宾汉流体稠密两相湍流流动中的二阶矩模型

建立的Bingham流体稠密两相流动的二阶矩-颗粒动力论湍流模型（USM-theta模型）既体现了两相的作用,又体现了屈服应力所引起的附加项,并提出了USM-theta模型下考虑浓度修正值影响的两相湍流流动的算法．利用该模型对圆管内Bingham流体的单相湍流流动、稠密液固两相的湍流流动进行了计算,并和五方程湍流模型进行了比较,结果表明该模型的预测效果更好．利用USM-theta模型对含颗粒的Bingham流体的两相湍流流动进行了模拟,随着屈服应力的增加,Bingham流体相与颗粒相在管道中心附近的主流速度减小．液固两相湍流和Bingham流体两相湍流的计算结果表明屈服应力引起的附加项对流动有很重要的影响．     

颗粒湍能输运方程的两相湍流模型和平面闭式两相射流的数值模拟

在现有的两相湍流数值模拟中,对颗粒湍流普遍采用以局部追随概念为基础的代数模型,其预报结果在很多情况下与实验不符。本文提出了以颗粒湍能输运方程为基础的κ-ε-kκ两相湍流模型,并以平面闭式两相射流为例进行了数值模拟,预报结果与实验符合良好,表明此模型明显优于k-ε-A.P.的颗粒湍流代数模型。     

两相湍流色噪声PDF模型数值模拟

1 引言湍流两相流动双流体模型的关键问题是两相湍流模型．徐江荣在文献[1]中用色噪声方法来处理两相流中的湍流,将流体脉动速度看着色噪声,从颗粒运动的朗之万方程出发,对流体脉动速度用扩维法来推导两个不同层次的PDF输运方程,通过高斯过程假设和高斯分部积分解决PDF方程的封闭问题,从而推导出双流体模型方程．这些方程都具     

固液两相流中的K－ε双方程湍流模式

建立了固液两相流中更一般的Ｋ－ε双方程湍流模式．模化了固相和液相的连续方程、动量方程及Ｋ方程和ε方程．该湍流模型考虑了固液两相间速度的滑移，颗粒间的作用及相间作用．使用本文所建立的湍流模型，数值预测了一管湍流中的沙水混合流动，其预测结果与实验结果比较一致．     

大涡旋的分类和模式理论的封闭*

本文依据前文[1]大小涡旋分开考虑的湍流模式,对局部产生的湍流大涡旋引进了新的几率分布,由此引入了一种新的平均过程和平均值.通过这种平均,我们就可以把局部产生的湍流大涡旋和外来的湍流大涡旋进行严格的区分.然后,再引进适当的辅助条件和根据外来干扰的实际情况确定了外来大涡旋的耗散尺度l_N,使得湍流脉动二阶矩的方程组封闭,同时对前文[1]中的一些扩散系数进行了适当的修改.最后,得到了一个封闭的、能够数值求解的方程组.     

不可压缩均匀各向同性湍流的统计理论

本文根据均匀各向同性湍流的涡旋结构理论,从Navier-Stokes方程出发,引进了准相似性条件,认为均匀各向同性湍流场在衰变过程中具有相似性,相似性尺度由表征湍流强弱的湍流脉动速度均方差q以及与特征涡旋尺度具有密切关系的湍流广义Taylor微尺度λ所决定,在对均匀各向同性湍流场计算中,假定湍流脉动在空间呈周期性,周期性尺度正比于λ。 本文对脉动速度等物理量用Fourier级数展开,将在物理空间上的计算转化到谱空间上,利用快速Fourier变换,采用前差格式和Leap-frog格式,对不同Reynolds数的均匀各向同性湍流场从衰变后期到前期进行了计算,得出了与实验较符合的结果。     

螺旋型旋风分离器两相流场的数值模拟

对螺旋型旋风分离器进行了两相流场的三维数值模拟．气体流场通过求解三维N-S方程得到,湍流模型采用了雷诺应力模型．计算结果表明,旋风分离器内部的流场分为两部分:螺旋通道内比较稳定的流场和筒体中心区域的复合涡结构流场．对颗粒运动轨迹的计算表明,颗粒在入口处的初始位置对颗粒分离有比较显著的影响．同时得到了不同入口速度下颗粒的分级效率曲线,并给出了气体流量对旋风分离器性能的影响,结果显示:气体流量的增加会提高分离效率,但同时导致压力损失的急剧增加．     

稳定分层流动中湍流统计特性和输运特性的数值研究

采用大涡模拟方法研究了上下两层温度不同的稳定分层湍流,主要对其中的湍流尺度特性、统计特性和湍流输运特性,以及标量场的时空演化进行了分析研究.研究结果表明:浮力尺度沿流动方向先增加然后趋于某个稳定值,剪切越强,浮力尺度越大;翻转尺度沿流向也是逐渐增大,剪切越强,翻转尺度越大;统计特性的分析发现,在较大的区域内温度脉动的平坦因子都偏离Gauss分布,强分层条件下温度脉动的统计特性与速度脉动的统计特性有较明显差别;湍流混合的过程开始于小尺度运动,最后扩展到大尺度运动.     

固液两相流中的K-ε双方程湍流模式*

建立了固液两相流中更一般的K-ε双方程湍流模式。模化了固相和液相的连续方程、动量方程及K方程和ε方程。该湍流模型考虑了固液两相间速度的滑移,颗粒间的作用及相间作用。使用本文所建立的湍流模型,数值预测了一管湍流中的沙水混合流动,其预测结果与实验结果比较一致。     

两相流中柱状固粒对流体湍动特性影响的研究

对含柱状固粒的两相流场,建立了包含柱状固粒对流场影响的流体脉动速度方程,在求解脉动速度方程的基础上,经平均得到流体的湍流强度和雷诺应力.将该方法用于槽流湍流场的求解,并与单相流实验结果进行了比较.计算中变化柱状固粒的参数,给出了固粒的体积分数、长径比、松驰时间对流场湍动特性的影响,说明粒子对流场的湍动特性起着抑制作用,其抑制的程度与粒子的体积分数、长径比成正比,与粒子的松弛时间成反比.     

Numerical simulation of UV disinfection reactors: Evaluation of alternative turbulence models

Six turbulence models, including standard k–ε, k–ε RNG, k–ω (88), revised k–ω (98), Reynolds stress transport model (RSTM), and two-fluid model (TFM), were applied to the simulation of a closed conduit polychromatic UV reactor. Predicted flow field and turbulent kinetic energy were compared with the experimental data from a digital particle image velocimetry (DPIV). All of the predicted flow fields were combined with a multiple segment source summation (MSSS) fluence rate model and three different microbial response kinetic models to simulate the disinfection process at two UV lamp power conditions. Microbial transport was simulated using the Lagrangian particle tracking method. The results show that the fluence distributions and the effluent inactivation levels were sensitive to the turbulence model selection. The level of sensitivity was a function of the operating conditions and the UV response kinetics of the microorganisms. Simulations with operating conditions that produced higher log inactivation or utilized microorganisms with higher UV sensitivity showed greater sensitivity to the turbulence model selection. In addition, a broader fluence distribution was found with turbulence models that predicted a larger wake region behind the lamps.     

Free energy fluctuations for a mixture of directed polymers

We study the free energy fluctuations for a mixture of directed polymers, which was first introduced by Borodin et al. (2015) to investigate the limiting distribution of a stationary Kardar-Parisi-Zhang (KPZ) equation. The main results consist of both the law of large numbers and the asymptotic fluctuation for the free energy as the model size tends to infinity. In particular, we find the explicit values (which may depend on model parameters) of normalizing constants in the fluctuation. This shows that such a mixture model is in the KPZ university class.     

圆管纤维悬浮湍流场中粒子运动的研究

利用从细长体理论出发得到的三维分段积分法和湍流简化方法模拟了大量纤维粒子在圆管湍流内的运动．统计了不同Re数下计算区域内的纤维的取向分布,计算结果与实验结果基本吻合,结果表明湍流的脉动速度导致纤维取向趋于无序,且随着Re数的增加,纤维取向的分布越来越趋于均匀．其后又考虑了纤维速度和角速度的脉动,二者都充分体现了流体速度脉动的影响,且纤维速度的脉动在流向上的强度大于横向,而其角速度的脉动在流向上的强度小于横向．最后统计了纤维在管道截面上的位置分布,说明Re数的增加加速了纤维在管道截面上的位置扩散．     

Simulating bistable biochemical systems by means of reactive multiparticle collision dynamics

Based on a reactive multiple particle collision method, we construct a mesoscopic dynamics model to simulate chemical system. The validity of the reactive multiple particle collision method under various conditions in a double-feedback bi-stable chemical system is studied. Then, we extend it to simulate diffusion-limited reactions with fast reaction rate in cellular environment. Using the improved method, we observe bi-stable behavior with randomly distributed reactants and spatial domain separation of opposite phases. The particle-based mesoscopic method is computationally efficient, although hydrodynamic interactions and fluctuation are both properly accounted for. Stochastic effects shown to play dominant roles in biochemical dynamics are also considered. The improved method could be used to explore a variety of reactions with disparate scale of reaction rates.     

Numerical simulation of indoor suspension particles based on v-f model

This paper presents a new algorithm for the prediction of indoor suspension particle dispersion based on a v²-f model. In order to handle the near-wall turbulence anisotropy properly, which is significant in the dispersion of fine particles, the particle eddy diffusivity is calculated using different formulae among regions of the turbulent core and in the vicinity of walls. The new algorithm is validated by several cases performed in two ventilated rooms with various air distribution patterns. The simulation results reveal that v²-f nonlinear turbulence model combined with a particle convective equation gives satisfactory agreement with the experimental data. It is generally found that the dynamic equation combined with the v²-f model can properly handle low Reynolds number (LRN) flows which are usually encountered in indoor air flows and fine particle dispersion.     

Ionization properties in the collision of Rydberg Cs atoms with CCl4 molecules

The experimental study of ionization in the collision of Rydherg Cs atoms with CCL molecules is conducted using the crossed atomic and molecular beam apparatus. It is found that the relative collision rate constants are not consistent with the prediction of the essentially free electron model in the range ofn=30–50. The possible reason for this discrepancy is discussed.     

Quantum transport theory and nonequilibrium chiral condensates

Quantum transport theory and disoriented chiral condensate (DCC) in the Nambu-Jona-Lasinio model are investigated. The kinetic equations are derived in covariant and equal-time formulations. It is found that DCC is closely connected with the fluctuation of quark spin density. For the longitudinally expanding system, which models a highenergy nucleus-nucleus collision, a DCC present initially is quickly dissolved due to the expansion and the collision processes. However, the quantum off-shell effect leads to a strong fluctuation of DCC. Project supported in part by the National Natural Science Foundation of China (Grant No. 19845001) and Cao Guangbiao Foundation of Tsinghua University.     

Evaluation of reduction behavior of blast furnace dust particles during in-flight process with experiment aided mathematical modeling

In-flight reduction technology is a flexible process that allows recycling of the fine iron bearing metallurgical dusts efficiently. In this work, a mathematical model, incorporating introduced experimental kinetic parameters, was developed to accurately evaluate the reduction behavior of blast furnace (BF) dust particles during flight. A detailed evaluation of particle residence time, thermal history and reduction degree conversion were used to eliminate the deviations related to the assumptions of constant particle velocity and temperature in the experiment. The results show that the particle velocity decreases along the longitudinal direction of the reactor for a long distance and reaches a constant low velocity at the middle part of the reaction zone. The calculated particle residence time is 0.15–0.44 s less than the experimentally estimated value. The particle temperature reaches the isothermal temperature at the 0.15 m position from the reaction zone bottom. An obvious transition of reduction degree of dust particle is found when particle temperature reaches over 1640 K. The prediction accuracy of the model was improved by using the optimized kinetic parameters, namely pre-exponential factor and activation energy.     

Numerical study of gas–solid flow in a cyclone separator

This paper presents a numerical study of the gas–powder flow in a typical Lapple cyclone. The turbulence of gas flow is obtained by the use of the Reynolds stress model. The resulting pressure and flow fields are verified by comparing with those measured and then used in the determination of powder flow that is simulated by the use of a stochastic Lagrangian model. The separation efficiency and trajectory of particles from simulation are shown to be comparable to those observed experimentally. The effects of particle size and gas velocity on separation efficiency are quantified and the results agree well with experiments. Some factors which affect the performance of cyclone were identified. It is shown that the collision between gas streams after running about a circle and that just entering occurred around the junction of the inlet duct and the cylinder of the cyclone, resulting in a short-circuiting flow. The combination of flow source and sink was distributed near the axis of cyclone, forming a flow dipole at axial section. Particles entering at different positions gave different separation efficiency. A particle with size exceeding a critical diameter, which was condition-dependant, would stagnate on the wall of cyclone cone. This was regarded as one of the main reasons for the deposition on the inner conical surface in such cyclones used in the cement industry.