特征线算子分裂有限元的圆柱绕流大涡模拟

为研究圆柱绕流流场特性,将大涡模拟与特征线算子分裂有限元相结合,建立了大涡模拟特征线算子分裂有限元模型,对单圆柱和串列双圆柱绕流问题进行了数值模拟,所得结果与现有研究结果吻合较好。模拟结果表明:对单圆柱绕流,随着雷诺数的增大,圆柱近尾流区上下交替的涡旋逐渐靠近通过圆柱几何中心的水平线,且涡脱落位置逐渐靠近圆柱。对Re=1 000的串列双圆柱绕流,临界间距在圆柱直径的2.25~2.5倍之间;当两圆柱间距小于临界间距时,上游圆柱后方无明显涡旋脱落,间隙处压力较稳定;大于临界间距时,有涡旋脱落,上游圆柱尾流区上下表面交替出现强负压区。     

并列旋转双圆柱流动特性的数值模拟

该文基于k - ε湍流模型,采用Galerkin有限元法对并列旋转双圆柱的绕流特性进行了数值模拟,计算的雷诺数为 1550.为了考查两圆柱旋转和间距的相互作用,文中采用三种间距比分别是T/D = 1.2,1.6和3.0 (T 为两圆柱中心之间的距离,D为圆柱直径) 和一系列不同的旋转速度比 (|α| ≤ 2).计算显示,当 |α| = 0,即圆柱不转动时,对应三种间距有三种典型的流型,单钝体流型对应小间距、偏流对应中等间距和对称流对应大间距;当 |α| 达到临界值时,涡脱落得到了有效的抑制,流动趋于稳定,升力系数和阻力系数的脉动值趋于零;平均升力系数和阻力系数随着 |α| 的增大分别增大和减小.     

亚临界雷诺数下波浪型圆柱绕流的数值模拟及减阻研究

采用大涡模拟数值方法对亚临界雷诺数Re=3000下波浪型圆柱的绕流现象进行研究分析。研究结果表明,波浪型圆柱的三维尾迹涡结构能得到很好的控制,它在轴向方向呈现周期性正负涡的交替分布特性。随着幅值的增大,波浪型圆柱表面的自由剪切层得以延展,使得旋涡的脱落发生在波浪型圆柱下游较远处,从而达到减阻的目的。波浪型圆柱与普通直圆柱相比,其幅值对平均阻力系数及脉动升力系数的减少及尾迹控制有着更重要的影响,最大减阻可达16%。     

流向振荡圆柱绕流中的涡脱落模态分析

本文用有限体积法对平行于均匀来流方向受迫振荡的圆柱绕流问题进行了二维数值模拟。雷诺数选取Re=200、855等几种亚临界雷诺数情况。研究了不同振幅和振动频率下的流场结构、涡脱落模态和一些重要流动参数如升阻力系数、Strouhal数的变化关系。西文计算表明，流向振荡圆柱绕流中涡脱落方式可划分为对称与反对称两种形态，在反对称的涡脱落现象中又观察到了三种基本模态，相互竞争的各种模态间的共同作用对流动特征起决定性作用。同时计算结果还验证了在圆柱绕流实验中观察到的频率锁定现象。     

均匀流中横向振荡圆柱绕流场的数值分析

针对工程中关心的振荡圆柱绕流问题,采用同位网格系统中的SIMPLE算法求解N-S方程,以及动网格技术模拟圆柱的运动边界。对均匀来流中的横向振荡圆柱绕流场进行数值模拟,计算了振荡圆柱升力系数、阻力系数随时间的变化曲线,以及不同相位时的流线矢量图和涡量等值线分布图。为验证模型的可靠性,对静水中的振荡圆柱流场进行模拟。与实验及数值计算结果吻合较好。     

线性剪切来流对方柱绕流特性影响的数值分析

为研究线性剪切来流对方柱绕流特性的影响,将对流出口边界条件引入特征线算子分裂有限元法中,建立了线性剪切来流的方柱绕流数值模型,并采用均匀来流的方柱绕流模拟结果验证了该模型计算方柱绕流问题的可靠性。数值模拟结果表明,剪切参数k对方柱绕流尾部形成的两排涡影响明显,涡脱落在k较大时被抑制,在k≥0.4时涡量最终形成一个类似三角形的区域;驻点随k的增加逐渐向高速侧分离点移动,在k≥0.3时驻点的位置不再发生变化;平均阻力系数在0.2≤k≤0.25时发生较大变化,平均升力系数在k≥0.2时为负。     

旋转方式对并列双圆柱绕流特性的影响

基于层流模型,使用有限体积法对并列旋转双圆柱绕流进行数值模拟计算。首先计算雷诺数为100时静止单圆柱绕流与旋转单圆柱绕流的流动性能,并与参考文献结果进行比较,验证计算的可靠性;然后,分析了两圆柱在中心间距比为3.0时圆柱不同旋转方式与旋转速度比对双圆柱绕流流动性能的影响。结果表明:逆流向旋转比顺流向旋转的临界转速更小,对涡旋脱落的抑制效果更好;两种旋转方式下时均升力系数绝对值均随旋转速度比的增大而增大;逆流向旋转下时均阻力系数随旋转速度比的增大而减小,顺流向旋转下时均阻力系数随旋转速度比的增大先增大后减小。     

用SST-DES和SST-URANS方法数值模拟亚临界雷诺数下三维圆柱绕流问题

基于剪切应力运输(shear stress transport,SST)的分离涡模拟(detached-eddy simulation,DES)在近壁面的流动区域采用SST模型,而在其他区域采用亚格子模型求解流场。本研究中分别应用SST-DES和SST-URANS方法对亚临界雷诺数Re=3900下的三维圆柱绕流进行了数值模拟,比较分析了两种数值方法下圆柱表面的时均压力分布、圆柱后方的漩涡脱落特征、圆柱下游流场的时均速度剖面分布等,并将数值模拟结果同前人物理试验比较,结果表明SST-DES方法在亚临界雷诺数下的三维圆柱绕流数值模拟问题上比SST-RANS方法具有更好的效果。     

低雷诺数60°交叉双圆柱直接数值模拟研究

交叉双圆柱在工程应用中较为广泛.为了解决交叉双圆柱绕流问题,该文应用OpenFOAM开源代码对Re=200,不同间隙比G(间隙/直径,G=0.5,1,2和4)条件下60°交叉双圆柱流场进行了直接数值模拟(DNS)研究,得到了两圆柱的时均阻力系数,斯特劳哈尔数(St)以及间隙附近的流线图、涡量图和Ω涡识别图.研究表明,随...     

多圆柱绕流旋涡脱落和流场形态概论

当流体绕多个圆柱流动时,圆柱的个数、相对位置和大小都明显影响着绕流流场结构。基于已有研究成果,归纳总结了流场中放置双圆柱、三圆柱、四圆柱和流场中主圆柱附近放置1个或2个附属小圆柱时,随圆柱相对位置变化,流场结构和旋涡脱落状态的演变规律。双圆柱相对位置分串列、并列和错置3种,其绕流流态按间距比和水流攻角对旋涡脱落形态的影响划分,流态在临界间距比上下变化很大;三圆柱呈“品”字布置,当间距比小于临界值时,出现明显的偏流现象,大于临界值后不再出现偏流现象;三圆柱呈倒“品”字布置,在任何间距比下,流动状态相对于来流几乎都是对称分布的;四圆柱绕正方形排列,实验研究发现上游两圆柱后没有旋涡形成和脱落,下游两圆柱旋涡脱落反相同步,而数值模拟得到上下游柱后都有明显的涡脱落;主圆柱尾流中存在附属小圆柱时,调整小圆柱的位置能够有效抑制尾流中的旋涡脱落。     

Vortex shedding in the flow around two side-by-side circular cylinders of different diameters

In this paper,the 3-D turbulent flow around two side-by-side circular cylinders of different diameters,at sub-critical Reynolds number(Re=3 900),is numerically simulated by the large eddy simulation(LES).The spacing ratios(T/D)between the two cylinders are considered in four cases(T/D=1.2,1.5,1.8 and 2.7)to study the vortex shedding and turbulent properties in the flow field.The main results are focused on the drag and lift coefficients,the vortex shedding frequency,the coherent structure,and the scale properties.It is shown that when T/D is equal to 1.2,the vortex shedding of the main cylinder is strongly suppressed by the small cylinder,the drag and lift coefficients of the main cylinder are smaller than those in other three cases.While T/D is equal to 1.5,the vortex shedding of the main cylinder can be improved,the drag and lift coefficients of the main cylinder are larger than those in other three cases.The empirical mode decomposition(EMD)method is applied to decompose the velocity signals traced by the LES.It is shown that there is a linear relationship between the mean period and the mode in the semi-log coordinates.The vortex shedding period of the main cylinder is consistent with the period of the restructured coherent structures quantitatively.     

Numerical simulation of low-Reynolds number flows past two tandem cylinders of different diameters

The flow past two tandem circular cylinders of different diameters was simulated using the finite volume method. The diameter of the downstream main cylinder （D） was kept constant, and the diameter of the upstream control cylinder （d） varied from 0.1D to D. The studied Reynolds numbers based on the diameter of the downstream main cylinder were 100 and 150. The gap between the control cylinder and the main cylinder （G） ranged from 0.1D to 4D. It is concluded that the gap-to-diameter ratio （G/D） and the diameter ratio between the two cylinders （d/D） have important effects on the drag and lift coefficients, pressure distributions around the cylinders, vortex shedding frequencies from the two cylinders, and flow characteristics.     

Numerical simulation of flow past twin near-wall circular cylinders in tandem arrangement at low Reynolds number

Fluid flow past twin circular cylinders in a tandem arrangement placed near a plane wall was investigated by means of numerical simulations. The two-dimensional Navier-Stokes equations were solved with a three-step finite element method at a relatively low Reynolds number of Re = 200 for various dimensionless ratios of 0.25 ≤ G/D ≤ 2.0 and 1.0 ≤ L/D ≤ 4.0, where D is the cylinder diameter, L is the center-to-center distance between the two cylinders, and G is the gap between the lowest surface of the twin cylinders and the plane wall. The influences of G/D and L/D on the hydrodynamic force coefficients, Strouhal numbers, and vortex shedding modes were examined. Three different vortex shedding modes of the near wake were identified according to the numerical results. It was found that the hydrodynamic force coefficients and vortex shedding modes are quite different with respect to various combinations of G/D and L/D. For very small values of G/D, the vortex shedding is completely suppressed, resulting in the root mean square (RMS) values of drag and lift coefficients of both cylinders and the Strouhal number for the downstream cylinder being almost zero. The mean drag coefficient of the upstream cylinder is larger than that of the downstream cylinder for the same combination of G/D and L/D. It is also observed that change in the vortex shedding modes leads to a significant increase in the RMS values of drag and lift coefficients.     

2-D eddy resolving simulations of flow past a circular array of cylindrical plant stems

In the present study, 2-D large eddy simulations(LES) are conducted for flow past a porous circular array with a solid volume fraction(SVF) of 8.8%, 15.4% and 21.5%. Such simulations are relevant to understanding flow in natural streams and channels containing patches of emerged vegetation. In the simulations discussed in the paper, the porous cylinder of diameter D contains a variable number of identical solid circular cylinders(rigid plant stems) of diameter d= 0.048 D. Most of the simulations are conducted at a Reynolds number of 2 100 based on the diameter D and the velocity of the steady uniform incoming flow. Though in all cases wake billows are shed in the regions where the separated shear layers(SSLs) forming on the sides of the porous cylinder interact, the effect of these wake billows on the mean drag is different. While in the high SVF case(21.5%), the total drag force oscillates quasi-regularly in time, similar to the canonical case of a large solid cylinder, in the cases with a lower SVF the shedding of the wake billows takes place sufficiently far from the cylinder such that the unsteady component of the total drag force is negligible. The mean amplitude of the oscillations of the drag force on the individual cylinders is the largest in a streamwise band centered around the center of the porous cylinder, where the wake to wake interactions are the strongest. In all cases the maximum drag force on the individual cylinders is the largest for the cylinders directly exposed to the flow, but this force is always smaller than the one induced on a small isolated cylinder and the average magnitude of the force on the cylinders directly exposed to the flow decreases monotonically with the increase in the SVF. Predictions of the global drag coefficients, Strouhal numbers associated with the wake vortex shedding and individual forces on the cylinders in the array from the present LES are in very good agreement with those of 2-D direct numerical simulations conducted on finer meshes, which suggests LES is a better option to numerically investigate flow in channels containing canopy patches, given that LES is computationally much less expensive than DNS at high Reynolds number. To prove this point, the paper also discusses results of 2-D LES conducted at a much higher Reynolds number, where the near-wake flow is strongly turbulent. For the higher Reynolds number cases, where the influence of the turbulence model is important, the effect of the sub-grid scale model and the predictive capabilities of the unsteady Reynolds averaged Navier-Stokes(RANS) approach to predict flow past porous cylinders are discussed.     

闸墩绕流影响因素及其作用规律分析

为探究某水利工程渡槽附近不平稳水面的形成原因并为制定消除措施提供参考,使用COMSOL Multiphysics软件对高雷诺数( Re =5×106)条件下,不同宽度的顺直流道、收缩渐变段和弯曲流道下游的圆柱绕流和不同形态的闸墩绕流进行了数值模拟研究,分析了流道布局和闸墩形状尺寸等因素对绕流流场和结构受力特性的影响。圆柱绕流分析结果表明:在计算参数范围内,随着顺直流道宽度的减小,圆柱的升阻力系数均逐渐增大;流道渐变段的截面收缩对圆柱绕流产生的影响不大;流道弯曲导致的来流偏流系数越大,圆柱受力的非对称性越明显,升阻力系数均值越大。闸墩绕流计算结果表明:无墩尾闸墩的绕流流场和受力特性与圆柱绕流类似,随着墩身长度增加,闸墩的平均阻力系数减小;带墩尾的闸墩呈流线型,其升阻力系数与尺寸相当的圆柱和无墩尾闸墩相比均显著减小;给闸墩设置合适的墩尾是解决闸墩绕流脱涡的有效措施。     

Large-eddy simulation of the flow past both finite and infinite circular cylinders at Re =3900

The flow past a finite circular cylinder with a height-to-diameter ratio of 1.5 and an infinite circular cylinder of the same diameter at a Reynolds number Re= 3 900 is investigated using the large eddy simulation(LES). The objective of the present study is to explore the differences of the flow mechanisms between the finite and infinite circular cylinders. It is shown that the free end of the finite circular cylinders affects the wake region significantly. The mean drag coefficient and the fluctuating lift coefficient of the finite circular cylinder are smaller than those of the infinite circular cylinder. The three-dimensional separation and the separated shear layer instability of the finite circular cylinder can obviously be observed. The existence of an arch vortex in the average flow downstream of the free end is demonstrated.     

Flow characteristics of the two tandem wavy cylinders and drag reduction phenomenon

This paper presents an extensive numerical study of 3-D laminar flow around two wavy cylinders in the tandem arrangement for spacing ratios (L/Dm ) ranging from 1.5 to 5.5 at a low Reynolds number of 1...