IMPROVED SUBGRID SCALE MODEL FOR DENSE TURBULENT SOLID-LIQUID TWO-PHASE FLOWS

The dense solid-phase governing equations for two-phase flows are obtained by using the kinetic theory of gas molecules.Assuming that the solid-phase velocity distributions obey the Maxwell equations,the collision term for particles under dense two-phase flow conditions is also derived. In comparison with the governing equations of a dilute two-phase flow,the solid-particle's governing equations are developed for a dense turbulent solid-liquid flow by adopting some relevant terms from the dilute two-phase governing equations.Based on Cauchy-Helmholtz theorem and Smagorinsky model, a second-order dynamic sub-grid-scale(SGS)model,in which the sub-grid-scale stress is a function of both the strain-rate tensor and the rotation-rate tensor,is proposed to model the two-phase governing equations by applying dimension analyses.Applying the SIMPLEC algorithm and staggering grid system to the two-phase discretized governing equations and employing the slip boundary conditions on the walls,the velocity and pressure fields,and the volumetric concentration are calculated.The simulation results are in a fairly good agreement with experimental data in two operating cases in a conduit with a rectangular cross-section and these comparisons imply that these models are practical.

Flow structures inside a large-scale turbulent fluidized bed of FCC particles:Eulerian simulation with an EMMS-based sub-grid scale model

Turbulent fluidized bed reactors are widely used in industry. However, CFD simulations of the hydrody- namic characteristics of these reactors are relatively sparse, despite the urgent demand from industry. To address this problem, Eulerian simulations with an EMMS-based sub-grid scale model, accounting for the effect of sub-grid scale structures on the inter-phase friction, are performed to study the hydrodynamics inside a large-scale turbulent fluidized bed of FCC particles. It is shown that the simulated axial and radial solid concentration profiles, entrained solid fluxes and standard deviation of the solid concentration fluc- tuation agreed well with experimental data available in the literature. In-depth analysis of time-averaged particle velocity and solid concentration shows that a dense-suspension upflow regime coexists with fast fluidization regime in this bed, which is reminiscent of the hydrodynamic characteristics in high-density circulating fluidized bed （CFB） risers, even though they are operated in different fluidization regimes. The Reynolds stresses in turbulent fluidized beds are anisotropic, but the degree of anisotropy is far less pro- nounced than the reported values in CFB risers. It was also found that the solid concentration fluctuation and axial particle velocity fluctuation are strongly correlated. 2009 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.

Solving Schrodinger Equation with Non-Uniform Grids by Scale Transformation Method

A new scale transformation method is used in solving the Schrodinger equation. With it, the uniform grids in the discretization in conventional metho d are changed into non-uniform grids. Consequently, in some cases, the computing quantity will be greatly reduced at keeping the required accuracy. The calcul ation of the quantized inversion layer in MOS structure is used to demonstrate t he efficiency of the new method.

Equality Testing for Soil Grid Unit Resolutions to Polygon Unit Scales with DNDC Modeling of Regional SOC Pools

Matching soil grid unit resolutions with polygon unit map scales is important to minimize the uncertainty of regional soil organic carbon(SOC) pool simulation due to their strong influences on the modeling.A series of soil grid units at varying cell sizes was derived from soil polygon units at six map scales,namely,1:50 000(C5),1:200 000(D2),1:500 000(P5),1:1 000 000(N1),1:4 000 000(N4) and 1:14 000 000(N14),in the Taihu Region of China.Both soil unit formats were used for regional SOC pool simulation with a De Nitrification-DeC omposition(DNDC) process-based model,which spans the time period from 1982 to 2000 at the six map scales.Four indices,namely,soil type number(STN),area(AREA),average SOC density(ASOCD) and total SOC stocks(SOCS) of surface paddy soils that were simulated by the DNDC,were distinguished from all these soil polygon and grid units.Subjecting to the four index values(IV) from the parent polygon units,the variations in an index value(VIV,%) from the grid units were used to assess its dataset accuracy and redundancy,which reflects the uncertainty in the simulation of SOC pools.Optimal soil grid unit resolutions were generated and suggested for the DNDC simulation of regional SOC pools,matching their respective soil polygon unit map scales.With these optimal raster resolutions,the soil grid units datasets can have the same accuracy as their parent polygon units datasets without any redundancy,when VIV < 1% was assumed to be a criterion for all four indices.A quadratic curve regression model,namely,y = – 0.80 × 10^(–6)x^2 + 0.0228 x + 0.0211(R^2 = 0.9994,P < 0.05),and a power function model R? = 10.394?^(0.2153)(R^2 = 0.9759,P < 0.05) were revealed,which describe the relationship between the optimal soil grid unit resolution(y,km) and soil polygon unit map scale(1:10 000x),the ratio(R?,%) of the optimal soil grid size to average polygon patch size(?,km^2) and the ?,with the highest R^2 among different mathematical regressions,respectively.This knowledge may facilitate the grid partitioning of regions during the investigation and simulation of SOC pool dynamics at a certain map scale,and be referenced to other landscape polygon patches' mesh partition.

New Technology for Grids and Scales Manufacturing in Optical Devices

Using the laser controlled thermocracking method, research results for the new technology of optical grids and scales manufacturing are given in this paper. The opportunity of grids and scales manufacturing is shown for a wide range of the sizes, scale’s pitches and its width: from 10 nanometers up to 10 microns with a backlight in various optical ranges.

Research on Control Method of Inverters for Large-scale Grid Connected Photovoltaic Power System

A grid-connected inverter controlling method to analyze dynamic process of large-scale and grid-connected photovoltaic power station is proposed. The reference values of control variables are composed of maximum power which is the output of the photovoltaic array of the photovoltaic power plant, and power factor specified by dispatching, the control strategy of dynamic feedback linearization is adopted. Nonlinear decoupling controller is designed for realizing decoupling control of active and reactive power. The cascade PI regulation is proposed to avoid inaccurate parameter estimation which generates the system static error. Simulation is carried out based on the simplified power system with large-scale photovoltaic plant modelling, and the power factor, solar radiation strength, and bus fault are considered for the further research. It’s demonstrated that the parameter adjustment of PI controller is simple and convenient, dynamic response of system is transient, and the stability of the inverter control is verified.

Analysis on the Effect of Shading on the Characteristics of Large-scale on-grid PV System in China

Because of rich solar resource and low land cost, a lot of large-scale ground-based grid-connected PV systems have been built in Northwest China. In this paper, some shading phenomena on a grid-connected PV system inNorthwest Chinaare classified and analyzed. Through the I-V curve test of PV modules, it can be seen that dust influence system performance of the grid-connected PV system. And the experimental results have shown that shading could affect the electrical properties of PV modules. Meanwhile, same shading area on different shading positions could have different impacts on the identical PV module.

Stability Enhancement of Small-Scale Power Grid with Renewable Power Sources by Variable Speed Diesel Power Plant

This paper proposes a power control method to improve a stability of a small-scale power grid with renewable energy sources. In an isolated small- scale power grid such as an island, diesel power plant is main power source which has an environmental burden and expensive running cost due to high priced fossil fuel. Thus, expanding installation of the renewable energy sources such as a wind power is strongly desirable. Such fluctuating energy sources, however, harm power quality of the small-scale power grid, and in addition, conventional power plant in the small-scale power grid cannot, in general, stabilize the grid system with such fluctuating power sources. In this study, Variable Speed Doubly-Fed Induction Generator (VS-DFIG) is proposed to be in-stalled at a diesel power plant instead of a conventional Fixed Speed Synchronous Generator (FS-SG), because quick control of a power balance in the small-scale power grid can be achieved by using the inertial energy of VS-DFIG. In addition, utilization of a Battery Energy Storage System (BESS) is also considered to assist cooperatively the VS-DFIG control. As a result of the simulation analysis by using the proposed method, it is verified that frequency fluctuations due to renewable energy source can be effectively reduced by quick power control of the VS-DFIG compared to the conventional FS-SG, and further control ability can be obtained by utilizing BESS. Moreover, the transient stability of a small-scale power grid during a grid fault can also be enhanced.

Impacts of the Thermal Effects of Sub-grid Orography on the Heavy Rainfall Events Along the Yangtze River Valley in 1991

A P - σ regional climate model using a parameterization scheme to account for the thermal effects of the sub-grid scale orography was used to simulate the three heavy rainfall events that occurred within the Yangtze River Valley during the mei-yu period of 1991. The simulation results showed that by considering the sub-grid scale topography scheme, one can significantly improve the performance of the model for simulating the rainfall distribution and intensity during these three heavy rainfall events, most especially the second and third. It was also discovered that the rainfall was mainly due to convective precipitation. The comparison between experiments, either with and without the sub-grid scale topography scheme, showed that the model using the scheme reproduced the convergence intensity and distribution at the 850 hPa level and the ascending motion and moisture convergence center located at 500 hPa over the Yangtze River valley. However, some deviations still exist in the simulation of the atmospheric moisture content, the convergence distribution and the moisture transportation route, which mainly result in lower simulated precipitation levels. Further analysis of the simulation results demonstrated that the sub-grid topography scheme modified the distribution of the surface energy budget components, especially at the south and southwest edges of the Tibetan Plateau, leading to the development and eastward propagation of the negative geopotential height difference and positive temperature-lapse rate difference at 700 hPa, which possibly led to an improved precipitation simulation over eastern China.

计及锁相环非线性特性的跟网型逆变器暂态稳定性分析

锁相环作为跟网型逆变器实现相位同步的关键环节,由于其本身所固有的非线性特性,在弱电网情况下,传统小信号模型可能存在不能准确预测锁相环暂态稳定性的问题。为保证受扰后锁相环的稳定运行,提出了一种基于非线性数学模型的稳定性分析方法。首先,根据锁相环工作原理,建立其二阶非线性微分方程,通过相平面法对锁相环输出特性进行定性分析,评估不同参数对锁相环暂态特性的影响;然后,基于等面积法则分析锁相环暂态失稳机理,鉴于传统等面积法则可能导致稳定性的误判,采用非线性动力学多尺度法对锁相环受扰后的暂态时域表达式进行求解,明确锁相环暂态稳定边界;最后,通过Matlab仿真和RT⁃LAB半实物平台验证了所得解析解和锁相环暂态稳定域的准确性,为弱电网条件下锁相环控制参数及电路参数选取提供一定参考。

Study on Computational Properties of Several VerticalGrids with a Nonhydrostatic Model in Comparisonto Analytical Solutions

Starting from nonhydrostatic anelastic equations a comparative investigation is performed of inertial-gravitational wave hundreds, tens and a few of kilometers in horizontal wavelength, which are ex amined on vertical grids available at present from the perspectives of frequency, vertical component of group velocity and the inappropriate range for a positive vertical component emerging, with the findings compared to analytical solutions. Evidence suggests that grids C’P and LZ are suitable for the study of the wave at the mentioned horizontal scales and the counterparts L and LY (LTS and CPTS) are applicable on ly to the horizontal scales of more (less) than lens of kilometers.

不均匀积雪致大跨网架结构局部损伤分析

网架结构管球连接处局部损伤是此类结构的重要安全隐患。在风致雪漂移引起的不均匀积雪荷载作用下,局部损伤会加剧,最终导致结构局部破坏。因此很有必要分析不均匀积雪荷载作用下带伤服役网架结构局部损伤劣化情况。采用CFD数值模拟技术,以一正放四角锥网架结构为研究背景,分析了在90°风向角、12 m/s风速下,持续降雪24 h中网架结构屋面不均匀积雪分布的变化情况,并建立可表征网架结构管球连接处存在裂纹损伤的多尺度模型,分析了网架结构节点存在不同裂纹尺寸局部损伤时,不均匀积雪致网架结构局部损伤劣化程度。结果表明:网架结构风致积雪不均匀程度非常显著,而且当网架结构管球连接处存在局部裂纹损伤时,在降雪中后期,有管球连接损伤的节点大多都出现了不同程度的裂纹扩展,节点为最不利分布的穿透型裂纹时,裂纹扩展最大为15.64 mm。说明在持续特大降雪这种极端荷载作用下,带伤服役网架结构局部损伤将进一步加剧,危及结构使用安全。

西安都市圈生态系统服务价值与人类活动强度的时空关系

人类活动是影响生态系统服务价值(ecosystem service value,ESV)变化的重要原因之一,对两者时空关系的研究有助于生态环境与社会经济协调发展。以西安都市圈为研究对象,基于1990、2000、2010和2018年4期土地利用数据,在2 km×2 km格网尺度下构建ESV评估模型、ESV流向损益模型、人类活动强度(human activity intensity,HAI)指数评估模型和双变量空间自相关模型,探讨ESV与人类活动强度的时空关系。结果表明:(1)1990—2018年间,西安都市圈ESV在时间尺度上呈持续下降趋势,近30年间共降低12.8亿元;在空间尺度上呈现以西安主城区为中心向外围递增的分布格局;(2)ESV增益主要源于耕地转为林地、水域和草地;ESV亏损主要源于耕地、建设用地的转入以及水域的转出;(3)人类活动强度整体呈现以西安主城区为中心向南北递减的空间分布格局,高影响区集中分布在西安主城区及其周边地区,低影响区分布于研究区南北两端;(4)ESV与人类活动强度呈负相关关系,两者聚类关系以低-高和高-低聚集方式为主,低-高和高-低聚集区域显著性水平高。期望通过对ESV与人类活动强度的空间关系分析为区域可持续发展提供理论支持。

文丘里喷嘴空化流大涡模拟亚格子模型对比研究

针对文丘里喷嘴不同机制下可压缩空化流开展了大涡模拟(LES)研究,并对比讨论了3种常用的亚格子模型的适用性。结果显示,各模型在预测回射流机制和凝结激波机制的流速与空化云脱落周期上表现良好。在空化云演化过程中,WALE模型在两种机制下的模拟吻合度最佳,SL模型预测溃灭过程提前,蒸汽相体积分数偏小;KET模型吻合度最差,溃灭过程以及周期内脱落存在时间延迟。初步发现回射流机制下的喉部压力功率谱密度分析服从-5/3标度律,凝结激波机制服从-7/3标度律。

基于动态格栅的湍流风实验模拟方法研究

开发一种基于动态格栅的湍流风实验模拟方法和装置,并通过调节格栅运动参数和来流风速,实现对湍流风特征参数的定量调控。实验分别研究罗斯贝数Ro、格栅雷诺数Re_(M)、格栅运动角加速度α、格栅运动周期长度T对湍流参数的影响规律。结果表明,湍流参数仅对罗斯贝数敏感,平均风速与湍流度均随罗斯贝数增加而增大,而湍流积分尺度则呈现先减小后增大的趋势。综合实验工况参数,得到湍流强度可调节范围为0.05~0.39,湍流积分尺度可调节范围为0.15~1.56 m,适用于缩尺比1∶20~1∶1200范围的风电机组模型实验。

交直流电网多时间尺度暂态仿真接口综述

为满足大规模交直流电网多时间尺度动态过程的准确、高效仿真需求,已有研究结合不同仿真建模理论深入讨论了多时间尺度分区仿真方法。对于多时间尺度分区仿真,不同分区间的仿真接口是决定仿真成败的关键。首先,分析了交直流电网的多时间尺度特性及分区多时间尺度仿真的分类。然后,梳理了机电-电磁暂态离线仿真接口设计的关键问题。进一步,归纳了多速率电磁暂态仿真接口和移频-常规电磁暂态混合仿真接口的研究现状。最后,对多时间尺度分区协同仿真接口设计的未来研究方向进行了展望。

泵喷推进器间隙流场尺度效应数值研究

由于泵喷推进器转子与导管之间有间隙的存在,内流场特性较复杂.为了探究尺度效应对泵喷推进器间隙流场的影响,采用非定常雷诺平均Navier-Stokes(URANS)方程和SST k-ω湍流模型,利用全结构化网格对计算域进行离散处理,滑移网格用于处理转子和其他部件之间的相对运动.首先通过网格不确定分析来验证本文数值方法的可行性,数值计算结果与模型试验数据进行了比较,两者吻合良好;然后对3种尺度泵喷推进器模型的敞水性能进行了数值计算,从涡量场和压力场的角度进行了分析.结果表明,全进速系数下,实尺度模型的效率会提高;实尺度模型的叶梢泄漏涡(TLV)涡量溃灭提前,涡量强度更低,并且TLV涡核中心压力系数更小,间隙间的脉动压力幅值更低.

基于XGBoost模型的多尺度武装冲突风险预测——以巴基斯坦为例

全球范围内的武装冲突已经成为国际社会不容忽视的焦点,如何结合多源地理空间数据研究,从更加精细的空间尺度上预测冲突爆发风险是亟需解决的问题。以巴基斯坦为例,提出一种基于XGBoost的多尺度武装冲突预测模型。首先,构建多尺度网格对数据进行划分,经时空栅格化处理后作为预测模型的主题数据集;其次,基于数据集构建地理环境、经济社会、时间推移、空间扩张、时空依赖和全要素等6个主题子模型;最后,结合不平衡样本处理和贝叶斯自动寻优,对武装冲突爆发的风险等级进行划分和预测。基于XGBoost的多尺度网格的预测可以实现更精细的空间划分,准确捕捉冲突爆发的地点和分布模式。模型可应用于不同层级,为制定不同地区的冲突风险政策提供参考。

基于交流储能的光伏柔直系统多时间尺度调压控制

由于光伏发电受光照强度等因素影响明显,导致其与模块化多电平换流器(modular multilevel converter,MMC)并网运行时功率不均衡,影响系统直流电压稳定性。该文针对光伏接入柔直系统的多时间尺度功率不平衡问题进行研究,提出光伏送电端交流侧储能方案及附加功率反馈环控制策略,协调消纳多时间尺度及交流故障场景下系统产生的不平衡功率。所搭建21电平35 k V光伏并网柔直模型的仿真结果表明,在光照强度突变至600 W/m2的情况下,采用附加功率反馈环控制策略时储能系统对直流电压波动的抑制水平可达11.9%,交流故障时其对直流电压波动的抑制水平可达23.7%,验证了该方案及控制策略的可行性。

基于锚框的远距离多尺度红外目标跟踪技术

在红外目标跟踪过程中,由于目标距离和视角的变化,红外目标的成像面积通常很小,使得红外图像中缺乏足够的像素信息,无法准确提取目标的特征,增加了目标跟踪的难度。因此,提出基于锚框的远距离多尺度红外目标跟踪技术。利用编码的方式对应追踪框图与真实框图,得到目标框中心坐标值。为保证计算的精准度,通过设定特征融合阈值判定提取信息与真实信息较高的重叠度,并按照锚框的边缘、中心以及顶点坐标对应划分像素网格,根据特征值输出分类输出向量,完成红外目标特征提取。由此采用损失函数给出锚框、目标框、实际框的类别损失以及候选框等不同类别损失函数,按照不同点在网格中的体素值,以编码形式逐一对比候选框和真实框数据,通过迭代实现远距离多尺度红外目标跟踪。实验结果表明,所提方法对远距离多尺度红外目标的识别效果较好,查全率曲线基本保持在0.9以上。说明所提方法具有良好的远距离多尺度红外目标跟踪效果。