Five-equation and robust three-equation methods for solution verification of large eddy simulation

This study evaluates the recently developed general framework for solution verification methods for large eddy simulation(LES) using implicitly filtered LES of periodic channel flows at friction Reynolds number of 395 on eight systematically refined grids.The seven-equation method shows that the coupling error based on Hypothesis I is much smaller as compared with the numerical and modeling errors and therefore can be neglected. The authors recommend five-equation method based on Hypothesis II, which shows a monotonic convergence behavior of the predicted numerical benchmark(S_C), and provides realistic error estimates without the need of fixing the orders of accuracy for either numerical or modeling errors. Based on the results from seven-equation and five-equation methods, less expensive three and four-equation methods for practical LES applications were derived. It was found that the new three-equation method is robust as it can be applied to any convergence types and reasonably predict the error trends. It was also observed that the numerical and modeling errors usually have opposite signs, which suggests error cancellation play an essential role in LES. When Reynolds averaged Navier-Stokes(RANS) based error estimation method is applied, it shows significant error in the prediction of S_C on coarse meshes. However, it predicts reasonable S_C when the grids resolve at least 80% of the total turbulent kinetic energy.     

基于样条修正的河口动力问题大涡模拟模式

该文采用三次样条函数对水位梯度进行修正,建立了珠江伶仃洋河口的大涡模拟湍流模式,通过对1978年7呈20早23点至78年7月22日23点的珠江伶仃洋河口动力过程的模拟表明,建立的模式能准确地模拟潮汐的动力过程,同时,由于三次样条插值能够自然地反映河岸零水位梯度条件,本文的方法能够更清晰地反映出河岸的局部涡结构。     

A new method of LES verification and validation for attached turbulent cavitating flow

The large eddy simulation(LES)is used to resolve the flow structure in the cavitating turbulent flow around the Clark-Y hydrofoil coupled with a homogeneous cavitation model.A new method is proposed in this paper to calculate the LES error of the time-averaged streamwise velocity for the LES verification and validation(V&V).From the instantaneous cavity patterns,it is demonstrated that the predicted results agree fairly well with the experimental data.With this new proposed method,the LES errors can be easily and effectively calculated with a limited mesh number,and the method might be used in the other applications of the LES V&V.Results of the LES errors obtained by the new method show that the relatively steady flow can be simulated with small errors,while the complex flow structures at the cavity shedding region might lead to an increase of errors in the LES modeling.In addition,the distributions of the resolved Reynolds stresses are used to estimate the influences of the cavitation on the turbulent fluctuations.Results indicate that the turbulent fluctuations for the cavitating flow are much larger in magnitude as compared to the cases without cavitation.     

贝叶斯框架下的大坝变形交互式时变预测模型及其验证

大坝变形是同一时刻内外多重风险因素综合作用的结果,应用时序分析方法挖掘历史监测数据潜在规律是变形预测的常用方法,现有时变预测模型不仅参数配置难度高,且难以融入专业知识,导致预测效果并不理想。本文提出一种耦合自动预测算法与大坝专业知识的交互式变形预测模型。该模型在贝叶斯框架下,以加法模型为基础重构各时序分解项作为模型底层,根据仿真结果甄选模型参数缺省值进行自动预测,通过结合参数化检测与直观参数配置实现交互式建模,并借助拟合可视化和统计指标准确反映预测误差来源,从而进一步修正参数以提高模型适用性。基于上述流程协同构建的大坝变形循环预测体系,以某混凝土坝多测点长期变形监测数据为例,对模型的准确性、鲁棒性和灵活性进行了有效验证与分析,为大坝变形安全预测与分析提供了新的模型和手段。     

2-D eddy resolving simulations of flow past a circular array of cylindrical plant stems

In the present study, 2-D large eddy simulations(LES) are conducted for flow past a porous circular array with a solid volume fraction(SVF) of 8.8%, 15.4% and 21.5%. Such simulations are relevant to understanding flow in natural streams and channels containing patches of emerged vegetation. In the simulations discussed in the paper, the porous cylinder of diameter D contains a variable number of identical solid circular cylinders(rigid plant stems) of diameter d= 0.048 D. Most of the simulations are conducted at a Reynolds number of 2 100 based on the diameter D and the velocity of the steady uniform incoming flow. Though in all cases wake billows are shed in the regions where the separated shear layers(SSLs) forming on the sides of the porous cylinder interact, the effect of these wake billows on the mean drag is different. While in the high SVF case(21.5%), the total drag force oscillates quasi-regularly in time, similar to the canonical case of a large solid cylinder, in the cases with a lower SVF the shedding of the wake billows takes place sufficiently far from the cylinder such that the unsteady component of the total drag force is negligible. The mean amplitude of the oscillations of the drag force on the individual cylinders is the largest in a streamwise band centered around the center of the porous cylinder, where the wake to wake interactions are the strongest. In all cases the maximum drag force on the individual cylinders is the largest for the cylinders directly exposed to the flow, but this force is always smaller than the one induced on a small isolated cylinder and the average magnitude of the force on the cylinders directly exposed to the flow decreases monotonically with the increase in the SVF. Predictions of the global drag coefficients, Strouhal numbers associated with the wake vortex shedding and individual forces on the cylinders in the array from the present LES are in very good agreement with those of 2-D direct numerical simulations conducted on finer meshes, which suggests LES is a better option to numerically investigate flow in channels containing canopy patches, given that LES is computationally much less expensive than DNS at high Reynolds number. To prove this point, the paper also discusses results of 2-D LES conducted at a much higher Reynolds number, where the near-wake flow is strongly turbulent. For the higher Reynolds number cases, where the influence of the turbulence model is important, the effect of the sub-grid scale model and the predictive capabilities of the unsteady Reynolds averaged Navier-Stokes(RANS) approach to predict flow past porous cylinders are discussed.     

水文模型不确定性及集合模拟总体框架

水文过程集合模拟将多种确定性和不确定性方法及物理模型和数值模型进行耦合集成,是当前减少水文过程模拟预报不确定性影响的一种有效方法。本文在回顾水文模型发展历程的基础上,从模型输入、模型参数和模型结构三方面的不确定性出发,剖析了水文模型不确定性的原因,最后对水文过程集合模拟的特点、总体框架及主要方法进行了评述。     

某大型综合医院污水处理系统问题与对策

以某大型综合医院污水处理系统为研究对象,分析了其水量变化规律和两种处理工艺的运行效果,研究了其化粪池、曝气生物滤池、生物接触氧化罐等设施存在的主要问题,并提出了相应的技术对策。     

Implicit large eddy simulation of unsteady cloud cavitation around a plane-convex hydrofoil

The present paper focuses on the erosive cavitation behavior around a plane convex hydrofoil. The Zwart-Gerber-Belamri cavitation model is implemented in a library form to be used with the OpenFOAM. The implicit large eddy simulation(ILES) is applied to analyze the three-dimensional unsteady cavitating flow around a plane convex hydrofoil. The numerical results in the cases under the hydrodynamic conditions, which were experimentally tested at the high speed cavitation tunnel of the école Polytechnique Fédérale de Lausanne(EPFL), clearly show the sheet cavitation development, the shedding and the collapse of vapor clouds. It is noted that the cavitation evolutions including the maximum vapor length, the detachment and the oscillation frequency, are captured fairly well. Furthermore, the pressure pulses due to the cavitation development as well as the complex vortex structures are reasonably well predicted. Consequently, it may be concluded that the present numerical method can be used to investigate the unsteady cavitation around hydrofoils with a satisfactory accuracy.     

上翻式拱形闸门的流激振动控制及原型观测验证

通过物理模型和数值模型相结合的方法,系统研究了大跨度上翻式拱形闸门水动力荷载、结构动力特性和流激振动特性,揭示了闸门结构的流激振动强烈共振现象及成因。针对存在的强烈振动的问题,通过减小闸门底缘下游倾斜面水平面投影面积、缩减闸门底部小横梁尺寸等方式来进行结构优化。优化后的工程现场原型观测结果显示,推荐布置方案有效控制了闸门的有害振动,运行平稳,安全可靠,验证了物理模型试验成果的正确性。     

用大涡模拟方法数值模拟Spar平台涡激运动问题

该文采用基于开源代码计算流体力学工具包Open FOAM开发的船舶与海洋工程水动力学求解器naoeFOAM-SJTU,对一座Spar模型的硬舱部分进行了涡激运动的数值模拟。采用大涡模拟方法数值模拟了不同折合速度下Spar模型的涡激运动情况,并与Finnigan等[1,2]的模型试验结果作了对比和分析,证明了求解器naoe-FOAM-SJTU在模拟涡激运动方面的可靠性。     

Verification and validation of URANS simulations of the turbulent cavitating flow around the hydrofoil

In this paper, we investigate the verification and validation(VV) procedures for the URANS simulations of the turbulent cavitating flow around a Clark-Y hydrofoil. The main focus is on the feasibility of various Richardson extrapolation-based uncertainty estimators in the cavitating flow simulation. The unsteady cavitating flow is simulated by a density corrected model(DCM) coupled with the Zwart cavitation model. The estimated uncertainty is used to evaluate the applicability of various uncertainty estimation methods for the cavitating flow simulation. It is shown that the preferred uncertainty estimators include the modified Factor of Safety(FS1), the Factor of Safety(FS) and the Grid Convergence Index(GCI). The distribution of the area without achieving the validation at the U v level shows a strong relationship with the cavitation. Further analysis indicates that the predicted velocity distributions, the transient cavitation patterns and the effects of the vortex stretching are highly influenced by the mesh resolution.     

Numerical modelling of odour dispersion around a cubical obstacle using large eddy simulation

In the present work two different large eddy simulation (LES) approaches, namely the Dynamic Smagorinsky model and the Wale model, are used to simulate the air flow and pollutant dispersion around a cubical obstacle. Results are compared with wind tunnel data (WT) and with results from the Smagorinsky LES model. Overall agreement was good between the different LES approaches and the WT results, both for the mean and fluctuating flow and concentration patterns. LES models can provide good estimates of concentration fluctuation intensity and enable the calculation of the intermittency factor. The model results indicate that LES is a viable tool for odour impact assessment.     

用大涡模拟方法计算尾水管内非定常周期性湍流

本文运用大涡模拟-双方程模型进行尾水管内部非定常周期性湍流计算。而且本文对尾水管复杂计算域的网格划分，提出一种更加合理的多块结构网格。计算结果用波形显示了尾水涡带和压力脉动的主要性质和运动特征。     

长江流域大型水库实施生态调度方法框架研究

为减缓水库运行对河流生态环境的影响,开展生态调度已成为流域管理的一项重要内容。结合长江流域水利工程与生态环境背景,探讨了水库生态调度相关概念和主要调度内容;分析了生态调度与常规兴利调度的关系;从生态影响辨析、生态目标设定、生态流量确定、调度方案设计、经济可行性、生态监测反馈等方面探讨实施生态调度的方法框架;最后探讨长江流域生态调度面临的主要困难。研究结果将为长江流域未来实施生态调度提供一定的参考。     

二维大涡模拟在双流道式污水泵叶轮流场分析中的应用

本文采用高斯型滤波函数对Navier—Stokes方程进行滤波处理，再引入亚格子雷诺应力模型推导出了在水力机械内部流场计算中实用的二维大涡模拟方程，并在贴体坐标系下得到计算平面二维大涡模拟方程及其离散形式，编制了数值计算程序，应用该程序计算了双流道式污水泵叶轮内部流场，得到了满意的速度分布和压力分布规律。在此基础上，我们对双流道式污水泵的叶轮进行了优化设计。     

Three-dimensional large eddy simulation and vorticity analysis of unsteady cavi- taring flow around a twisted hydrofoil

Large Eddy Simulation (LES) was coupled with a mass transfer cavitation model to predict unsteady 3-D turbulent cavitating flows around a twisted hydrofoil. The wall-adapting local eddy-viscosity (WALE) model was used to give the Sub-Grid Scale (SGS) stress term. The predicted 3-D cavitation evolutions, including the cavity growth, break-off and collapse downstream, and the shedding cycle as well as its frequency agree fairly well with experimental results. The mechanism for the interactions between the cavitation and the vortices was discussed based on the analysis of the vorticity transport equation related to the vortex stretching, volumetric expansion/contraction and baroclinic torque terms along the hydrofoil mid-plane. The vortical flow analysis demonstrates that cavitation promotes the vortex production and the flow unsteadiness. In non-cavitation conditions, the streamline smoothly passes along the upper wall of the hydrofoil with no boundary layer separation and the boundary layer is thin and attached to the foil except at the trailing edge. With decreasing cavitation number, the present case has σ= 1.07, and the attached sheet cavitation becomes highly unsteady, with periodic growth and break-off to form the cavitation cloud. The expansion due to cavitation induces boundary layer separation and significantly increases the vorticity magnitude at the cavity interface. A detailed analysis using the vorticity transport equation shows that the cavitation accelerates the vortex stretching and dilatation and increases the baroclinic torque as the major source of vorticity generation. Examination of the flow field shows that the vortex dilatation and baroclinic torque terms increase in the cavitating case to the same magnitude as the vortex stretching term, while for the non-cavitating case these two terms are zero.     

A framework for understanding energy for water

Abstract

This article offers a framework for understanding how energy is used to meet water demand in countries. Specifically, the relationships between energy use and water scarcity, the location of renewable water resources, and aggregate water demand are explored. The article also examines how policy options such as water price reforms, agriculture subsidies and crop elimination may influence the energy use and energy intensity of water withdrawals. Conclusions suggest that while policy options exist, certain uncontrollable factors such as severe water scarcity or substantial freshwater abundance limit the ability of some countries to significantly improve the aggregate energy efficiency of water provision.     

大中型水电站工程物资核销模式及重点探讨

由业主统一提供主材是国内大中型水电站工程物资供应管理的主流模式。业主供材通常数量庞大,对工程质量及造价的影响也十分显著。为此,必须建立统供物资流向规范管控机制,而物资核销就是其中不可或缺的重要环节及内容。从锦屏水电站物资核销实践出发,对物资核销的缘起、特点、模式以及重点等方面进行梳理分析,提出了核销工作需要重点关注并着力解决的主要问题。研究结果对水电站物资核销管理有一定的借鉴意义。