扩压叶栅二维流动分离的数值模拟

通过数值模拟,研究了二维扩压叶栅内流动分离及涡脱落的特征.定常计算结果表明:随着叶片负荷的增加,流场中奇点数目明显增加.非定常计算结果表明;在二维扩压流场中存在尾缘脱落涡和吸力面脱落涡两种脱落涡,它们的脱落频率随着冲角和马赫数的变化而变化.     

电磁力控制湍流边界层分离圆柱绕流场特性数值分析

在亚临界区高雷诺数Re=1.4×105下,采用脱体涡模拟结合湍流分离的方法对弱电解质中电磁力作用下湍流边界层分离圆柱绕流场及其升(阻)力特性进行了数值模拟和分析.结果表明,电磁力可以提高圆柱体湍流边界层内的流体动能,延缓圆柱体湍流边界层的流动分离,减弱圆柱体湍流绕流场中在流向和展向上大尺度漩涡的强度,减小圆柱体阻力时均值及其升力脉动幅值.当电磁力作用参数大于某个临界值后,湍流边界层流动分离消失,在圆柱体尾部产生射流现象,从而电磁力对圆柱体产生净推力作用,出现负阻力现象,而且升力脉动幅值接近于零,出现圆柱体升力消失现象.     

基于转捩模型及声比拟方法的高精度圆柱分离涡/涡致噪声模拟

基于七阶加权紧致非线性格式(WCNS-E8T7),结合延迟分离涡模拟(DDES)和Ffowcs WilliamsHawkings声比拟方法,对亚临界雷诺数下单圆柱、圆柱-翼型的分离涡/涡致噪声问题进行了数值模拟.针对亚临界雷诺数下圆柱尾迹中的转捩问题,发展了基于γ-Reθ模型高精度转捩-延迟分离涡模拟(Tran-DDES)方法,并与传统基于全湍流剪切应力输运(SST)模型的高精度DDES方法进行了对比.单圆柱模拟结果表明:传统SST-DDES方法会造成平均流场的回流区增大,压差阻力偏小等问题;而添加转捩模型的Tran-DDES方法与实验符合得很好.圆柱尾迹中添加翼型后,翼型对圆柱附近流场产生影响,使SST-DDES方法造成的圆柱后回流区偏大的问题减弱,并与Tran-DDES模拟结果差异变小.但在脉动量预测以及脉动产生的噪声预测方面, Tran-DDES方法仍与实验符合得更好.     

数值模拟圆柱绕流旋涡运动及尾流不稳定性分析

1引言流体绕过圆柱所产生的非定常旋涡运动以及由此引起的流动不稳定性在理论和实践上都具有重要的意义。数值模拟圆柱绕流旋涡产生及演化过程，探讨圆柱尾流涡街产生的机制，控制尾迹不同速度型以抑制涡街的产生，避免涡激振动在工程上造成破坏作用具有重大实际意义。为使问题简化，本文以二维圆柱绕流作为研究对象。该流动涉及到非定常分离，旋涡的形成、运动及发展，流动不稳定性质改变等许多未完全解决的问题。B。had等山对圆柱突然起动问题作了一系列实验研究。文献门对二维圆柱绕流问题作了系统数值研究。本文采用文献门提出的差分格…     

基于Omega涡识别方法的离心泵内非定常流动数值评价

离心泵内部非定常流动与流场中生成的旋涡密切相关.本文采用Omega涡识别方法研究离心泵流场涡结构及非定常流动特性,阈值选择为0.51.分析得出LES湍流模型相对比其他两种湍流模型涡识别结果更为细致,对叶片流道中的小涡结构可以很好地捕捉.设计工况下,LES模型识别出的涡面积数值是DDES模型的1.15倍,是SSTk-ω模型的1.55倍.压力脉动受流场空间位置与工况影响较大,其主要激励为叶频,并且压力脉动强度与涡面积密切相关.叶轮出口处的涡结构周期性脱落,在隔舌处会撞击隔舌而破裂.离心泵动静干涉的产生与叶轮出口处的涡结构脱落有关.     

壁面换热和压力梯度条件下边界层演化的LES研究

本文采用大涡模拟(LES)方法对壁面换热边界条件下有压力梯度的分离边界层流动问题进行了数值模拟研究,对不同壁面热边界条件下分离边界层进行了统计特性及瞬态流场分析.结果表明:与绝热壁面条件相比,等温冷壁面条件(0.8倍来流总温)下附着边界层内的速度剖面比绝热壁面更加饱满,边界层形状因子降低、分离泡也被显著抑制,其时均分离泡尺寸无论是在流向还是法向均更小;等温冷壁面条件下分离剪切层展向涡脱落的位置提前,展向涡卷起的频率降低,分离剪切层内相干结构尺度整体减小,促使分离边界层转捩加速和再附位置前移。     

针对典型分离流动数值模拟的自适应耗散调节方法

分离流动是一种复杂湍流现象,其中微小尺度结构的发展演化有着重要的影响,但是湍流数值模拟中数值方法的固有耗散会抑制小尺度流动结构的发展.因此,本文结合延迟分离涡模拟方法,基于五阶耗散紧致格式,通过引入流场相关的调节因子,构造自适应耗散调节方法,使其在大涡模拟区域降低耗散影响以提高对小尺度流动结构的辨识能力,在雷诺平均区域恢复正常耗散水平以保持稳定求解.本文首先通过近似色散关系和涡输运算例说明该方法可有效降低数值耗散影响,同时具有更高分辨率的涡保持能力;随后通过各向同性衰减湍流和平板槽道流动展示了该方法可有效提高对微小流动结构的辨识能力,同时在存在大梯度的流场中可保持求解稳定性;最后求解Re=3900亚临界状态下的圆柱绕流,通过对比流场云图、时均速度和雷诺应力分布曲线,说明了该方法通过减小分离区耗散影响,可有效提高对典型分离流动求解的准确性.     

DES模型在压气机亚音转子中的应用探讨

本文运用分离涡模拟(DES)方法研究了不同工况下压气机亚音转子的流动情况,分析了其时均与瞬时流场中顶部间隙泄漏流动和根部角区的流动分离.通过与S-A模型计算结果的对比表明,DES模型在模拟顶部泄漏流动及二次泄漏、泄漏流在转子下游与尾迹的干涉时能够捕捉到更强的旋涡结构,在模拟转子根部角区的分离时也能获得更为丰富的流动现象.对不同工况的DES计算表明负荷的上升会使泄漏涡形成的位置向上游移动,从而导致并加剧二次流动,并对叶栅下游泄漏涡与尾迹的干涉产生影响.对设计工况下瞬时流场的分析表明,泄漏涡在叶栅下游体现出周期性的强弱变化,近叶根分离区也体现出明显的非定常性.     

旋风分离器内涡核摆动的特性研究

涡核摆动是旋风分离器流体运动的一种非稳定现象,会对流场造成扰动,导致颗粒返混进而影响分离效率.为了探究涡核摆动的规律和机理,本文对单入口蜗壳式旋风分离器内的流场进行研究,运用PDPA测量流场的基本数据,采用Reynolds应力输运模型对其流动进行数值模拟,并定义旋流不稳定性指数TⅡ来分析流动的不稳定性.结果表明,截面Z/D=0.71上瞬时切向速度波动经Fourier变换后,存在一个集中分布的频率,此频率由涡核摆动导致,而且数值模拟结果与实验测量数据吻合较好。通过对涡核中心的运动轨迹分析,发现在整个旋风分离器空间内部都存在涡核摆动现象;在同一时刻,各个截面的涡核中心偏离几何中心的情况并不相同,涡核区域也不一致;各个截面的涡核中心的连线并不在一个平面或者直线上,而是摆动的。此外,分析TⅡ曲线发现气流从不对称的入口结构进入旋风分离器内部,涡核中心开始偏离几何中心,不稳定性逐渐扩大;随着旋流运动向下,入口结构不对称对旋流的影响变小,不稳定性又逐渐减小;流动最终在Z/D=8之后趋于稳定.     

非定常尾迹控制叶栅分离研究

本文采用大涡模拟对某大转角叶栅的非定常分离流动及其在非定常尾迹作用下的分离控制机理进行了数值分析。主要捕捉了两个特征频率:分离泡不稳定频率f_shear和尾缘脱落涡频率f_shed,研究了不同的激励频率、尾迹移动方向、随机脉动等激励特征控制流动分离的效果。结果显示:特定外部频率强化了分离剪切层中的K-H展向涡结构,f_shed能同时影响分离区域和尾涡区域,f_shear只能作用于分离区域;尾迹从吸力面向压力面移动时,分离结构表现出对来流周期性更明显的响应;进口随机脉动对破坏K-H展向涡结构非常有效。     

High-definition PIV analysis on vortex shedding in the cylinder wake

A high-definition analysis based on flow topology is made on the vortex motions under natural and lock-on conditions in the near-wake region of a circular cylinder, where two-dimensional flow fields in the wake-transition regime are measured by a time-resolved PIV system. The Reynolds stress distributions are examined in view of the mean separation streamline and the trajectory ofthe vortex center. It is shown that, by the lock-on, the Reynolds stresses become stronger and their dispositions match well with the shortened wake bubble, indicating perfect synchronization of shedding to the oscillatory forcing flow in the near field, which causes increased lift and drag forces.     

A study on drag reduction of a rotationally oscillating circular cylinder at low Reynolds number

The flow field around a rotationally oscillating circular cylinder in a uniform flow is studied by using a particle image velocimetry to understand the mechanism of drag reduction and the corresponding suppression of vortex shedding in the cylinder wake at low Reynolds number. Experiments are conducted on the flow around the circular cylinder under rotational oscillation at forcing Strouhal number 1, rotational amplitude 2 and Reynolds number 2,000. It is found from the flow measurement by PIV that the width of the wake is narrowed and the velocity fluctuations are reduced by the rotational oscillation of the cylinder, which results in the drag reduction rate of 30%. The mechanism of drag reduction is studied by phase-averaged PIV measurement, which indicates the formation of periodic small-scale vortices from both sides of the cylinder. It is found from the cross-correlation measurement between the velocity fluctuations that the large-scale structure of vortex shedding is almost removed in the cylinder wake, when the small-scale vortices are generated at the unstable frequency of shear layer by the influence of rotational oscillation.     

Lagrangian analysis of the formation and mass transport of compressible vortex rings generated by a shock tube

In order to understand the mass transport and the dynamic genesis associated with a compressible vortex formation,a dynamic analysis of compressible vortex rings (CVRs) generated by shock tubes by using the framework of Lagrangiancoherent structures (LCSs) and finite-time Lyapunov exponents field (FTLE) is performed. Numerical calculation is performed to simulate the evolution of CVRs generated by shock tubes with 70 mm, 100 mm, and 165 mm of the driver sectionat the circumstances of pressure ratio = 3. The formation of CVRs is studied according to FTLE fields. The mass transportduring the formation is obviously seen by the material manifold reveled by FTLE fields. A non-universal formation numberfor the three CVRs is obtained. Then the elliptic LCSs is implemented on three CVRs. Fluid particles separated by ellipticLCSs and ridges of FTLE are traced back to t = 0 to identify the fluid that eventually forms the CVRs. The elliptic LCSsencompass around 60% fluid material of the advected bulk but contain the majority of the circulation of the ring. The otherparts of the ring carrying almost zero circulation advect along with the ring. Combining the ridges of FTLE and the ellipticLCS, the whole CVR can be divided into three distinct dynamic parts: vortex part, entrainment part, and advected part. Inaddition, a criterion based on the vortex part formation is suggested to identify the formation number of CVRs.     

Study of interactions between liquid vortices and bubbles by simultaneous particle image velocimetry and laser-induced fluorescence tracking

Extensive experimental research has been conducted using the particle image velocimetry (PIV), laser-induced fluorescence (LIF) imaging and backlit photographic recordings to study the complex interactions between coherent vortex structures created in the shear layer of jets and the bubbles. Triggering of the naturally-developing instabilities of the shear layer by a thin, pulsed annular flow surrounding the jets allowed the creation of large, orderly structures with controllable frequency and phase. Synchronization of the triggering with data acquisition permitted phase averaging of the data and revealed several interesting phenomena. In particular, the evolution of large vortices and bubble fields could be tracked and the interactions could be studied. The horizontal and vertical velocity components of the liquid and bubble fields and the vertical velocity of both the vortex and bubble rings that were created were measured by the PIV. LIF and image recordings have been combined to visualize bubble trapping inside large eddy structures.     

基于间断有限元方法的并列圆柱层流流动特性

基于高阶的间断有限元方法, 数值模拟低马赫数下并列圆柱的可压缩层流流动, 捕捉并列圆柱流场中的漩涡结构, 以便分析并列圆柱尾流的流动特性. 针对二维圆柱的边界形式, 采用曲边三角形单元构造二维圆柱的曲面边界, 以适应高阶离散格式的精度. 在验证方法合理性的基础上, 分析圆柱间距及雷诺数对漩涡脱落及受力特性的影响规律. 研究结果表明: 并列圆柱的间距是影响流场流动特性的一个主要因素, 它会改变圆柱漩涡脱落的形式. 随着圆柱间距的增加, 上下圆柱的平均阻力系数及平均升力系数的绝对值随之显著下降. 雷诺数对于平均阻力系数的影响相对较小. 但随着雷诺数的增加, 上下圆柱的平均升力系数会随之降低, 而漩涡的脱落频率会随之增大.     

Computing Lagrangian coherent structures from their variational theory

Using the recently developed variational theory of hyperbolic Lagrangian coherent structures (LCSs), we introduce a computational approach that renders attracting and repelling LCSs as smooth, parametrized curves in two-dimensional flows. The curves are obtained as trajectories of an autonomous ordinary differential equation for the tensor lines of the Cauchy-Green strain tensor. This approach eliminates false positives and negatives in LCS detection by separating true exponential stretching from shear in a frame-independent fashion. Having an explicitly parametrized form for hyperbolic LCSs also allows for their further in-depth analysis and accurate advection as material lines. We illustrate these results on a kinematic model flow and on a direct numerical simulation of two-dimensional turbulence.     

过渡流下近壁插入圆柱对壁面强化传热的影响

本文利用基于复合网格系统的计算方法,对Re=50~1200的近壁插入圆柱流场进行数值模拟,研究过渡流状态下在壁面附近插入圆柱对下游壁面传热强化的影响。并基于低速循环水槽流动实验台,采用粒子成像测试法(PIV)对Re=100~500的近壁插入圆柱流场进行可视化实验研究,验证了数值模拟方法的可靠性。研究结果表明:近壁插入圆柱流场在Re=100时进入过渡流状态;Re直接影响圆柱尾流中周期性涡脱和壁面涡岛的发生位置及其洗刷效应的大小,随着Re的增大,洗刷效应明显增强,因而,过渡流范围内Re越大,圆柱下游壁面传热强化越大。     

Flow and magnetic structures in a kinematic ABC-dynamo

Dynamo theory describes the magnetic field induced by the rotating, convecting and electrically conducting fluid in a celestial body. The classical ABC-flow model represents fast dynamo action, required to sustain such a magnetic field. In this letter,Lagrangian coherent structures(LCSs) in the ABC-flow are detected through Finite-time Lyapunov exponents(FTLE). The flow skeleton is identified by extracting intersections between repelling and attracting LCSs. For the case A = B = C = 1, the skeleton structures are made up from lines connecting two different types of stagnation points in the ABC-flow. The corresponding kinematic ABC-dynamo problem is solved using a spectral method, and the distribution of cigar-like magnetic structures visualized.Inherent links are found to exist between LCSs in the ABC-flow and induced magnetic structures, which provides insight into the mechanism behind the ABC-dynamo.     

Lagrangian coherent structures are associated with fluctuations in airborne microbial populations

Many microorganisms are advected in the lower atmosphere from one habitat to another with scales of motion being hundreds to thousands of kilometers. The concentration of these microbes in the lower atmosphere at a single geographic location can show rapid temporal changes. We used autonomous unmanned aerial vehicles equipped with microbe-sampling devices to collect fungi in the genus Fusarium 100 m above ground level at a single sampling location in Blacksburg, Virginia, USA. Some Fusarium species are important plant and animal pathogens, others saprophytes, and still others are producers of dangerous toxins. We correlated punctuated changes in the concentration of Fusarium to the movement of atmospheric transport barriers identified as finite-time Lyapunov exponent-based Lagrangian coherent structures (LCSs). An analysis of the finite-time Lyapunov exponent field for periods surrounding 73 individual flight collections of Fusarium showed a relationship between punctuated changes in concentrations of Fusarium and the passage times of LCSs, particularly repelling LCSs. This work has implications for understanding the atmospheric transport of invasive microbial species into previously unexposed regions and may contribute to information systems for pest management and disease control in the future.