The Decay of Swirling Flows in a Type of Cross-Section-Varying Pipes

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静电旋风分离器气相流场的数值模拟

对一内部安装电晕极的切向进口旋风分离器,以三维贴体坐标为基础,应用Bradshaw的修正k-ε湍流模型,用非交错的SIMPLE算法对静电旋风分离器的气相流场进行求解,计算结果与实验结果进行了对比。分析了电晕极的不同安装位置对旋风分离器流场的影响。从流场的角度来看,电晕极安装在筒体与排气管之间并靠近排气管的位置有利于提高静电旋风分离器的分离效率。     

Computational Modelling of Turbulent Mixing in Confined Swirling Environment Under Constant and Variable Density Conditions

A high-intensity swirling flow in a model combustor subjected to large density variations has been examined computationally. The focus is on the Favre-averaged Navier–Stokes computations of the momentum and scalar transport employing turbulence models based on the differential second-moment closure (SMC) strategy. An updated version of the basic, high-Reynolds number SMC model accounting for a quadratic expansion of both the pressure–strain and dissipation tensors and a near-wall SMC model were used for predicting the mean velocity and turbulence fields. The accompanied mixing between the annular swirling air flow and the central non-swirling helium jet was studied by applying three scalar flux models differing mainly in the model formulation for the pressure-scalar gradient correlation. The computed axial and circumferential velocities agree fairly well with the reference experiment [So et al., NASA Contractor Report 3832, 1984; Ahmed and So, Exp. Fluids 4 (1986) 107], reproducing important features of such a weakly supercritical flow configuration (tendency of the flow core to separate). Although the length at which the mixing was completed was reproduced in reasonable agreement with the experimental results, the mixing activity in terms of the spreading rate of the shear/mixing layer, that is its thickness, was somewhat more intensive. Prior to these investigations, the model applied was validated by computing the transport of the passive scalar in the non-swirling (Johnson and Bennet, Report NASA CR-165574, UTRC Report R81-915540-9, 1981) and swirling (Roback and Johnson, NASA Contractor Report 168252, 1983) flow in a model combustor.     

高精度格式WCNS-E-5的Fourier分析与应用

对高精度加权紧致非线性格式WCNS-E-5进行了理论分析和应用研究。首先采用Fourier方法分析了WCNS-E-5与WCNS-5及其它高阶格式(迎风偏置格式EUW-5与标准格式)的差分误差特性,研究了它们在分辨效率方面的特性和相位误差在多维方向上的各向异性特性。分析结果显示WCNS-E-5与WCNS-5在色散与耗散特性方面优于EUW-5,分辨效率也普遍高于EUW-5和格式,而且它们的相位速度在多维方向上传播所表现的各向异性特性与其它高阶格式一致。WCNS-E-5的高精度特性与WCNS-5的一致,但在计算过程中少了三对角矩阵求解因而它的效率更高,于是采用WCNS-E-5数值模拟了二维/三维复杂流场,算例包括单涡斜向运动、二维Rie-mann问题以及存在分离的大攻角钝锥流动问题。计算结果体现了WCNS-E-5具有很低的数值耗散误差,它对激波、膨胀波和接触面等间断具有很好的捕捉能力,得到的图像清晰光滑,准确再现了真实流动现象。     

Disappearing Wakes and Dispersion in Numerically Simulated Flows Through Tube Bundles

Flow through a staggered array or bundle of parallel rigid cylinders of diameter D is computed with the help of a three-dimensional direct numerical simulation (DNS) at various values of Reynolds number between 50 and 6000. Two different spacings L of the tubes, i.e. L/D= 2 and L/D= 3, have been considered. When Re 500 the flow is laminar. In that case the converging flow between a pair of adjacent cylinders brings the oppositely signed vorticity at the two edges of the wake closer together behind the upstream cylinder so that the vorticity decreases quickly due to cancellation by diffusion. At Re 6000, when the flow is highly turbulent, the wake vorticity disappears rather by turbulent diffusion. This disappearance of the wakes in the closely packed flows (i.e. L/D 2) causes the mean flow in a cell, which consists of the region around a single cylinder, to be effectively independent of that in other cells. Another consequence is that the mean velocity field can be very well approximated by potential flow except in a thin boundary layer along the cylinder and a short wake behind it. The results have been applied to the transport of scalars in closely packed arrays. As in other complex flows, the dispersion of the scalars is dominated by the divergence and convergence of the streamlines around the cylinder rather than by the wake turbulence. Approximate expressions are derived for this topologically influenced dispersion in terms of the geometry of the array. The fact when most of the flow in the array can be approximated by a potential flow, allows us to introduce a fast approximate calculation method to compute the dispersion.     

展向喷嘴湍流横向射流大涡模拟及其统计分析

采用大涡模拟方法数值模拟了展向椭圆喷嘴的湍流横向射流,对其大尺度结构的时空演化和湍流脉动速度场的时间序列分析、频谱分析、PDF分析以及时、空截面上的统计平均特性进行分析.结果表明,在射流出口附近的下游核心区中速度脉动剧烈,显现出明显的湍流特征.除了三维涡环脱落、扭曲、变形、摆动所对应频率之外,还存在很宽的湍流基频,它与在喷嘴出口附近产生的三维涡环的时空演化过程密切相关.由于展向椭圆喷嘴的湍流横向射流中的三维涡环快速脱落和强相互作用导致射流尾迹中的强湍流脉动,展向椭圆喷嘴湍流横向射流的PDF空间演化特征结构复杂.在射流核心区的湍流偏应力变化平缓,其统计平均值分布接近左右对称.展向椭圆喷嘴的湍流横向射流脉动速度场具有极为复杂的统计行为,与流向椭圆喷嘴相比具有更好的掺混能力.     

A 1D-Averaged Model for Stable and Unstable Miscible Flows in Porous Media with Varying Péclet Numbers and Aspect Ratios

This paper deals with reducing the number of spatial dimensions of the models used to solve stable and unstable miscible flows in saturated and homogeneous porous media. Unstable miscible displacements occur when a fluid displaces another fluid of higher viscosity, with which it can fully mix. Stable flows occur if the displaced fluid is less viscous than the displacing one. First, a 1D-averaged model is identified, capable of accurately describing unstable flows at high Péclet numbers. Second, another 1D-averaged model is determined, capable of accurately predicting miscible displacements at low Péclet numbers. Third, a new model is proposed, for any Péclet number and for both stable and unstable flows, as a combination of the previous two models. This combination involves three parameters whose values depend on the dimensionless numbers of the problem, namely, the viscosity ratio M, the Péclet number P_e, the aspect ratio A, and the dispersion length ratio ε. These parameters are computed for several values of M, P_e, A with ε=1 by matching results from direct 2D simulations, obtained from a numerical model previously validated against experimental data. It is found that a 1D-averaged model combining an extended version of the Todd–Longstaff model and the diffusive term of the 1D-miscible model forms an accurate general model for miscible displacements in homogeneous porous media. This paper also provides a large set of data computed from high-resolution 2D simulations of unstable miscible displacements.     

The journal Communications in Numerical Methods in Engineering with Biomedical Applications becomes the International Journal for Numerical Methods in Biomedical Engineering (IJNMBE) from 1st January 2010

Communications in Numerical Methods in Engineering, founded by Roland W. Lewis in 1985, will change its title to the ‘International Journal for Numerical Methods in Biomedical Engineering’ and has a revised Aims and Scope. Copyright © 2009 John Wiley & Sons, Ltd.     

Analysis of Displacement Variances in Stochastic Nonuniform Flows by Means of a First Order Analytical Model and Comparison with Monte-Carlo Simulations

Flow, particle displacement and particle arrival time statistics in 2D nonuniform flows, without microdispersion, are studied both theoretically, in the framework of stochastic modelling, and numerically by means of Monte-Carlo simulations.Turning, radial convergent and dipolar flow fields are considered. These three types of flow are numerically investigated in heterogeneous media with different levels of heterogeneity. Monte-Carlo simulations show (1) that the scale separation hypothesis, frequently used in fluid mechanics, is justified for one-point flow statistics; and (2) how displacement variances, and consequently the dispersivities defined as their spatial derivatives, depend on the type and the amplitude of flow nonuniformity: in none of the investigated cases does the assumption of scale separation hold for displacement, except in the turning flow when the spatial scale associated to the nonuniformity is much greater than the correlation scale of transmissivity.The theoretical approach of displacement and arrival time statistics relies on the analysis of particle trajectory. Displacement variances expressions are derived by the perturbation method for each type of flow and for different approximation orders. The proposed expressions of displacement variances are, on the whole, in good agreement with the numerical results. On the other hand, the uniform flow approximation – commonly used for the interpretation of tracer experiments – chosen such as to satisfy the mean arrival time to the pumping well, gives the best prediction of the breakthrough curves.     

Experimental and computational study of the developed flow field in a flat plate integrated collector storage (ICS) solar device with recirculation

The flow structure in a flat plate integrated collector storage device, with recirculation of the storage water, is studied experimentally and theoretically. To facilitate flow visualization, an experimental device was constructed by transparent material (Plexiglas). Flow velocities and fluctuations are measured, using a LDV system. A three-dimensional CFD-model was developed using the FLUENT code. The standard k–ω model is selected as the most appropriate. The model is validated, with good agreement, against experimental measurements. Furthermore, copper tubes, in the form of embedded heat exchanger, are placed inside the device and another similar 3D model was developed. The model was used to examine the behavior of the system, when the service water enters the heat exchanger, thus being indirectly heated by the stored hot water. It is shown that the outlet temperature of the service water is enough higher, when recirculation occurs.     

内管自转且公转时环管幂律流的受力分析

数值模拟了环管中内管偏心自转且公转时由轴向压力所驱动的幂律流体充分发展层流,分析了内管上的流体作用力。结果表明,内管偏心自转时流体作用力具有推动内管作和自转同向公转的效果。当只有外力矩驱动内管自转时,由于流体的作用,随内管线密度的不同,内管能达到的受力平衡态也不同:线密度较小时内管仅能在同心自转时达到受力平衡;线密度较大时内管能在作具有不变角速度和偏心率公转时达到受力平衡,且内管线密度越大,对应的受力平衡的公转的偏心率也越大。     

Heat and fluid flow in cylindrical and conical annular flow-passages with through flow and inner-wall rotation

The flow and heat transfer in cylindrical and conical annular flow-passages with through flow and inner-wall rotation have been numerically simulated by using the large eddy simulation with a Lagrangian dynamic subgrid-scale model. Inlet through-flow Reynolds number was 1000 and the Taylor number was set at 0, 1000, 2000, and 4000. In the conical flow passage, when the inner-wall rotation speed was increased, at first spiral vortices in the downstream region and then much more complicated vortices appeared. The vortices for Ta = 4000 changed the structure in both through-flow and wall-normal directions in the downstream half of the passage. The flow structure and heat transfer of the conical case were completely different from those of the cylindrical case. It was because of the three factors: the expansion of the flow passage, the rotation radius change in the through-flow direction, and the centrifugally driven through-flow. The last factor is due to the acute angle between the centrifugal and through-flow directions.     

水流冲击管道内滞留气团现象的VOF模型仿真分析

针对复杂管道系统内水流冲击滞留气团现象,采用VOF模型(Volume of Fluid Model)进行了数值模拟计算,并与一维模型进行了比较计算分析,结果表明:系统的最大压力并不总是气团的最大压力,有可能还会叠加水体对管壁的撞击而形成的突然升高压力.与实验实测结果的比较分析表明:采用VOF模型,能够较精细地仿真水流冲击滞留气团现象的气团形态、流场结构以及压力分布等的变化过程,其压力数值计算结果与实验实测基本吻合,其计算误差明显小于现有一维模型的计算误差,是深入研究该复杂瞬变流现象的有效方法.     

液固两相流冲洗油管道的冲蚀磨损特性数值模拟及分析

以液固两相流冲洗油管道为研究对象,采用Realizableκ-ε湍流模型、随机轨道模型,结合液固两相流冲蚀磨损试验,建立修正的冲蚀磨损数理模型,数值预测典型工况下冲洗油管道内速度、压力、冲蚀磨损率等流动参数分布情况,分析了冲蚀磨损的形成机制.研究结果表明:受曲率半径影响,冲洗油管道冲蚀磨损速率随曲率半径的增加而减小;由于颗粒惯性及管内二次流影响,弯头中间区域外侧壁面和出口直管段内侧面磨损较为严重,三通管件的最大冲蚀磨损率位于孔口处,数值预测结果与失效解剖案例吻合.本文建立的冲蚀磨损定量预测方法,适用于压力管道的风险评定及寿命预测.     

Numerical simulation for laminar flow and heat transfer of gas in rectanglar micropassages with constant wall heat flux

Theoretical investigations were performed on the developed laminar flow and convective heat transfer characteristics for incompressible gases flow through rectanglar micropassages with constant wall heat flux. Mathematical models were proposed for considering the change in viscosity and thermal conductivity of gas in the wall-adjacent region from the kinetic theory. The dimensionless velocity distribution and corresponding pressure drop, the dimensionless temperature distribution and corresponding heat transfer characteristics were both simulated numerically, and the results were compared to other report simulations [10–12] with brief discussions.     

Usability of explicit filtering in large eddy simulation with a low‐order numerical scheme and different subgrid‐scale models

Based on a priori tests, in large eddy simulation (LES) of turbulent fluid flow, the numerical error related to low‐order finite‐difference‐type methods can be large in comparison with the effect of subgrid‐scale (SGS) model. Explicit filtering has been suggested to reduce the error, and it has shown promising results in a priori studies and in some simulations with fourth‐order method. In this paper, the effect of explicit filtering on the total simulation error is studied together with a second‐order scheme, where the numerical error should be even larger. The fully developed turbulent channel flow between two parallel walls is used as a test case. Rather simple SGS models are applied, because these models are most likely used in practical applications of LES. Explicit filtering is here applied to the non‐linear convection term of the Navier–Stokes equations, four three‐dimensional filter functions are applied, and the effect of filtering is separated from the effect of SGS modelling. It is shown that the effect of filtering is rather large and smooth filters introduce an additional error component that increases the total simulation error. Finally, filtering via subfilter‐scale modelling is applied, and it is shown that this approach performs better. However, the large‐frequency components of the resolved flow field are not as effectively damped as when the non‐linear convection term is filtered. Copyright © 2007 John Wiley & Sons, Ltd.