Modeling of Fluid Turbulence Modification Using Two-time-scale Dissipation Models and Accounting for the Particle Wake Effect

Presently developed two-phase turbulence models under-predict the gas turbulent fluctuation, because their turbulence modification models cannot fully reflect the effect of particles. In this paper, a two-time-scale dis- sipation model of turbulence modification, developed for the two-phase velocity correlation and for the dissipation rate of gas turbulent kinetic energy, is proposed and used to simulate sudden-expansion and swirling gas-particle flows. The proposed two-time scale model gives better results than the single-time scale model. Besides, a gas tur- bulence augmentation model accounting for the finite-size particle wake effect in the gas Reynolds stress equation is proposed. The proposed turbulence modification models are used to simulate two-phase pipe flows. It can prop- erly predict both turbulence reduction and turbulence enhancement for a certain size of particles observed in ex- periments.

A nonlinear k_p-ε_p particle two-scale turbulence model and its application

A particle nonlinear two-scale kp-εp turbulence model is proposed for simulating the anisotropic turbulent two-phase flow. The particle kinetic energy equation for two-scale fluctuation, particle energy transfer rate equation for large-scale fluctuation, and particle turbulent kinetic energy dissipation rate equation for small-scale fluctuation are derived and closed. This model is used to simulate gas-particle flows in a sudden-expansion chamber. The simulation is com- pared with the experiment and with those obtained by using another two kinds of tow-phase turbulence model, such as the single-scale k-ε two-phase turbulence model and the particle two-scale second-order moment （USM） two-phase turbulence model. It is shown that the present model gives simulation in much better agreement with the experiment than the single-scale k-ε two-phase turbulence model does and is almost as good as the particle two-scale USM turbu-lence model.

A two-scale second-order moment two-phase turbulence model for simulating dense gas-particle flows

A two-scale second-order moment two-phase turbulence model accounting for inter-particle collision is developed, based on the concepts of particle large-scale fluctuation due to turbulence and particle small-scale fluctuation due to collision and through a unified treatment of these two kinds of fluctuations. The proposed model is used to simulate gas-particle flows in a channel and in a downer. Simulation results are in agreement with the experimental results reported in references and are near the results obtained using the sin- gle-scale second-order moment two-phase turbulence model superposed with a particle collision model （USM-θ model） in most regions.

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.

EFFECT OF EMPIRICAL COEFFICIENTS ON SIMULATION IN TWO-SCALE SECOND-ORDER MOMENT PARTICLE-PHASE TURBULENCE MODEL

A two-scale second-order moment two-phase turbulence model accounting for inter-particle collision is developed, based on the concept of particle large-scale fluctuation due to turbulence and particle small-scale fluctuation due to collision. The proposed model is used to simulate gas-particle downer reactor flows. The computational results of both particle volume fraction and mean velocity are in agreement with the experimental results. After analyzing effects of empirical coefficient on prediction results, we can come to a conclusion that, inside the limit range of empirical coefficient, the predictions do not reveal a large sensitivity to the empirical coefficient in the downer reactor, but a relatively great change of the constants has important effect on the prediction.

Derivation of a second-order model for Reynolds stress using renormalization group analysis and the two-scale expansion technique

With the two-scale expansion technique proposed by Yoshizawa,the turbulent fluctuating field is expanded around the isotropic field.At a low-order two-scale expansion,applying the mode coupling approximation in the Yakhot-Orszag renormalization group method to analyze the fluctuating field,the Reynolds-average terms in the Reynolds stress transport equation,such as the convective term,the pressure-gradient-velocity correlation term and the dissipation term,are modeled.Two numerical examples：turbulent flow past a backward-facing step and the fully developed flow in a rotating channel,are presented for testing the efficiency of the proposed second-order model.For these two numerical examples,the proposed model performs as well as the Gibson-Launder （GL） model,giving better prediction than the standard k-ε model,especially in the abilities to calculate the secondary flow in the backward-facing step flow and to capture the asymmetric turbulent structure caused by frame rotation.

A continuum traffic flow model with the consideration of coupling effect for two-lane freeways

A new higher-order continuum model is proposed by considering the coupling and lane changing effects of the vehicles on two adjacent lanes. A stability analysis of the proposed model provides the conditions that ensure its linear stability. Issues related to lane changing, shock waves and rarefaction waves, local clustering and phase transition are also investigated with numerical experiments. The simulation results show that the proposed model is capable of providing explanations to some particular traffic phenomena commonly observable in real traffic flows.

径向井辅助前置液酸压裂缝扩展数值模拟

基于扩展有限元及双尺度连续模型理论建立径向井辅助前置液酸压裂缝扩展模型,引入偏离系数作为量化评价指标,分析各因素对裂缝形态的影响,并利用灰色关联分析方法明确主控因素。结果表明:径向井对前置液酸压裂缝扩展具有显著引导作用,裂缝优先沿径向井方向起裂并扩展一定距离,而后逐渐偏向最大水平主应力方向;径向井方位角、水平地应力差和地层弹性模量越小,径向井长度越长,偏离系数越小,径向井引导效果越好,其中水平地应力差为主控因素;当径向井方位角为15°时,前置液酸压裂缝沿着径向井方向扩展14.32 m后偏向最大水平主应力方向,径向井的引导效果最好,偏离系数为0.005。

中央空调负荷精细化调控双层优化技术研究

针对当前中央空调系统粗放式调节不能准确释放调控潜力的问题,深入分析中央空调各子系统工作原理与调控方式,提出基于双层优化的中央空调负荷精细化调控技术方案。首先,面向实际工程实践需求建立建筑湿热标度模型;然后,建立以舒适度和经济性优化目标函数为主的上层调控模型,采用Shapley值法对温湿设定值的进行分配;接着,建立以各子系统优化组合能耗最低为优化目标函数的下层调控模型,采用蚁群算法对优化目标函数进行求解;之后,通过求解双层模型,制定包括末端风机、冷冻水泵以及冷水机组等组件的精细化调控策略;最后,通过多个算例验证所提中央空调负荷精细化调控技术方案的正确性与有效性。

桥梁结构中E-GFRP单向板徐变性能与双尺度均匀化数值评估

由于玻璃纤维增强复合材料(GFRP)组分材料中的树脂属于高分子材料,桥梁工程界对GFRP结构的徐变性能十分担忧,阻碍了其推广应用。该文聚焦于桥梁工程E型玻璃纤维增强复合材料(E-GFRP)单向板,基于双尺度均匀化数值模拟方法,从纤维和树脂的徐变性能评估了E-GFRP单向板的徐变性能,并与试验结果进行比较验证了预测结果的准确性。在此基础上,分析了应力水平、纤维体积率及持荷形式对E-GFRP单层板徐变性能的影响,结果表明,应力水平越高、纤维体积率越小,单层板的徐变粘滞应变越大。在此基础上,提出了不同纤维体积率下E-GFRP单层板粘滞应变模型,准确表述了应力水平、持荷时间和徐变应变之间的关系。最后,提出了E-GFRP单层板徐变断裂时间预测公式,可为E-GFRP结构的长期性能预测提供参考。

An improved large eddy simulation of two-phase flows in a pump impeller

An improved large eddy simulation using a dynamic second-order sub-grid-scale （SGS） stress model has been developed to model the governing equations of dense turbulent particle-liquid two-phase flows in a rotating coordinate system, and continuity is conserved by a mass-weighted method to solve the filtered governing equations. In the cur- rent second-order SGS model, the SGS stress is a function of both the resolved strain-rate and rotation-rate tensors, and the model parameters are obtained from the dimensional consistency and the invariants of the strain-rate and the rotation-rate tensors. In the numerical calculation, the finite volume method is used to discretize the governing equations with a staggered grid system. The SIMPLEC algorithm is applied for the solution of the discretized governing equations. Body- fitted coordinates are used to simulate the two-phase flows in complex geometries. Finally the second-order dynamic SGS model is successfully applied to simulate the dense turbu-lent particle-liquid two-phase flows in a centrifugal impeller. The predicted pressure and velocity distributions are in good agreement with experimental results.

“双规模化”与土地规模经营稳定性

近年来,农地经营规模化在政策支持下快速发展,但粮食规模化经营的风险集聚可能增加经营不稳定性,加剧粮食市场波动,威胁国内粮食安全。研究从风险分散和经营稳定性视角阐释农地经营规模化与作业服务规模化结合形成的“双规模化”经营模式影响粮食生产规模户经营稳定性的理论逻辑,并用来自黑龙江、河南、四川和浙江四省粮食生产规模户2015年和2018年两期跟踪调查数据实证检验上述影响与机制。研究发现,“双规模化”经营模式具有显著的风险分散功能,可明显提高粮食生产规模户的经营稳定性,且规模户经营规模越大,其稳定农地规模经营的作用越强。也有证据表明粮食生产规模户采纳“双规模化”经营模式有利于其维持和扩张经营规模。建议政府相关部门除了积极为农地规模经营风险管理提供多元工具和公共政策,还可创造条件鼓励经营主体采用“双规模化”经营模式。

Macroscopic damping model for structural dynamics with random polycrystalline configurations

In this paper the macroscopic damping model for dynamical behavior of the structures with random polycrystalline configurations at micro-nano scales is established. First, the global motion equation of a crystal is decomposed into a set of motion equations with independent single degree of freedom （SDOF） along normal discrete modes, and then damping behavior is introduced into each SDOF motion. Through the interpolation of discrete modes, the continuous representation of damping effects for the crystal is obtained. Second, from energy conservation law the expression of the damping coefficient is derived, and the approximate formula of damping coefficient is given. Next, the continuous damping coefficient for polycrystalline cluster is expressed, the continuous dynamical equation with damping term is obtained, and then the concrete damping coefficients for a polycrystalline Cu sample are shown. Finally, by using statistical two-scale homogenization method, the macroscopic homogenized dynamical equation containing damping term for the structures with random polycrystalline configurations at micro-nano scales is set up.

基于二维象限分析的大学生在线学习行为研究——以混合学习环境下的SPOC实践为例

在线学习环境下学生的全景式行为表现反映着学生的真实学习投入状况,进而折射出线上教学的质量。研究中以混合学习环境下的SPOC实践为例,从在线学习行为的内隐性和外显性特征两个维度构建学习行为分析的象限图,试图对学习者的在线学习投入表现、学习效果以及内隐性和外显性交互影响作用的异同进行较全面的分析。研究发现:(1)在学习投入水平上,作业类学习任务达到较高的行为投入,而自主选择类的行为投入较低。(2)在线学习投入的学习效果上,全面投入类行为的学习效果要优于主动投入类行为和被动投入类行为的学习效果;而主动投入类行为和自我管理类行为的学习效果又要优于规律投入类行为和绩效投入类行为。(3)从学习行为内隐性与外显性交互影响的作用来看,当行为内隐性和外显性都呈现为高投入时,学习效果与行为投入趋于一致;当行为内隐性和外显性高低投入不一致时,学习效果并不理想。

高敏感儿童量表21条目版测评中学生的效度和信度

目的:引进高敏感儿童量表21条目版(HSC-21)并检验其效度和信度。方法:选取中学生1150人,检验HSC-21的结构效度、效标效度及内部一致性信度,使用中国大五人格问卷(CBF-PI)神经质、开放性、外向性分量表以及行为抑制-激活系统量表(BIS/BAS)的BIS分量表为效标工具。2周后选取其中初中生100人完成重测。结果:验证性因子分析显示HSC-21为三因子双因子模型(χ^(2)/df=2.90,CFI=0.93,TLI=0.91,RMSEA=0.04);HSC-21总分及低感觉阈值得分与CBF-PI神经质、开放性得分及BIS得分均呈正相关(r=0.38、0.15、0.40,均P<0.01)。总量表的Cronbachα系数为0.81,易刺激性、低感觉阈值、审美敏感性3个因子的Cronbachα系数为0.70、0.69、0.77;总量表的重测信度(ICC)为0.70,3个因子的ICC为0.68、0.72、0.66。结论:高敏感儿童量表21条目版(HSC-21)测评中学生群体显示出良好的效度和信度,可以作为环境敏感性水平的测量工具。

A TWO-SCALE SECOND-ORDER MOMENT PARTICLE TURBULENCE MODEL FOR GAS-PARTICLE FLOW

A two-scale second-order moment two-phase turbulence model was developed and used to simulate gas-particle flow in a sudden-expansion chamber and a channel. The simulation results were in agreement with the experimental results, and the results were compared with those of the single-scale second-order moment two-phase turbulence model. Several improved features show that the two-scale model is to a certain extent better than the single-scale model, which may be attributed to the fact that particle turbulence is well characterized by the two-scale turbulence model.

基于多尺度注意力机制的两阶段文物图像修复方法

文物常因保存或物理修复手段不当而受到损坏,使用虚拟技术对其进行修复很重要,而现有传统图像修复技术和基于深度学习的修复方法主要针对结构纹理简单、破损区域较小的图像或是破损区域规则的自然图像,无法直接应用于文物图像。针对文物图像结构纹理复杂、破损区域不规则及现存文物图像数据集较小等问题,以山水画图像修复为例提出了一种基于多尺度注意力机制的两阶段文物图像修复方法。首先基于全局注意力机制对文物图像的整体结构和基础色调进行粗粒度修复,然后使用局部注意力机制和残差模块对文物图像的小型结构和细节纹理进行局部细粒度修复,并在粗粒度修复的结果上使用上下文注意力机制从文物图像远距离精确借用信息,对图像的大型结构和纹理进行全局细粒度修复,最后将局部和全局的修复结果进行特征融合,实现文物图像的修复。针对文物图像特殊的破损类型,修复的文物图像伪迹较少,色彩均匀,结构纹理清晰,相比对比方法,在峰值信噪比上平均提高了3.76 dB,在结构相似性上平均提高了0.034。实验结果的主观和客观分析表明,与主流图像修复方法相比,在语义合理性、信息准确性和视觉自然性上都具有一定优势,在文物修复领域有较大应用价值。

Two-phase air-water flows: Scale effects in physical modeling

Physical modeling represents probably the oldest design tool in hydraulic engineering together with analytical approaches. In free surface flows, the similitude based upon a Froude similarity allows for a correct representation of the dominant forces, namely gravity and inertia. As a result fluid flow properties such as the capillary forces and the viscous forces might be incorrectly reproduced, affecting the air entrainment and transport capacity of a high-speed model flow. Small physical models operating under a Froude similitude systematically underestimate the air entrainment rate and air-water interfacial properties. To limit scale effects, minimal values of Reynolds or Weber number have to be respected. The present article summarizes the physical background of such limitations and their combination in terms of the Morton number. Based upon a literature review, the existing limits are presented and discussed, resulting in a series of more conservative recommendations in terms of air concentration scaling. For other air-water flow parameters, the selection of the criteria to assess scale effects is critical because some parameters （e.g., bubble sizes, turbulent scales） can be affected by scale effects, even in relatively large laboratory models.

An operational two-layer remote sensing model to estimate surface flux in regional scale: Physical background

Based on the improved interaction mechanism of two-layer model, this paper proposed Pixel Component Arranging and Comparing Algorithm (PCACA) and theoretically positioning algorithm, estimated the true temperature of mixed pixel in four extreme points in combination with the measurements of dry and wet points in calibration fields and improved the reliability of positioning dry and wet line. A new two-layer energy-separation algorithm was proposed,which was simple and direct without resistance network parameters for each pixel. We also proposed a new thought about the effect of advection. The albedo of mixed pixel was also separated with PCACA. In combination with two-layer energy-separation algorithm, the net radiation of mixed pixel was separated to overcome the uncertainty of conventional energy-separation algorithm using Beer's Law. Through the validation of retrieval result, this method is proved to be feasible and operational. At the same time, the uncertainty of this algorithm was objectively analyzed.

基于二尺度模型的星载SAR海面场景成像仿真

根据海浪随机理论进行了粗糙海面建模,利用海面的二尺度模型,构造了包含涌浪在内的随机粗糙海面,同时考虑了不同尺度波浪对海面的影响。基于此海面模型,利用Bragg散射模型和Kirchhoff近似并考虑海面运动所产生的速度聚速效应,模拟了不同情况下海浪的极化SAR图像。最后对海浪参数与SAR参数对极化图像的影响做了分析,证明了模拟的正确性与有效性。