气固两相平面混合层的直接数值模拟

本文采用直接数值模拟(DNS)方法研究了气固两相混合层的涡量场和颗粒扩散,分析了湍流混合层中涡结构的卷起和配对过程,并讨论了大尺度涡结构的配对过程对平均速度、雷诺应力的影响。同时分析了混合层中不同Stokes数的颗粒在涡结构的作用下的混合和扩散。结果显示Stokes数为1的颗粒主要分布在流场大尺度涡结构的外边界,而Stokes数为0.01在涡结构的作用下,在流场中充分混合。     

自由热射流流场的光学不均匀性数值研究

 通过建立物理模型，求解层流N-S方程，用数值方法模拟了二维热射流流场的有序大涡结构，计算了流场不同流向位置的光程值，分析了射流流场混合层的光学特性，得出了气动光学流场由于流场中大尺度涡结构的存在所引起的光学不均匀性，大涡结构中奇点所导致的光学畸变最大，在一定条件下大涡结构有散焦作用。     

自耦合射流强化掺混的数值研究

针对同向两股气流掺混的某一特定物理模型,对自耦合射流作用下的气流掺混过程进行了数值模拟研究.与未施加自耦合射流激励的掺混过程相比,自耦合射流周期性的吹/吸诱导流场内产生复杂的大尺度涡系结构,使得同向两股气流形成对流型混合,有效缩减了主流核心区长度;相对于在燃烧室单侧放置激励器的方式,双侧自耦合射流激励时温度均匀分布线较...     

超音速射流火焰的DNS研究:计算方法与涡结构

介绍了可压缩反应流的计算方法,然后计算了超音速射流火焰,对超音速射流火焰中的涡结构进行了分析。认为超音速射流火焰中的涡结构特征,与亚音速射流中有本质差异,且涡结构在燃料与组分的混合中起到了重要作用,进而直接影响火焰结构和燃烧效率。     

射流温度场与拟序结构间相互作用的大涡模拟

本文分别对常温空气和高温空气的平面自由射流进行了大涡模拟,采用分步投影法求解动量方程,亚格子项采用标准Ｓｍａｇｏｒｉｎｓｋｙ亚格子模式模拟,并同时求解了标志物输运方程以实现数值流场显示。模拟结果给出了湍流的瞬态发展过程以及流动中拟序结构的发展演变过程,通过对常温射流和高温射流瞬态流场的比较揭示了射流中温度场与拟序结构间相互作用的细节过程。     

气固两相三维圆柱绕流的直接数值模拟

本文采用直接数值模拟(DNS)方法研究了气周两相三维圆柱绕流的涡量场和颗粒扩散,并着重讨论了圆柱绕流中卡门涡街的形成和涡结构的转捩过程。同时分析了圆柱绕流中不同Stokes数的颗粒在涡结构作用下的横向扩散。结果显示Stokes数为1的颗粒主要分布在流场大尺度涡结构的外边界,而Stokes数为0．01的颗粒在涡结构的作用下,在流场中充分混合。     

水流冲击超声速气体射流实验研究

通过水洞实验对有水流速度影响的水下超声速气体射流进行实验研究,通过高速摄像系统记录射流形态演变过程,采用动态测力系统测量射流演变过程中射流周围环境压力的脉动特征.对剪切层涡旋结构进行分析,得到水流冲击射流的剪切涡流动形态演化和压力脉动特征.实验结果表明,射流主体形态的非定常运动依赖于水流速度,无水流速度时,射流主体受到重浮力影响向上弯曲较大,并且可以捕捉到射流的振荡诱导喷管口平面处主频为200 Hz的压力脉动,当存在水流速度时,射流主体向下游发展过程中的偏斜程度较小,射流与水流相互作用形成剪切涡,在水流作用下射流主体向下游发展过程中卷入射流剪切层,与射流主体掺混形成较大尺度的涡结构,喷管口平面处主频消失.     

圆管潜射流生成偶极子涡特性实验

利用溢流恒压装置产生具有稳定出流速度的圆管潜射流, 结合染色液流态显示方法, 在多种射流无量纲潜深d/H、雷诺数Re以及限制数C的组合下, 实验研究了该潜射流动量在有限深密度均匀流体中的演化特性, 其中d为射流潜深, H为水深. 研究表明, 当C<1时潜射流表现为深水特征, 而当1≤ C<2时潜射流表现为过渡特征, 在这两种情况下均不产生任何形式的大尺度相干结构; 当2≤ C<10时潜射流表现为浅水特征, 而C≥10时潜射流表现为极浅水特征, 在这两种情况下均产生大尺度的偶极子涡结构. 对极浅水特征潜射流, 在各种无量纲潜深下, 偶极子涡结构的无量纲形成时间t_f^*与无量纲射流时间T_inj^*均满足相同的正比例关系; 对浅水特征潜射流, 当d/H=0.5时, t_f^*与T_inj^*满足某种线性关系, 但对其他无量纲潜深, t_f^*与T_inj^*之间无明显规律. 关键词： 圆管潜射流 限制数 偶极子涡结构 形成时间     

带根部间隙压气机静叶流道流动结构研究

本文采用数值模拟的方法对某低速压气机静叶流道内的流动结构进行了研究。设计工况下,带根部间隙的静叶流道中主要存在三个大尺度旋涡结构,分别为上通道涡、刮削涡和集中脱落涡,这些涡系在生成和发展过程中相互作用,流道内总奇点数先增加后减小,但都严格遵循了流道奇点数拓扑法则。     

三维射流中颗粒碰撞的直接数值模拟

采用硬球模型对三维气同两相射流中Stokes数为10的中等颗粒的碰撞行为进行了直接数值模拟,以初步考察两相流动中颗粒碰撞的特件.颗粒的跟踪采用单向耦合的Lagrangian方法,计算分析了颗粒碰撞随空间、时间的演化及其对颗粒分布不均匀性的影响.模拟结果表明颗粒碰撞主要分布在流场巾颗粒局部浓度较高的区域;由于射流初期大尺度涡结构的影响,颗粒的浓度分布最为不均,因此碰撞次数在这一时期随时间呈线性增加,达到最大值后逐渐回落趋于平缓.此外,对网格中颗粒个数分布的矩的统计发现,碰撞对颗粒分布不均匀性的影响随时间呈现不同的特性.     

Large Eddy Simulation of a Vortex Ring Impacting a Bump

A vortex ring impacting a three-dimensional bump is studied using large eddy simulation for a Reynolds number Re=$4$x$10^4$ based on the initial diameter and translational speed of the vortex ring. The effects of bump height and vortex core thickness for thin and thick vortex rings on the vortical flow phenomena and the underlying physical mechanisms are investigated. Based on the analysis of the evolution of vortical structures, two typical kinds of vortical structures, i.e., the wrapping vortices and the hair-pin vortices, are identified and play an important role in the flow state evolution. The boundary vorticity flux is analyzed to reveal the mechanism of the vorticity generation on the bump surface. The circulation of the primary vortex ring reasonably elucidates some typical phases of flow evolution. Further, the analysis of turbulent kinetic energy reveals the transition from laminar to turbulent state. The results obtained in this study provide physical insight into the understanding of the mechanisms relevant to the flow evolution and the flow transition to turbulent state.     

Visualisation and analysis of large-scale vortex structures in three-dimensional turbulent lid-driven cavity flow

In this paper, large eddy simulation (LES) of a three-dimensional turbulent lid-driven cavity (LDC) flow at Re = 10,000 has been performed using the multiple relaxation time lattice Boltzmann method. A Smagorinsky eddy viscosity model was used to represent the sub-grid scale stresses with appropriate wall damping. The prediction for the flow field was first validated by comparing the velocity profiles with previous experimental and LES studies, and then subsequently used to investigate the large-scale three-dimensional vortical structures in the LDC flow. The instantaneous three-dimensional coherent structures inside the cavity were visualised using the second invariant (Q), Δ criterion, λ₂ criterion, swirling strength (λ_ci) and streamwise vorticity. The vortex structures obtained using the different criteria in general agree well with each other. However, a cleaner visualisation of the large vortex structures was achieved with the λ_ci criterion and also when the visualisation is based on the vortex identification criteria expressed in terms of the swirling strength parameters. A major objective of the study was to perform a three-dimensional proper orthogonal decomposition (POD) on the fluctuating velocity fields. The higher energy POD modes efficiently extracted the large-scale vortical structures within the flow which were then visualised with the swirling strength criterion. Reconstruction of the instantaneous fluctuating velocity field using a finite number of POD modes indicated that the large-scale vortex structures did effectively approximate the large-scale motion. However, such a reduced order reconstruction of the flow based on the large-scale vortical structures was clearly not as effective in predicting the small-scale details of the fluctuating velocity field which relate to the turbulent transport.     

Counter rotating vortex pair structure in a reacting jet in crossflow

This paper analyzes the time averaged flow structure of a reacting jet in cross flow (RJICF), emphasizing the structure of the counter-rotating vortex pair (CVP) by using simultaneous tomographic particle image velocimetry (TPIV) and hydroxyl radical planar laser induced fluorescence (OH-PLIF). It was performed to determine the extent to which heat release, and the associated effects of gas expansion and baroclinic vorticity production, impact the structure of the CVP. These results show the clear presence of a CVP in the time averaged flow field, whose trajectory lies below the jet centerline on either side of the centerplane. Consistent with other measurements of high momentum flux ratio JICF in nonreacting flows, there is significant asymmetry in strength of the two vortex cores. The strength and structure of the CVP was quantified with vorticity and swirling strength (λ_ci), showing that some regions of the flow with high shear are not necessarily accompanied by large scale bulk flow rotation and vice-versa. The OH PLIF measurement allows for correlation of the flame position with the dominant vortical structures, showing that the leeward flame branch lies slightly above, as well as, in the region between the CVP cores.     

湍流边界层中下扫流与“反发卡涡”

用氢气泡法观测湍流边界层的下扫流和有关的流动结构.实验中发现一种新型涡结构，它的特征与典型的发卡涡正好相反.发卡涡的头部指向下游，而它的头部指向上游； 发卡涡的两腿之间，由于涡的诱导产生上升流，而它则在其两腿之间，由于涡的诱导产生下扫流. 关键词： 湍流边界层 流动显示 流动结构 发卡涡     

高雷诺数双螺旋涡尾迹演化特性分析

螺旋状尾迹涡是直升机悬停旋翼流场的主导特征之一,其时空演化特性对旋翼气动性能具有重要影响.为了揭示悬停状态下旋翼尾迹涡的演化特征,对两桨叶刚性旋翼在高雷诺数悬停状态下的双螺旋状尾迹涡开展数值研究,采用基于流场特征的网格自适应技术,结合低耗散迎风/中心混合格式以及延迟脱体涡模拟方法对Caradonna-Tung旋翼在桨尖马赫数为0.439、桨尖雷诺数为1.92×10~6的悬停流场进行了高分辨率计算.基于欧拉和拉格朗日两种描述方法对计算结果进行了分析,揭示了双螺旋尾涡系统的演化特性:后缘尾涡面在桨尖附近的反向卷起及其与下游桨尖涡的相互作用是影响涡系稳定性以及涡-涡相互作用的重要因素;涡龄小于720°时,在固连于桨叶上的旋转坐标系中观察,涡系具有时空稳定性,涡管中心处轴向涡量随涡龄按照幂函数规律衰减.在固连于漩涡中心的局部极坐标系中,周向速度分布以及涡核半径随涡龄的变化与理论涡模型相符合,环量随涡龄的变化显示了漩涡的生长、平衡及耗散等演化阶段;模态分析结果表明,除点涡模态外,来流与点涡的复合模态在漩涡演化过程中对流动特征的转变有重要影响;涡系轴截面速度场的拉格朗日拟序结构直观地显示了漩涡场的时空演化过程,揭示了漩涡配对和共旋穿越等流动特征,同时也展示了后缘尾涡面卷起现象在漩涡演化过程中的作用.     

Coherence and Reynolds stresses in the turbulent wake behind a curved circular cylinder

The turbulent wake behind a curved circular cylinder is investigated based on data obtained from a direct numerical simulation. Here, emphasis is placed in the assessment of two approaches for simplified modelling: reduced-order modelling (ROM) and Reynolds-averaged Navier–Stokes equations. To this end, the instantaneous vortical structures, the proper orthogonal decomposition (POD) of the flow, and relevant Reynolds stress components have been analysed. The results show that despite the complexity of the instantaneous vortical structures, the wake dynamics are governed by the quasi-periodic shedding of primary vortices. Between 24% and 50% of the kinetic energy in the POD is captured by the two most energetic modes, and about 200 modes are needed to capture 90% of the kinetic energy. These findings suggest that, as long as the large-scale structures of the von Kármán vortex shedding are concerned, the present case can be approached by ROM; but a detailed representation of the flow dynamics without an eddy viscosity model that accounts for the unresolved turbulent fluctuations would require a large amount of degrees of freedom. Concerning the Reynolds stresses, their magnitude varies considerably depending on the depth at which they have been sampled. This dependence is related to the strength of the vortex shedding, and the intensity of the secondary flows induced by the curvature of the cylinder. As a consequence of the combination of these two effects, the correlation between streamwise and vertical velocity fluctuations is highest in the wake behind the midspan of the curved cylinder, and the correlation between cross-flow and vertical velocity fluctuations reaches large values in the lower wake.     

The evolution of synthesised vortices in turbulent boundary layer

Particle image velocimetry technique has been used to investigate the evolution of synthesised vortical structures in the turbulent boundary layer. Synthetic jet actuator is implemented on the flat plate surface to synthesise various vortical structures by operating the actuator at varying operating parameters. The vortices are issued into the boundary layer and their evolution and subsequent interaction with the relatively less energetic near wall fluid is studied. The investigation is based on the quantitative measurements that are made both on the central and parallel lateral planes. Finally, the enhancement of the wall shear stress resulted in by the passing vortices is measured to evaluate the effectiveness of the actuator towards flow separation control.     

A study on large coherent structures and noise emission in a turbulent round jet

A moderate Reynolds number,and high subsonic turbulent round jet is investigated by large eddy simulation.The detailed results(e.g.mean flow properties,turbulence intensities,etc.)are validated against the experimental data,and special attention is paid to study motions of coherent structures and their contributions to far-field noise.Eulerian methods(e.g.Q-criteria andλ2criteria)are utilized for visualizing coherent structures directly for instantaneous flow fields,and Lagrangian coherent structures accounting for integral effect are shown via calculating fields of finite time Lyapunov exponents based on bidimensional velocity fields.All visualizations demonstrate that intrusion of three-dimensional vortical structures into jet core occurs intermittently at the end of the potential core,resulting from the breakdown of helical vortex rings in the shear layer.Intermittencies in the shear layer and on the centerline are studied quantitatively,and distinctively different distributions of probability density function are observed.Moreover,the physical sound sources are obtained through a filtering operation of defined sources in Lighthill’s analogy,and their distributions verify that intrusion of vortical structures into the core region serves as important sound sources,in particular for noise at aft angles.The facts that intermittent behaviors are caused by motions of coherent structures and correlated with noise generation imply that to establish reasonable sound sources in active noise production region based on intermittent coherent structures is one of the key issues for far-field noise prediction.