基于数值模拟方法的自然通风器风载体型系数研究

基于Fluent软件平台,采用混合网格划分,选用Realizable k-ε湍流模型,对某自然通风器的风场进行三维数值模拟研究.选择0°、45°、90°、135°和180°等风向,每个风向考虑39m/s风速,此外对0°、180°两风向还考虑了57m/s风速.通过模拟,得到了主通风器导流片的风载体型系数.经计算,0°,180°两个风向的风场对导流片抗风影响较大,基于此对这两个风向的计算结果进行了详细分析.在数值模拟过程中引人多孔介质阶跃模型模拟次通风器导流片,在降低模型复杂程度和减少计算量的同时,保证了数值模拟结果的可靠性.分析表明,基于数值模拟方法得到的风载体型系数在不同风向和风速的情况下呈现一定的变化规律性,为确定自然通风器类结构的风载体型系数提供了合理依据.     

超声速多喷流干扰流场特性研究

研究了旋成体上超声速来流与超声速横向多喷流相撞产生的层流干扰流场特性. 数 值方法针对三维可压缩Navier-Stokes方程按二阶精度Roe格式进行离散,采用基于多区对 接网格技术的有限体积法. 数值模拟结果描述了多喷流干扰流场的空间结构以及激波/边界层 干扰引起的分离范围,探讨了沿流向等间距排列的喷口个数对表面和空间流场结构以及压力 分布的影响规律. 结果表明,第一喷口对多喷流干扰流场主要结构和喷口上游表面分离范围 起主导作用. 其中三喷流流场数值模拟的对称面激波结构与实验纹影结果进行对比,符合较 好.     

垂向双蝠鲼变攻角滑翔水动力性能研究

为了探究垂向间距和攻角对双蝠鲼在沿垂向分布集群滑翔时的水动力性能影响,根据蝠鲼的实际外形建立了蝠鲼计算模型,设置了4种间距排布即0.25, 0.5, 0.75, 1倍体厚排布以及9种攻角状态即-8°～8°,随后借助Fluent软件进行了双蝠鲼变攻角、变垂向间距的集群滑翔数值模拟,结合流场压力云图以及速度云图对集群系统平均升/阻力以及集群中各单体的升/阻力进行了分析.数值计算结果表明:双蝠鲼沿垂向分布在攻角范围为-8°～8°进行集群滑翔时系统平均阻力均高于单体滑翔时所受阻力.单体在集群滑翔过程中获得减阻收益,当双蝠鲼以负攻角集群滑翔时,下方蝠鲼阻力减小,且垂向间距越小,减阻效果越明显;当以正攻角集群滑翔时,上方蝠鲼获得减阻收益.当双蝠鲼以负攻角滑翔时,系统平均升力大于单体滑翔时所受升力;当双蝠鲼以负攻角滑翔时,系统平均升力小于单体滑翔时所受升力,系统平均升力几乎不受垂向间距影响.下方蝠鲼升力始终大于上方蝠鲼升力,但随着垂向间距的增大,升力差距逐渐减小.     

并列圆柱绕流的格子Boltzmann数值模拟

采用非均匀不可压格子Boltzmann模型对低雷诺数下并列圆柱绕流进行了数值模拟,给出了数值计算结果,分析了间距g对圆柱尾流及升力、阻力的影响,并在此基础上得到了4种尾迹模式.此外,研究了流场的初始扰动对流动分岔现象的影响,发现在适当的扰动下可以很快得到同步同相的尾流.对Re=160和200下圆柱的升、阻力进行了对比,结果表明升力和阻力受间距g的影响大于雷诺数.     

正方形排列四圆柱体绕流特性及互扰效应研究

基于计算流体动力学理论,运用大涡模拟方法对雷诺数Re=3900三维正方形排列四圆柱体结构群的绕流问题进行数值计算,主要分析来流攻角与间距比两个参数对四圆柱体结构群流体参数及流场模态的影响。结果表明:来流攻角与间距比均对四圆柱体结构群绕流特性有较强的影响;来流攻角θ=0°、22.5°、45°下,临界间距比分别为3.5、4.0、3.0;间距比的变化会导致下游圆柱表面压力系数分布发生改变;另一方面,间距比较小时,四圆柱体结构之间的互扰作用均以临近效应为主;随间距比增大,上游圆柱尾流对下游圆柱有显著影响,其互扰作用会转变尾激效应。     

水平分布三气泡运动特性研究

为研究孔径、间距对三气泡运动特性的影响规律,本文利用高速相机和粒子图像测速仪观察三喷嘴气泡的运动过程,分析间距与孔径对三气泡的轨迹、速度、偏移量的影响,并与单气泡和双气泡进行对比,获得间距与孔径对气泡的影响规律以及流场对气泡的运动影响。实验结果表明,气泡数目的增多会使气泡运动轨迹更加复杂;气泡间距和孔径的增大或减小,对气泡的速度、流场、偏移量和运动轨迹都会产生影响。     

非定常空化流场结构的实验研究

为深入研究非定常空化流场结构,本文用实验方法研究了绕Clark-Y型水翼的非定常空化流动现象.实验在空化水洞中进行,采用高速摄像技术观测了云状空化阶段的非定常空穴形态,并应用粒子成像测速系统(PIV)对绕水翼空化流场的速度场和涡量场等流动特性进行了同步的实验分析.研究表明:空化现象对流场结构有着重要的影响,在无空化和空...     

高温燃气喷流/主流相互干扰实验研究

报道关于高温燃气自由喷流(热喷流)、燃气喷流/主流干扰流对气动热环境影响的实验研究结果. 其意义在于: 抽象出高超声速飞行器实际飞行时燃气喷流及其干扰流的物理模型, 为高超声速飞行器防热需求提供实验依据. 实验主流由脉冲风洞提供,燃气喷流用氢氧燃烧驱动路德维希管的方式产生. 利用脉冲风洞驱动段压力信号自动控制热气源的产生以保证风洞主流与燃气喷流同步, 利用氢气、氮气和氧气的不同比例实现燃气喷流的热力学相似. 实验技术上完成了高温燃气喷流系统的参数采集与系统状态标定; 实验内容上开展了压缩拐角平板模型的气动热实验研究, 通过实验比较了只有主流流场、只有热喷流流场和既有主流流场又有热喷流流场(即干扰流场)3种工况的热流分布. 实验研究发现,热喷流/主流相互干扰会对压缩拐角平板上某一范围内的气动热环境造成显著影响, 热流峰值较无喷流流场高出一个量级.      

重力和接触角对表面张力贮箱内液体流动的影响

采用流体体积函数方法数值模拟板式表面张力贮箱内液体的流动过程. 主要考虑了不同重力加速度和接触角等因素的影响. 发现在接触角等于10° 的前提下, 重力加速度小于10-2g₀, 液体沿着外侧导流叶片爬升至贮箱顶部; 当重力加速度小于10-3g₀ 时, 重力加速度的减小对液体爬升速度的影响较小. 在爬升过程中, 导流叶片附近液体的速度呈逐渐减小的趋势, 且重力加速度越小, 初始速度越大. 接触角对液体的爬升高度也会产生影响,接触角越小, 液体爬升至顶部的时间越短; 当接触角大于45° 时, 外侧导流叶片附近的液面不能爬升至顶端. 另外, 接触角越大, 液面高度的斜率越小,导流叶片附近液体的初始速度也越小.      

爆轰波拐角传播三维数值模拟

为了研究爆轰波在拐角中的传播特性以及拐角爆轰低压流场现象,用LSDYNA 3D程序对三种常见的拐角装药的爆轰波传播特性进行了数值模拟,清晰显示了爆轰波因拐角处爆轰波传播面积的变化而产生的衰减 增长过程,讨论了侧向三通中爆速的变化情况,并得到了拐角处低压爆轰区的尺寸。     

Measurement and calculation of guide vane performance in expanding bends for wind-tunnels

 The design of guide vanes for use in expanding bends was investigated both experimentally and numerically. The primary application in mind is the use of expanding corners in wind-tunnels for the purpose of constructing compact circuits with low losses. To investigate the performance of guide vanes in realistic situations expansion ratios between 1 and 53 were tested in the experiments. These were carried out in an open wind-tunnel specially built for the present purpose. The experimental results demonstrated that suitably designed guide vanes give very low losses and retained flow quality even for quite substantial expansion ratios. For wind-tunnel applications expansion ratios around 1.3 seem appropriate, Optimization of a guide vane design was done using a two-dimensional cascade code, Mises. A new vane optimized for an expansion ratio of 43 gave a two-dimensional total pressure-loss coefficient as low as 0.041 for a chord Reynolds number of 200,000. Received: 7 April 1997/Accepted: 11 August 1997     

Transient simulation of a pump-turbine with misaligned guide vanes during turbine model start-up

Experimental studies of a model pump-turbine S-curve characteristics and its improvement by misaligned guide vanes （MGV） were extended to prototype pump turbine through 3-D transient flow simulations. The unsteady Reynolds-averaged Navier-Stokes equations with the SST turbulence model were used to model the transient flow within the entire flow passage of a reversible pump-turbine with and without misaligned guide vanes during turbine model start-up. The unstable S-curve and its improvement by using misaligned guide vane were verified by model test and simulation. The transient flow calculations were used to clarify the variations of pressure pulse and internal flow behavior in the entire flow passage. The use of misaligned guide vanes can eliminate the S-curve characteristics of a pump-turbine, and can significantly increase the pressure pulse amplitude in the entire flow passage and the runner radial forces during start-up. The MGV only decreased the pulse amplitude on the guide vane suction side when the rotating speed was less than 50% rated speed. The hydraulic reason is that the MGV dramatically changed the flow patterns inside the entire flow passage, and destroyed the symmetry of the flow distribution inside the guide vane and runner.     

Hybrid simulations of the near field of a split-vane spacer grid in a rod bundle

Three-dimensional, unsteady simulations of isothermal turbulent flows in a rod bundle with a split-vane spacer grid have been performed using a segregated turbulence model, which is a combination of scale adaptive simulations and large eddy simulations. These simulations have been conducted within the framework of an international blind CFD benchmark exercise. A finer mesh than that submitted to the benchmark exercise was used for the present study, which improved the agreement of the turbulence predictions with the measurements. For the first time, several vortices were identified in the vicinity of the vanes. The strongest among these vortices, which was a central vortex in the core of each subchannel, was generated by the vane pair in each subchannel. Each vane also created a strong stream across the gap between two rods and towards an adjacent subchannel. Axial profiles of turbulent kinetic energy in each subchannel core exhibited two peaks, a low peak in the near-vane zone and a larger peak one hydraulic diameter downstream of the vanes.     

Correlation of swirl number for a radial-type swirl generator

An experimental investigation was undertaken to derive a new correlation for the swirl number of a radial-type swirl generator under various Reynolds numbers and various vane angle conditions. A radial-type swirl generator with 16 rotary guide vanes was used to generate an annular swirling jet flow. The Reynolds numbers ranged from 60 to 6000, and the vane angles from 0° to 56°. Quantitative measurements for the velocities were made by using an optical method of laser-Doppler anemometry (LDA). Three-component velocity profiles of axial, radial, and azimuthal components at the swirling jet exit were measured for various flow conditions. A flow visualization method using smoke-wire and still photography was also applied to observe the flow patterns of the recirculation region behind the circular bluff body. Under low Reynolds number conditions, the swirl strength was found to be strongly dependent on the Reynolds number as well as on the guide vane angle. Based on the experimental results, a modified swirl number S is derived to characterize the swirling flow, which is useful for the design of a swirl generator.     

Selection of wires and wire spacing for X-wires

X-wires are commonly used to obtain two components of the fluctuating velocity in a turbulent flow. Data obtained from the experiments of others and from our own experiments is used to provide design information that can be used as a guide in the selection of diameter, length and spacing of the wires that are used to make such X-wires. The selection is based on an expectancy that the errors in the measured data will then not exceed 4%. Selections for lower expected errors can also be made. Although the experiments have been carried out in specific flows, the results are reported in terms of the Kolmogorov flow scale and it is expected that they will therefore have general applicability.     

Experimental study and large eddy simulation for the turbulent flow around four cylinders in an in-line square configuration

The turbulent flows around four cylinders in an in-line square configuration with different spacing ratios of 1.5, 2.5, 3.5 and 5.0 have been investigated experimentally at subcritical Reynolds numbers from 11,000 to 20,000. The mean and fluctuating velocity distributions were obtained using the laser Doppler anemometry (LDA) measurement. The digital particle image velocimetry (DPIV) was employed to characterize the full field vorticity and velocity distributions as well as other turbulent quantities. The experimental study indicated that several distinct flow patterns exist depending on the spacing ratio and subcritical Reynolds number for turbulent flow. The three-dimensional numerical simulations were also carried out using the large eddy simulation (LES) at Reynolds number of 15,000 with the spacing ratio of 1.5 and 3.5. The results show that the LES numerical predictions are in good agreement with the experimental measurements. Therefore, the three-dimensional vortex structures and the full field instantaneous and mean quantities of the flow field such as velocity field, vorticity field, etc., which are very difficult to obtain experimentally, can be extracted from the simulation results for the deepening of our understanding on the complex flow phenomena around four cylinders in in-line configuration.     

A numerical and experimental investigation of the interactions between a non‐uniform planar array of incompressible free jets

The interaction between multiple incompressible air jets has been studied numerically and experimentally. The numerical predictions have been first validated using experimental data for a single jet configuration. The spreading features of five unequal jets in the configuration of one larger central jet surrounded by four smaller equi‐distant jets, have been studied, for different lateral spacing ratios of 1.5, 2.0 and 2.5 and a central jet Reynolds number of 1.24×105 (corresponding to a Mach number of 0.16). Flow of five equal jets has also been simulated, for the sake of comparison. The jet interactions commence at an axial distance of about 3–4 diameters and complete by an axial distance of about 10 diameters for the lowest spacing ratio of 1.5. For larger spacing ratios, the length required for the start and completion of jet interaction increase. Peripheral jets bend more towards the central jet and merge at a smaller distance, when their sizes are smaller than that of the central jet. The entrainment ratio for multiple jets is higher than that for a single jet. Excellent agreement is observed between the experimental data and theoretical predictions for both mean flow field and turbulent quantities, at regions away from the jet inlet. The potential core length and initial jet development, however, are not predicted very accurately due to differences in the assumed and actual velocity profiles at the jet inlet. Copyright © 2004 John Wiley & Sons, Ltd.     

4D Magnetic resonance velocimetry for mean velocity measurements in complex turbulent flows

An adaptation of a medical magnetic resonance imaging system to the noninvasive measurement of three-component mean velocity fields in complex turbulent engineering flows is described. The aim of this paper is to evaluate the capabilities of the technique with respect to its accuracy, time efficiency and applicability as a design tool for complex turbulent internal geometries. The technique, called 4D magnetic resonance velocimetry (4D-MRV), is used to measure the mean flow in fully developed low-Reynolds number turbulent pipe flow, Re=6400 based on bulk mean velocity and diameter, and in a model of a gas turbine blade internal cooling geometry with four serpentine passages, Re=10,000 and 15,000 based on bulk mean velocity and hydraulic diameter. 4D-MRV is capable of completing full-field measurements in three-dimensional volumes with sizes on the order of the magnet bore diameter in less than one hour. Such measurements can include over 2 million independent mean velocity vectors. Velocities measured in round pipe flow agreed with previous experimental results to within 10%. In the turbulent cooling passage flow, the average flow rates calculated from the 4D-MRV velocity profiles agreed with ultrasonic flowmeter measurements to within 7%. The measurements lend excellent qualitative insight into flow structures even in the highly complex 180° bends. Accurate quantitative measurements were obtained throughout the Re=10,000 flow and in the Re=15,000 flow except in the most complex regions, areas just downstream of high-speed bends, where velocities and velocity fluctuations exceeded MRV capabilities for the chosen set of scan parameters. General guidelines for choosing scanning parameters and suggestions for future development are presented.     

双层滑片回转运动机构的力学特性

建立双层滑片运动机构的力学模型，并全面分析其力学特性，通过分析发现，与机械摩擦损失相比，双层滑片之间相对运动产生的粘性磨擦损失很小，可以忽略不计，采用双层滑片能够明显改善滑片端部的密封效果，减轻滑片的磨擦磨损，但滑片应力有所增加，强度有所降低，厚度比对双层滑片的受力及摩擦磨损影响很小，只对强度产生较大影响，从增加强度和便于加工的角度考虑，优化出双层滑片机构厚度的比的适宜值。