建筑群环境风场的特性及模拟--风环境模拟研究之一

建筑风环境是研究建筑物与城市风场相互作用的一门空气动力学与大气科学的交叉学科、将空气动力学惯用的风洞实验和流场显示研究手段，直接用于城市大气边界层和建筑物周边流场变化规律的研究．充分利用数值计算的广度和深度与风洞实验相结合，相互验证、相互补充、弥补了目前实验手段尚无法实现的研究．本以北京商务中心区(CBD)为研究背景，论述了建筑风环境中大气边界层的模拟和具体建筑物风场的显示方法，并与数值计算进行对比，取得了非常一致的结果．为该地区的城市规划和环境保护，提出了科学依据．     

变高度圆柱诱导的激波边界层干扰

采用分区方法及Ｒｏｅ三阶流通量差分分裂格式求解雷诺平均Ｎ－Ｓ方程,湍流附加黏性系数用Ｂａｌｄｗｉｎ－Ｌｏｍａｘ模型计算,数值模拟了高超声速条件下变高度圆柱诱导的激波边界层层干扰,其流场的主要特性均与实验结果一致或规律相同,结果清晰地展示了流场结构以及气动载荷分布随柱高度的变化特征,产说明激波碰撞和旋涡运动都可能导致飞行器表面局部气动载荷的增加。     

激波风洞内超燃冲压发动机三面压缩进气道流场实验观测

主要进行了超燃冲压发动机三面压缩进气道的实验观测。利用来流马赫数4.5的直通式激波风洞,考察了三组具有不同压缩角度的进气道模型内部的流场情况。实验观测手段为油流法、丝线法和高速纹影,同时,辅以数值模拟以有助于流场细节分析。纹影照片展示了进气道内部以激波边界层相互作用为主要影响因素的流场复杂结构,数值模拟也显示了相近的结果。油流技术与丝线法显示了近壁面处的流动图像,照片中可见激波、分离线、再附线等分界线位置。根据实验结果,可以推测唇口激波与进气道内边界层的相互作用及其引起的壁面分离是影响进气道内流动的主要因素。同时,尝试了利用抽吸方法减弱激波与边界层相互作用诱发的壁面流动分离,并取得一定结果。     

Simulation of unsteady hypersonic combustion around projectiles in an expansion tube

The temporal evolution of combustion flowfields established by the interaction between wedge-shaped bodies and explosive hydrogen-oxygen-nitrogen mixtures accelerated to hypersonic speeds in an expansion tube is investigated. The analysis is carried out using a fully implicit, time-accurate, computational fluid dynamics code that we recently developed to solve the Navier-Stokes equations for a chemically reacting gas mixture. The numerical results are compared with experimental data from the Stanford University expansion tube for two different gas mixtures at Mach numbers of 4.2 and 5.2. The experimental work showed that flow unstart occurred for both the Mach 4.2 cases. These results are reproduced by our numerical simulations and, more significantly, the causes for unstart are explained. For the Mach 5.2 mixtures, the experiments and numerical simulations both produced stable combustion. However, the computations indicate that in one case the experimental data were obtained during the transient phase of the flow; that is, before steady state had been attained. Received 7 February 2000/ Accepted 20 February 2001     

65°三角翼亚音速复杂流场计算和数据可视化

提出一种基于非结构混合网格和有限体积法的有效计算策略,对第二期国际涡流试验项目(second international vortex flow experiment,VFE-2)的尖前缘65°三角翼在马赫数0.4,迎角20.3°,雷诺数2×10~6条件下的亚音速复杂流场结构进行数值模拟,重点探讨了基于计算数据进行该类型复杂涡系干扰表面和空间流场关键特征提取和数据可视化问题.通过与相关试验类比,建立了与先进试验流动显示技术相比拟的定性和定量分析方法,为三角翼这类复杂流场结构的精细分析奠定了技术基础.采用上述方法,细致分析了亚音速三角翼的大迎角复杂旋涡流场结构,得到了与试验一致的结论.研究证实:在大迎角条件下,三角翼流动物理复杂,黏性效应耦合严重,只有通过N-S方程计算才能准确地捕捉主涡和二次涡的发展.     

近空间飞行器的气动复合控制原理及研究进展

简单回顾与讨论了在飞行器穿越大气层时,使用气动复合控制方案的必要性与可能性.其中最复杂情况为发动机喷流推力与舵面空气动力共同使用所形成的复合控制.喷流与外流相撞引起的强干扰形成了十分复杂的干扰流场,文中介绍了复杂流动形成的原因、流场结构的特点以及干扰引起的流场改变影响了飞行器性能的预估.基于3种研究途径:理论建模与数值模拟技术、地面试验模拟技术、飞行试验技术的研究,以及它们的发展及互相验证, 用来预估飞行器的性能.为了保证地面模拟与真实飞行之间存在相似关系, 研究相似准则的作用,及分析目前的模拟能力, 涉及到许多空气动力学界至今尚未解决的难题,为了解决这些困难对今后的研究及应用提出了多方面的需求.      

Flow visualization and LDV measurements of laminar flow in a helical square duct with finite pitch

Flow visualization and LDV measurements are performed on laminar flow in a helical square duct with finite pitch. The experimental observations are compared to results of numerical calculations employing the finite-volume method and assuming a fully developed flow. Good agreement is found between measured and computed velocity profiles. This suggests that the physical velocity components used in the numerical calculations are suitable to describe the flow. It is further demonstrated that the contravariant velocity components employed by some authors may lead to results that are difficult to interpret. Two stable solution branches are detected in the numerical calculations. For Reynolds numbers between the stable branches, unsteady and fully developed computations predict an oscillating flow between a two-vortex and a four-vortex structure. In the experiments, the flow normally retained a stable two-vortex structure in the numerically predicted unstable regime. However, by disturbing the flow at the duct inlet, a four-vortex flow that showed similarities to the computed flow could occasionally be obtained. For Reynolds numbers above 600, unsteady flow behavior was observed both experimentally and numerically, which might be an early sign of transition. In the experiments, Gdrtler-like extra vortices emerged spontaneously from the outer wall without disturbing the flow at the inlet. The same phenomenon was observed in the numerical calculations, assuming an unsteady and fully developed flow, but the extra vortices appeared with a lower frequency than in the experiments.     

Interferometry and reconstruction of strongly refracting fields in two-dimensional boundary layer flow

 Optical methods like interferometry as non-intrusive experimental techniques are used for fine analysis of flowfields. The accuracy of the measuring method is very important for the applicability of its results to CFD validation. A common evaluation method to reconstruct interferograms is based on the assumption, that the object ray propagates along a straight line. But the strong bending of rays that occurs, e.g. in supersonic boundary layer flow, cannot be neglected without losses in reconstruction quality. Since the reconstruction of a two-dimensional boundary layer flow can be considered as an one-dimensional problem, the phase difference of the object and reference ray at the interferogram can be related analytically to the refractive-index distribution using a Taylor series expansion. The resulting interferometric equation is an ordinary non-linear second-order differential equation, which can be integrated by numerical methods. By application of this interferometric equation on the one hand, the error in the “classical” interferometry resulting from the ray bending neglection can be estimated. On the other hand, the accuracy in evaluation of interferograms of two-dimensional boundary layer flow can be improved by solving this equation. Received: 23 May 1996 / Accepted: 21 September 1996     

Vortex shedding and evolution induced by a solitary wave propagating over a submerged cylindrical structure

The shedding and evolution of the vortical structures generated by a solitary wave propagating over a submerged cylindrical structure are investigated experimentally and numerically. The cylindrical structure consists of two concentric cylinders and represents a simplified model for an offshore submerged intake structure typically used in coastal power plants. Flow visualization by dye injection is used to identify the dominant vortical structures near the structure. The flow visualization results show an unexpected flow reversal that causes shedding of secondary vortical structures. The experimental results are used to check a three-dimensional volume of fluid-large eddy simulation (VOF-LES) numerical model. The VOF-LES model is then used to further study the flow structure. A total of six dominant vortical structures generated by the wave motion are identified, followed by two more generated by the flow reversal. In summary, this paper provides the vorticity evolution for a complex fluid–structure interaction problem and a three-dimensional numerical simulation tool has also been validated, which can be extended to study more complex geometries and wave conditions.     

Direct numerical simulation of flow over a forward-facing step – Flow structure and aeroacoustic source regions

A direct numerical simulation of the flow over a forward-facing step at a Reynolds number of 8000 based on the step height is presented. Calculations were performed using second-order finite volume discretisation in space on co-located meshes. A hybrid calculation approach based on Lighthill’s acoustic analogy is explained. Results of the simulation are intended to be used as a database for the validation of different discretisation schemes for the flow computation and simulation approaches for the calculation of sound radiation using a hybrid approach. Turbulent statistics are presented along with aeroacoustic source regions. Strong and weak forms of the aeroacoustic source term are presented and compared. For visualization purposes, the strong form is more suitable, whereas for the calculation of sound radiation both forms can be used. From the visualization of the aeroacoustic sources, it can be seen that they mainly concentrate on the region of the leading edge of the step and the shear layer close to the step.     

Direct numerical simulation of turbulent pipe flow

Fully developed turbulent pipe flow at low Re-number is studied by means of direct numerical simulation (DNS). In contrast to many previous DNS's of turbulent flows in rectangular geometries, the present DNS code, developed for a cylindrical geometry, is based on the finite volume technique rather than being based on a spectral method. The statistical results are compared with experimental data obtained with two different experimental techniques. The agreement between numerical and experimental results is found to be good which indicates that the present DNS code is suited for this kind of numerical simulations.     

流动显示技术及其现代发展概况

本文回顾了流动显示发展的历史,以及它在流体力学研究中所起的作用。根据获取流动信息的方式不同,流动显示技术可分为两大类型。作为近代技术发展的例子,三维非对称流干涉图的定量计算、片光技术、散斑照相术、激光诱导萤光法、多普勒干涉图象显示速度场、光电信号转换激光干涉仪、以及图象处理和计算机产生数字流谱技术均作了简要的介绍。     

有压管道双孔板水流流场的数值模拟

利用数值计算的方法对多级孔板消能器内湍流流场进行模拟，并将所得的结果与实验进行了比较。这些比较表明，数值计算的方法是可靠的。     

基于波动法的框式结构功率流主动控制方法与实验研究

为了研究扰动影响下框式结构中的功率流传播与主动控制,首先采用波动方法建立了框式结构的动力学模型并获得了其在扰动下的精确动力学响应,进一步得到结构中传播的功率流,并以此为目标函数,优化得到了最优控制力的大小与相位,然后对结构施加最优控制力,实现了框式结构的功率流主动控制。对框式结构功率流主动控制方法进行了数值计算分析与实验验证。结果表明：采用波动方法计算框式结构的动力学响应精确可靠;通过数值计算与实验研究可知功率流主动控制可以明显降低框式结构全频域内的抖动,验证了基于波动方法功率流主动控制的正确性与有效性。     

Effects of body shape and viscoelasticity on the slow flow around an obstacle

A study was made of the motion of axisymmetrical objects in viscous and viscoelastic fluids within a cylindrical tank with the assumption of negligible inertial effects. A numerical treatment based on the Stokes equations of motion and an optimization technique enabled the details of the velocity and rate-of-deformation fields for a Newtonian fluid to be predicted. The influence of the shape of various bodies, some even with concave surfaces, was examined. The corresponding experiments were carried out with viscous and viscoelastic fluids using a visualization technique. A correlation between the main flow characteristics and the rheological behaviour of the fluids was established.     

High-resolution numerical model for shallow water flows and pollutant diffusions

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Simulation and Modelling of a Skewed Turbulent Channel Flow

A time-dependent three-dimensionally skewed flow is investigated using direct numerical simulations of the incompressible Navier-Stokes equations. The effect on the instantaneous and mean turbulent field is investigated. Instantaneous flowfields reveal that the skewing has the effect of initially reducing the strength and height of quasi-streamwise vortices of both signs of rotation with respect to the skewing. A mechanism for this process is put forward. The mean flowfields show drops in turbulence quantities such as turbulence kinetic energy. In addition to this, two-equation turbulence modelling of the flow is carried out. This highlights a deficiency, in that the standard turbulence models are unable to capture the drop in turbulence intensity due to the skewing. A modification based on the exact dissipation equation is found to significantly improve the model behaviour for this flow. This revised version was published online in July 2006 with corrections to the Cover Date.     

旋转翼面尾迹的流态显示技术

本文介绍了一种可用于显示旋转弹翼面在非旋转和旋转过程中拖出的涡及其轨迹的流态显示技术。本方法在烟线技术的基础上,采用新的发烟装置,既提高了发烟的浓度,延长了发烟的时间,并不增加任何附加力矩而使发烟部件随旋转弹体一起转动,从而实现了在旋转过程中进行动态的流态显示。文中对于应用此方法得到的流态的真实性进行了分析讨论,分析表明用此方法显示的流态具有一定的真实性,可以提供定性的旋转翼面尾涡流动特性。     

Experimental and numerical study on the falling film flow characteristics outside circular tube applied in floating liquefied natural gas (FLNG) under offshore conditions

Spiral wound heat exchanger (SWHE) relying on falling film evaporation and boiling is often used for FLNG. The performance of SWHE can be impacted strongly by the motion of the FLNG caused by the wave and typhoon. The falling film characteristics of SWHE outside circular tube are studied experimentally and numerically by a visualization experimental device based on the high-speed camera and a numerical model based on the dynamic grid. The results show that the wave crest of the liquid film moves to the titled side under offshore conditions. The evolution process of falling film flow pattern outside circular tube with the tilt angle of 9° can be divided into four stages: droplet formation and migration, liquid column formation and migration, liquid column coalescence, liquid sheet formation. A correlation permitting the prediction of the falling film flow pattern outside circular tube and the other one permitting the prediction of the average film thickness of circular tube are developed respectively based on the experimental and numerical data.