混合层流动拟序结构的大涡模拟

采用大涡模拟方法对空间发展的二维平面混合层进行了数值模拟 ,动量方程采用分步投影法求解 ,亚格子项采用标准Smagorinsky亚格子模式模拟 ,压力泊松方程采用修正的循环消去法快速求解 ,同时求解了标志物输运方程以实现数值流场显示。模拟结果给出了混合层流动的瞬态发展过程以及流动中拟序结构的发展演变过程 ,成功地模拟了混合层发展中的各种瞬态细节过程 ,如涡的卷起、增长 ,涡与涡之间的配对、合并过程 ,以及大涡破碎为小涡的级联过程 ,为各种以混合层流动为原型流动的射流、尾流等工业流动的控制和优化提供了理论基础。     

混合层强化混合的数值研究

受 Ｗａｎｇ ＆ Ｆｉｅｄｌｅｒ（１９９７）的实验的启发，采用高阶精度的差分格式，通过数值模拟的方法，研究了二维混合层及限于两平板间的二维混合层（二维受限混合层）入口处加振动对提高混合层混合效率的作用．计算结果表明：对二维混合层，振动的频率越低，在混合层中产生的大尺度涡结构的尺度越大，在频率很低时，涡具有相似性；对限于两平板间的二维混合层，在一定的振动频率下，混合层中产生的涡较大而且破碎得也较好，这将有利于混合．这一结论与 Ｗａｎｇ ＆ Ｆｉｅｄｌｅｒ（１９９７）的实验观测到的结果是一致的．     

混合层中的流向涡与Rayleigh离心不稳定

用有限差分方法求解三维Ｎａｖｉｅｒ－Ｓｔｏｋｅｓ方程和连续性方程，对时间发展的平面混合层中流向涡的产生原因进行了分析．将Ｒａｙｌｅｉｇｈ的轴对称无粘离心不稳定理论推广应用于分析混合层的二维基本流，并导出一无量纲量Ｒａｙ＝－（ｒ／νθ）νθ／ｒ，其中νθ为一流体质点相对于平均速度的速度，ｒ为该质点迹线的曲率半径．当Ｒａｙ＞１．０时就会发生离心不稳定．采用这一判别式后发现混合层中展向涡的周围，尤其是在辫带区，的确存在离心不稳定区域，而过去的实验结果也表明三维不稳定产生于展向涡之间的辫带区．因此有理由认为，除非雷诺数特别小，离心不稳定是流向涡产生和发展的主要原因     

刚杆状聚合物和混合层中拟序结构的相互影响

用港方法（即Galerkin方法）求解多球刚杆模型位型空间分布函数的扩散方程，用拟谱方法求解物理空间的N－S方程，同时得到了加入刚杆状聚合物分子的二维混合层中拟序结构的演变过程和聚合物分子的最可几取向．计算结果较为合理地描述了聚合物分子和拟序结构的相互影响．||关键词##4多球刚杆分子模型;;混合层;;拟序结构;;谱方法     

速度分布对分层流中底部边界混合层结构的影响

对分层流中海底边界混合层发展问题提出了一个新的垂直积分模型。在这个模型里，新建立的能量方程中出现了一个表征速度大小随高度变化规律的参数，从而得出了被Ｐｏｌｌａｒｄ等人文章中应用的但被本文修正过的Ｒｉｃｈａｎｄｓｏｎ数封闭模型，对在水平乎底上的混合层成长问题，给出了分析解。从而把以往的公式都统一到本文的结果中，而且给出了以往海底边界混合层公式的理论依据与适用范围，这个结果与Ｗｅａｔｈｅｒｌｙ与Ｍａｒｔｉｎ（１９７８）用Ｍｅｌｌｏｒ与Ｙａｍａｄａ（１９７４）的高阶模型所得的数值解结果是基本一致的，并与现场实验相符。     

平面湍流混合层的准相似性理论

本文应用周培源的理论,给出了不可压缩平面湍流混合层的一级近似解,我们考虑连续性方程,平均运动方程和二阶脉动速度关联方程,忽略二阶脉动速度关联方程中的三阶关联项,并引进自模拟假设,得到了和实验符合的平均速度,二阶脉动速度关联项和湍流微尺度等的理论计算结果。     

可压缩气固混合层中离散相与连续相的相互作用研究

尽管已有许多文献采用数值模拟方法研究两相流问题,但主要是集中不可压流动方面.本文采用Eul-er-Lagrange颗粒-轨道双向耦合模型对时间模式下含有固粒的二维可压缩混合层流动进行了研究.气相流场采用非定常全Navier-Stokes方程描述,并应用具有空间三阶精度的WNND(Weighted Non-Oscillatory, Contai-ning No Free Parameters and Dissipative)格式进行数值高散.固相方程采用二阶单边三点差分离散.在考虑流场对固粒作用的同时,也计及颗粒对流场的反作用.主要研究混合层大尺度涡对颗粒扩散特性的影响及颗粒对流场结构的影响问题.在对流马赫数为0.5时,研究不同Stokes数颗粒在连续流场中的扩散特性,而在对流马赫数为0.8时研究了不同Stokes数颗粒对流场小激波结构的影响.     

空间模式下可压缩气固两相混合层流动特性研究

采用Euler-Lagrange颗粒-轨道双向耦合模型对空间模式下含有固粒的二维可压缩混合层流场进行了研究。气相流场采用具有空间三阶精度的WNND格式进行数值模拟,固相方程采用单边三点差分离散。在考虑流场对固粒作用的同时,也计及了固粒对流场的反作用。在对流马赫数为0.5时,研究了颗粒相对密度、颗粒尺寸、Stokes数等因素对粒子运动和流场结构的影响。研究结果表明:在可压缩空间模式混合层中,固粒的Stokes数仍然是主要影响参数之一;相同Stokes数下不同密度的固粒对流场的干扰不同,轻固粒对流场的干扰明显要小。     

低对流马赫数平面混合层的声波产生机理

为了详细揭示低对流马赫数下平面混合层的声波产生机理,基于高精度混合格式,对空间发展的平面混合层的流场结构及其声场特性进行了直接模拟.数值结果描述了对流马赫数为0.4时混合层失稳产生的涡串以及涡发展过程中所存在的各种声源,其中包括常规的单涡四极子声源、双涡以及多涡合并过程产生的四极子声源,另外,还发现因K-H不稳定引起混合层振动而产生的偶极子声源,声波以平面波的形式向周围传播.     

化学反应对超音速混合层发展的影响

超音速混合层流动发展的研究对于了解超音速燃烧过程具有重要意义.基于耦合详细化学反应动力学机理的高精度数值模拟,分析了化学反应对超音速混合层发展过程的影响.主要分析了在两种燃烧状态下化学反应对混合层的演化过程和混合层厚度的影响,此外从涡动力学角度,分析了化学反应对混合层厚度的影响机理.     

NUMERICAL SIMULATION OF BROWNIAN COAGULATION AND MIXING OF NANOPARTICLES IN 2D RAYLEIGH-B?NARD CONVECTION

采用泰勒展开矩方法对二维瑞利-贝纳德热对流系统(1×10⁶ ≤Ra ≤1 ×10⁸) 中纳米颗粒群的混合和凝并特性进行了数值模拟. 结果显示颗粒群随时间演化经历了扩散阶段、混合阶段、充分混合阶段3 个阶段, 随着颗粒群混合和凝并的进行, 颗粒数目浓度减少, 颗粒群的平均体积增大; 得到了颗粒分布函数各特征量与温度相关系数以及各特征量的空间分布标准偏差在3 个阶段的不同特征; 得到了颗粒分布函数各阶矩以及平均体积长时间演化的渐近行为, 结果与零维渐近解析解一致. 最后, 本文进一步研究了无量纲数(包括瑞利数Ra, 斯密特数Sc_M, 达姆科勒数Da) 对颗粒群达到自保持分布时间的影响, 发现该时间随着Ra和Sc_M的增大呈对数率减小, 随着Da的增大呈线性增大     

某区放射性环境地质评价研究

在某区拟建隧洞沿线的条带状区域进行了放射性环境地质调查。共采集和分析岩土(芯)放射性核素镭、钍、钾样品111组,地下水和地表水样品5组;现场测定岩土放射性样品392组。结果表明,在该区域内,某煤矿巷道中煤层和砂岩接触带煤、砂岩中镭-226比活度出现高值异常,相应的内照射指数和外照射指数也超过相关标准中规定的放射性核素限量,该异常带环境γ辐射剂量率也较高。镭-226的富集可能源于煤中有机质对放射性核素的吸附和解吸过程。建议在隧道选线过程中尽力避让这一地段。     

多孔介质中对流的周期性解与混沌

研究多孔介质内部有热源的对流传热．用高阶差分研究在不同的渗流瑞利（Ｒａｙｌｅｉｇｈ）数Ｒａ下对流随时间进展的演化情况（为比较起见也适当考察倾斜角的影响）．Ｒａ计算到大约１６０００．结果表明：在Ｒａ较小时对流是稳定的，Ｒａ增大到４６００出现了非稳定的但为周期性的解．随着Ｒａ进一步增大，出现一些混沌窗口．对于有侧斜角的情形，还出现阵发性     

Heat transfer in a second-order fluid for flow around a circular cylinder

The temperature distribution within the thermal boundary layer region due to the flow of an incompressible second-order fluid around a heated circular cylinder, maintained at a constant temperature higher than that of the fluid at infinity, has been obtained near the forward stagnation point by series expansion. The graph of the Nusselt number Nu for the Prandtl number P = 25 and the Eckert number E = 0.1 indicates that the non-Newtonian effect is to increase the heat flux from the cylinder to the liquid in the region 0° ? θ ? 15.7° and to decrease it in the region 15.7° < θ ? 0₀ where θ is the angular distance on the cylinder measured from the forward stagnation point. The critical point θ₀ at which Nu = 0, that is, where the effect of the frictional heating balances the effect of the temperature difference and there is no heat flux either from the cylinder or from the liquid, shifts towards the forward stagnation point with the increase of non-Newtonian effects.     

Interferometric study of buoyancy-driven convection in a differentially heated circular fluid layer

The present study is concerned with buoyancy-driven convection experiments in a circular horizontal differentially heated layer of air. The radius-to-height ratio of 14, and Rayleigh numbers of 5,861 and 12,124 have been considered. A Mach–Zehnder interferometer has been used to visualize the convection patterns in the fluid layer. The fluid layer has been imaged at view angles of 0, 45 and 90°. Results obtained show that fringe patterns appropriate for a cavity square in plan are seen in the fluid layer during the early stages of the experiments. After the passage of the initial transients, steady fringes have been observed in the fluid layer for a Rayleigh number of 5,861. At Ra=12,124, a dominant pattern was detectable combined with mild unsteadiness. The steady thermal field at Ra=5,861 displayed symmetry with respect to the viewing angle. A stronger three dimensionality was seen at the higher Rayleigh number. The average steady state Nusselt numbers were found to vary with view angle from 1.91 to 2.04 at Ra=5,861 and 2.28 to 2.43 at Ra = 12,124. The cavity-averaged Nusselt numbers are in good agreement with the available correlations.     

ELASTIC PERFECTLY-PLASTIC FIELDS AT A RAPIDLY PROPAGATING CRACK-TIP

All the stress components at a rapidly propagating crack-tipin an elastic perfectly-plastic material are the functions of θ only.Making use of this condition and theequations of steady-state motion.stress-strain relations and yield conditions,we obtain thegeneral solutions in both the cases of anti-plane and in-plane strain.Applying these twogeneral solutions to propagating Mode III and Mode I cracks.respectively,the elasticperfectly-plastic and the perfectly-plastic fields at the rapidly propagating tips of Mode IIIand Mode I crocks are derived.     

同位网格摄动有限体积格式求解浮力驱动方腔流

利用对流扩散方程的摄动有限体积格式，在Rayleigh数从10$^{3}$ 到10$^{8}$的范围内对浮力驱动方腔流动问题作了数值模拟. 对流扩散方程的摄动 有限体积格式具有一阶迎风格式的简洁形式，使用相同的基点，重构近似精度高，特别是两 相邻控制体中心到公共界面的距离相等或不相等，PFV格式公式相同等优点. 在数值模拟中， 无论均匀网格还是非均匀网格均获得与DSC方法、自适应有限元法、多重网格法等Benchmark 解相符较好的数值结果，证明UPFV格式对高Rayleigh数对流传热问题的适用性和有效性.     

Heat convection from a cylinder performing steady rotation or rotary oscillation – Part II: Rotary oscillation

This paper deals with the problem of combined (forced and natural) convection from a horizontal cylinder performing oscillating rotary motion in a quiescent fluid of infinite extent. While forced convection is caused by cylinder oscillation, the natural convection is caused by the buoyancy driven flow. The heat transfer process is governed by Rayleigh number, Ra, Reynolds number, Re, and the dimensionless frequency of oscillation, S. The study covers Ra up to 10³, Re up to 400 and S up to 0.8. The results showed that, for the same Ra, the time-averaged rate of heat transfer lies in between two limiting values. The first, is the steady state heat rate due to natural convection from a fixed cylinder and the second is the steady state heat rate from a cylinder rotating steadily at a speed equal to the maximum speed of rotational oscillation. The smaller the value of Re the nearer the time-averaged Nusselt number to that of fixed cylinder at the same Ra and the higher Re the lower the average Nusselt number. The effect of frequency is only limited to changing the amplitude of the fluctuating Nusselt number. Received on 15 December 1997     

Heat transfer characteristics of internally heated liquid pools at high Rayleigh numbers

Detailed numerical analysis is presented for buoyancy driven flow of a Newtonian fluid contained in a square enclosure for high Rayleigh (Ra) numbers. Natural convection is due to internal heating sources, which are assumed to be uniformly distributed within the enclosure. All walls of the cavity are maintained at constant temperature. Flow and heat transfer characteristics are investigated for a Ra number range of 10⁷ to 10¹² while Prandtl (Pr) number is taken to be 7.0. Governing equations (in primitive variables) are discretised using control volume technique based on staggered grid formulation. These equations are solved using SIMPLER algorithm of Patankar. Flow and heat transfer characteristics, streamlines, isotherms and average wall Nusselt (Nu) number, are presented for whole range of Ra number considered. Finally, present results for average wall Nu numbers are compared with experimental observations obtained from open literature. It is concluded that both results are in very good agreement, which confirmed the accuracy of the scaling used for present investigation. Received on 15 November 1999