湍流边界层中重粒子弥散的随机模型

在重粒子轨道模型的基础上,引入了Saffman力,并考虑了粒子-固壁碰撞和粒子-粒子碰撞的影响,建立了重粒子运动方程,耦合湍流脉动的随机方程,发展了重粒子弥散的随机模型,并在湍流边界层中考察该模型.将数值计算结果与实验结果进行比较,同时考察了Saffman力和粒子碰撞对计算结果的影响.     

Investigation of Turbulent Transition in Plane Couette Flows Using Energy Gradient Method

The energy gradient method has been proposed with the aim of better understanding the mechanism of flow transition from laminar flow to turbulent flow. In this method, it is demonstrated that the transition to turbulence depends on the relative magnitudes of the transverse gradient of the total mechanical energy which amplifies the disturbance and the energy loss from viscous friction which damps the disturbance, for given imposed disturbance. For a given flow geometry and fluid properties, when the maximum of the function $K$ (a function standing for the ratio of the gradient of total mechanical energy in the transverse direction to the rate of energy loss due to viscous friction in the streamwise direction) in the flow field is larger than a certain critical value, it is expected that instability would occur for some initial disturbances. In this paper, using the energy gradient analysis, the equation for calculating the energy gradient function $K$ for plane Couette flow is derived. The result indicates that $K$ reaches the maximum at the moving walls. Thus, the fluid layer near the moving wall is the most dangerous position to generate initial oscillation at sufficient high $\operatorname{Re}$ for given same level of normalized perturbation in the domain. The critical value of $K$ at turbulent transition, which is observed from experiments, is about 370 for plane Couette flow when two walls move in opposite directions (anti-symmetry). This value is about the same as that for plane Poiseuille flow and pipe Poiseuille flow (385-389). Therefore, it is concluded that the critical value of $K$ at turbulent transition is about 370-389 for wall-bounded parallel shear flows which include both pressure (symmetrical case) and shear driven flows (anti-symmetrical case).     

三种湍流模式数值模拟直角弯管内三维分离流动的比较

采用有限体积法数值离散雷诺时均Navier Stokes方程,模拟了方形截面90°大曲率弯曲管道内的三维湍流流动.用3种湍流模式(标准k-ε湍流模式、RNG k-ε湍流模式、Realizable k-ε湍流模式)求解该问题.给出了数学模型和计算结果,并与实验数据进行了比较.结果表明,采用RNG k-ε湍流模式并结合两层壁面模型的处理,能有效准确地求解强曲率影响的管道内及近壁区湍流的流动.     

各向异性颗粒PDF输运方程及其颗粒湍流扩散

根据Minier的模型,获得颗粒所见流体脉动速度的朗之万模型,并推导颗粒所见流体湍流脉动速度自关联函数,该函数具有各向异性的特征,进一步由流体脉动速度朗之万模型和颗粒运动方程获得相应的颗粒PDF输运方程.为描述在网格生成湍流中的颗粒扩散运动,将颗粒PDF输运方程作相应的简化,获得一个具有颗粒湍流各向异性的解析解,将方程的解与Wellsand Stock的颗粒湍流扩散实验结果进行比较,获得了很好的结果.揭示了具有各向异性特征的颗粒PDF输运方程的优越性.     

壁面在展向作周期运动的槽道湍流的大涡模拟

分别采用3种亚格子模式:传统的Smagorinsky模式、动力Smagorinsky模式和Cui(2004)基于Kolmogorov方程所提出的新模式,对壁面在展向作周期运动的槽道湍流进行了大涡模拟,以考察这3种模式对平均运动为三维、非定常的湍流流动的模拟能力.通过对湍流基本统计量的分析,发现动力模式和新模式都可以较好地预测这种三维非定常的湍流流动;对相位平均的湍流统计量,动力模式的结果略优于新模式;传统的Smagorinsky模式对这种流动的预测结果是最差的.     

甲醇喷雾湍流反应流的JPDF数值模拟

喷雾湍流燃烧过程中,液滴、湍流和化学反应之间强烈耦合,物理化学机理非常复杂。本文将速度-标量-频率联合概率密度函数JPDF输运方程方法应用于两相喷雾湍流反应流问题,利用火焰面模型解耦流动和化学反应动力学的耦合关系,建立起相应的数值计算模型。采用Monte-Carlo数值计算方法,针对澳大利亚悉尼大学Masri等人以甲醇为燃料所进行的湍流喷雾燃烧值班火焰这一试验进行了数值模拟,通过与Fluent下的计算结果及试验结果的对比分析,验证了本文所建模型的准确性。     

Turbulent gas-dispersed flow in a pipe with sudden expansion: numerical simulation

A mathematical model was developed to simulate two-phase gas-dispersed flow moving through a pipe with axisymmetric sudden expansion. In the model, the two-fluid Euler approach was used. The model is based on solving Reynolds-averaged Navier — Stokes equations for a two-phase stream. In calculating the fluctuating characteristics of the dispersed phase, equations borrowed from the models by Simonin (1991), Zaichik et al. (1994), and Derevich (2002) were used. Results of a comparative analysis with previously reported experimental and numerical data on two-phase flows with separation past sudden expansion in a plane channel and in a pipe are given. This work was supported by the President of the Russian Federation through the Foundation for Young Candidates of Sciences under Grant MK-186.2007.8 and by the Russian Foundation for Basic Research (Grants Nos. 05-08-33586 and 06-08-00967).     

湍流扩散火焰局部熄火和再燃现象的PDF模拟

对一个值班湍流CH4／O2／N2射流扩散火焰(Sandia Flame D)进行了数值模拟研究．所采用的数学物理模型包括双尺度的k—ε湍流模型,标量联合的概率密度函数(PDF)输运方程方法,甲烷氧化的ARM简化化学反应机理(包含16种组分,12步总包反应)和欧几里德最小生成树(EMST)小尺度混合模型．将计算结果和实验数据进行了比较,不仅对于平均量,对于标量的散点分布和条件概率密度分布也是如此．计算结果表明文中采用的模型不仅能够预测宏观的火焰结构,而且预测了湍流燃烧中复杂的局部熄火和再燃过程．     

Experimental Study of CuO/Water Nanofluid Turbulent Convective Heat Transfer in Square Cross-section Duct

Ducts with a square cross-section are widely used in many industrial applications because of their high compactness, easy forming, and low pressure drop. But the thermal performance of a duct will be reduced when the circular cross-sectional shape is not used. In this study, the convective heat transfer for a CuO/water nanofluid through a square cross-section duct in the turbulent flow regime has been investigated. The Nusselt number of nanofluids for different nanoparticle concentrations, as well as various Peclet numbers, was obtained. The results show considerable enhancement in the heat transfer coefficient and Nusselt number by increasing the nanoparticle concentrations as well as the Peclet number.     

Investigations on the Effect of Backward-Facing and Forward-Facing Steps on Turbulent Mixed-Convection Flow Over a Flat Plate

The effects of backward-facing and forward-facing steps on a turbulent buoyancy-dominated mixed-convection flow over a flat plate are examined experimentally. Air velocity and temperature distributions and their turbulent fluctuations are measured simultaneously by using a two-component laser-Doppler velocimeter and a cold wire anemometer, respectively. The experiment was carried out for a step (backward-facing/forward-facing) height of 22 mm, a temperature difference, ΔT, of 30°C between the heated walls and the free-stream air (corresponding to a local Grashof number Gr _xi = 6.45 × 10¹⁰), and a free-stream velocity of 0.48 m/s. It was found that the introduction of backward- and forward-facing steps increases the turbulence intensity of the velocity and temperature fluctuations downstream of the step. The present results also reveal that the maximum local Nusselt number occurs in the vicinity of the reattachment zone, and it is approximately twice for the case of the backward-facing step and three times for the case of the forward-facing step than that of the flat plate value at similar flow and thermal conditions.     

普通单模光纤色散补偿传输的实验研究

利用自行设计的悬臂梁调节机构,对１２ｃｍ长的均匀光纤光栅进行中心波长和啁啾的同时线性调谐后,将其应用于２０Ｇｂ／ｓ、１００ｋｍ普通单模学纤的色散补偿实验中,取得了良好的补偿效果。     

Heat Transfer and Pressure Drop Characteristics in Turbulent Flow Through a Tube

An experimental investigation has been carried out for turbulent flow through a tube with perforated strip inserts. Strips were of mild steels with circular holes of different diameters. Flow varies, with ranging Reynolds numbers from 15,000 to 47,000. Air velocity, tube wall temperatures, and pressure drops were measured for a plain and strip-inserted tube. The heat transfer coefficient and friction factor were found to be 2.80 times and 1.8 times, respectively, that of the plain tube. The heat transfer performance was evaluated and found to be 2.3 times that of the plain tube based on constant blower power.     

湍流参数对凝汽器数值模拟的影响

通过对一凝汽器的二维数值模拟,研究了湍流黏性系数和湍流扩散系数对凝汽器流动与传热特性的影响。结果表明,湍流参数的取值,对凝汽器不凝结率的计算影响较大,对壳侧压降也有一定影响。在凝汽器数值模拟中,采用两方程的RNGκ-ε模型米确定湍流参数,在管束区内部形成湍流参数较大的区域,能更准确描述凝汽器内的复杂流动。与常湍流参数的...     

可调谐激光器腔内色散棱镜及其组合

分析了色散棱镜的特性及其顶角设计问题。讨论了色散棱镜和扩束棱镜在布鲁斯特角入射状态下，可组合使用的形式及单程角色散。提出将色散棱镜与扩束棱镜组合使用，可减少色散棱镜个数而获得更大角色散的方法。并据此设计了两个色散棱镜组，在Ｔｉ∶Ｓ激光器中得到实际使用。     

Experimental Study of Nanofluid Convective Heat Transfer for Implementation of Dispersion Model Considering Non-Uniform Particle Distribution

Thermal dispersion model has been used here to simulate heat transfer of water–Al₂O₃ nanofluid. A new form for dispersion thermal conductivity has been introduced in which non-uniform concentration distribution is applied on the model. It was observed that the non-uniformity of concentration increases at greater Reynolds numbers and average concentrations. An experimental set-up was made, and an experimental study was conducted to find the empirical coefficient in the dispersion thermal conductivity. The obtained results show that the developed dispersion model is able to properly simulate heat transfer of the nanofluid and provides more accurate results in comparison with a homogenous model.     

粒子群优化算法在自适应偏振模色散补偿中的性能研究

反馈控制算法是偏振模色散的自适应补偿器的关键组成部分,将粒子群优化算法(PSO)引入到偏振模色散自适应补偿系统中。该算法的优点是具有快速收敛到全局最佳值的能力、避免搜索陷入局部极值的能力、抗噪声能力和多自由度控制能力。理论上分析了粒子群优化算法的两个分类———全局邻居结构粒子群优化(GPSO)和局部邻居结构粒子群优化(LPSO)在搜索全局最佳值方面的能力优劣,给出了局部邻居结构粒子群优化算法成功率达100%的三种邻居拓扑结构。实验表明:在补偿一阶偏振模色散时,全局邻居结构和局部邻居结构搜索全局最佳的成功率都能满足要求,全局邻居结构算法收敛速度快。而在补偿二阶偏振模色散时,全局邻居结构成功率降低,而局部邻居结构仍可以满足要求。     

亚微米颗粒湍流团聚特性的实验研究

燃煤电厂亚微米颗粒物排放是造成大气环境污染和影响人体健康的重要因素。本文搭建亚微米颗粒湍流团聚实验平台,通过场发射扫描电镜(FESEM)对亚微米颗粒的团聚形貌进行微观表征以及扫描电迁移率粒径谱仪(SMPS)对亚微米颗粒团聚前后的粒径分布特性进行宏观测量。结果表明:亚微米颗粒湍流团聚形成多分叉颗粒树聚团结构,进口流速为0.8?5.3m/s条件下,PM1和PM0.1的脱除效率分别为5.6%?18.6%和7.0%?21.4%,且PM1和PM0.1的脱除效率曲线随速度变化呈现二次函数规律。     

基于F-P腔标准具的色散补偿器件的研究

介绍了标准具的结构,分析了基于标准具补偿色散的原理,讨论了该方案在光纤通信系统中的应用前景。     

Turbulent kinetic energy spectrum in very anisothermal flows

In this Letter, we find that the Kolmogorov scaling law is no longer valid when the flow is submitted to strong dilatational effects caused by high temperature gradients. As a result, in addition to the nonlinear time scale, there is a much shorter “temperature gradients” time scale. We propose a model that estimates the time scale of the triple decorrelation incorporating the influences of the temperature gradient. The model agrees with the results from the thermal large-eddy simulations of different Reynolds numbers and temperature gradients. This Letter provides a better understanding of the very anisothermal turbulent flow.