study the sensitivity of the results to the uniformity of the biofloc distribution, respectively. The results showed that to get a better biofloc distribution, the bubble size should be range in 1 mm and 2 mm, and as the HRT decreasing, the distribution of biofloc distribution will be more homogeneous (0.45 ≤ HRT ≤ 0.56 h). This paper provides an essential data set for determining the bubble size and HRT in production, as well as evaluating the accuracy of various CFD models for capturing the complex flow field in a BFT rearing tank.     

EN—1对砒砂岩固化土坡面径流水动力学特征的影响

为探讨砒砂岩地区工程边坡径流的侵蚀规律,通过室内模拟冲刷试验,对添加EN-1固化剂后不同固化剂掺量、养护龄期和压实度的边坡径流水动力学特征进行了研究.结果表明:在试验设计的固化剂掺量、养护龄期和压实度处理条件下,砒砂岩固化土坡面径流流速随固化剂掺量、养护龄期和压实度的增大而增大,且径流流速随冲刷的进行呈现出波动减小的趋势.径流雷诺数Re随固化剂掺量、养护龄期和压实度的增大而减小,且Re有随冲刷的进行而逐渐增大的波动变化趋势,而弗劳德数Fr的变化趋势正好与Re相反.除压实度在较小处理冲刷后期时径流流态为紊流和缓流外,其他处理都是层流和急流.径流Darcy - Weisbach阻力系数f随固化剂掺量、养护龄期和压实度增大而减小,且f与输沙率有一定的正相关关系.     

Numerical study of dense solid-liquid flow in hydrodynamic vortex separator applied in recirculating biofloc technology system

Hydrocyclone separation technique has commonly been applied in engineering for solid-liquid separation. In aquaculture, since Hydrodynamic Vortex Separator (HDVS) is able to control flocs concentration effectively, the Recirculating Biofloc Technology (RBFT) has been gradually acknowledged (Zhu et al., 2016). In order to operate RBFT system at maximum performance, the removal efficiency of HDVS at different Hydraulic Retention Time (HRT) must be fully predictable. The aim of this work is to investigate the relationship between separation efficiency and HRT of HDVS applied in RBFT system, through Computational Fluid Dynamics (CFD) method. For this, a three-dimensional unsteady transient model was developed to simulate the hydrodynamics in HDVS based on the commercial software Workbenching 17.0. A Two Fluid Model (TFM) using kinetic theory of granular flow (KTGF) has been developed to describe the dense solid-liquid (flocs-water) flow field of HDVS in RBFT system. Additionally, the Reynolds averaged Navier-Stokes (RANS) equations with Reynolds stress turbulence model (RSM) are solved by use of the finite volume method based on the SIMPLE pressure correction algorithm in the computational domain. Finally, pilot-scale studies were conducted to evaluate the accuracy and precision of simulation models applied. The results indicate that the flocs removal efficiency of HDVS obtained from tests were less than that from the simulation results at three different HRTs. But it decreased with the decline of HRT both for experiment and simulation. Additionally, the CFD model proposed can enhance the fundamental understanding of the effects of HRT on separation efficiency and at the same time, probably coupled with physical experiment whenever necessary, offer a more reliable way for optimum control of HDVS under varying conditions. Based on the simulation results, the flocs management in RBFT system is briefly discussed.