基于POD方法开缝圆柱绕流流场的研究

利用粒子成像测速技术（particle image velocimetry,PIV）,在水槽中探究缝隙对圆柱流场结构的影响,应用频谱分析和本征正交分解（proper orthogonal decomposition,POD）方法,研究了开缝圆柱流场相干结构.实验Reynolds数范围内,缝隙的"吹吸"作用从根本上改变了圆柱绕流近区尾流结构,前6阶模态形态是流场中最主要的相干结构.第1,2阶模态形态控制着圆柱绕流流场涡街相继脱落过程,1或2阶模态系数为尾迹涡的固有频率;第3,4阶模态形态控制着脱落旋涡沿流向方向能量运输;第5,6阶模态形态中的同向涡旋结构作用于旋涡缓慢脱离柱体这一过程,并对旋涡能量起着衰减作用.      

DMD和POD对超燃冲压发动机凹腔流动的稳定性分析

开式凹腔作为超燃冲压发动机中增加掺混和稳焰的装置, 其流动稳定性的研究对深入理解凹腔增加掺混和稳焰机理以及凹腔的设计有着重要的学术意义和工程应用价值.基于大涡模拟方法对超燃冲压发动机开式凹腔流动进行数值模拟, 分别采用动力学模态分解(dynamic mode decomposition, DMD)和本征正交分解方法(proper orthogonal decomposition, POD)对自激振荡流动进行稳定性分析. DMD方法可准确提取凹腔的振荡频率, 与Rossiter模型以及压力脉动FFT分析得到的频率吻合较好, 且DMD中对应Rossiter前3阶频率的模态在流动中的主导作用顺序也与FFT分析结果一致, 自激振荡中RossiterⅢ模态占据主导作用, 同时DMD方法对Rossiter 3阶以上模态频率的预测能力明显强于FFT分析方法.在对低频的提取方面, DMD方法比Rossiter模型更具有优势.与前6阶Rossiter模态对应DMD模态均缓慢收敛, 主要表现为剪切层中的分离涡结构和中部及下游区域中的涡结构.前3阶不稳定模态中的分离涡结构主要集中在中部剪切层以及后缘附近区域. POD方法中较少的模态包含流场绝大部分的能量.但是, 通过POD方法提取的模态频率在分辨率上效果不佳, 提取到最低频率为Rossiter 3阶模态对应的频率, 且模态中均存在次频, 次频与主频之间的耦合导致模态的形态相差较大.另外, 与DMD方法相比POD方法无法判断所提取的模态的稳定性.      

Dynamic analysis of bubble-driven liquid flows using time-resolved particle image velocimetry and proper orthogonal decomposition techniques

Abstract  

An experimental study to evaluate dynamic structures of flow motion and turbulence characteristics in bubble-driven water flow in a rectangular tank with a varying flow rate of compressed air is conducted. Liquid flow fields are measured by time-resolved particle image velocimetry (PIV) with fluorescent tracer particles to eliminate diffused reflections, and by an image intensifier to acquire enhanced clean particle images. By proper orthogonal decomposition (POD) analysis, the energy distributions of spatial and temporal modes are acquired. Time-averaged velocity and turbulent kinetic energy distributions are varied with the air flow rates. With increasing Reynolds number, bubble-induced turbulent motion becomes dominant rather than the recirculating flow near the side wall. Detailed spatial structures and the unsteady behavior of dominant dynamic modes associated with turbulent kinetic energy distributions are addressed.     

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.     

Snapshot POD analysis of transient flow in the pilot stage of a jet pipe servo valve

The spatial evolution of a turbulent flow in the pilot stage of a jet pipe servo valve at the inlet pressure and deflection angle of the jet pipe is investigated using a large eddy simulation (LES). The pressure of the same flow field is measured by a high frequency dynamic pressure sensor in the experiments and is compared with the results of the LES, as well as their root-mean-square (RMS) and fast Fourier transform (FFT) results. The results of experiments and LES are in good agreement, indicating that LES is able to predict the flow dynamics. Velocity datasets based on LES are utilised to conduct the snapshot proper orthogonal decomposition (snapshot POD) technique. The snapshot POD analysis results of the first 4 modes show a full ability to directly visualise details of the coherent structures. The influences of the inlet pressure and deflection angle of the jet pipe are also discussed. Under different inlet pressures, the velocity eigenfunctions of the first mode are similar, while the locations and strengths of the vortices in high modes are different. The Lamb-Oseen vortices that affect the trajectory of jet streams are observed in the vicinity of the entrances of receiver channels only in the first mode, and several spindly vortices appear in the region of ?5?y/n?相似文献   

飞行器大攻角复杂流动的POD和DMD对比分析

基于非结构/混合网格、耗散自适应2阶混合格式以及脱体涡模拟（detached eddy simulation,DES）方法开展了现代战斗机模型复杂分离流动的数值模拟,并与有限的平均气动力试验数据进行了对比,结果表明计算具有合理性,在此基础上进一步应用本征正交分解（proper orthogonal decomposition,POD）和动力学模态分解（dynamic mode decomposition,DMD）方法对数值模拟流场的非定常特性进行了对比分析.研究表明飞行器背风区流场由一对边条涡的螺旋运动主导,旋涡破裂前在横向空间截面上流场是中性稳定的,同时主涡核的运动是多频耦合的.POD和DMD的对比分析则表明:两者模态配对的方式不同,但主要模态之间具有一定相关性;POD模态中包含多种频率的运动,而且能量较集中于主模态,流场重构效率更高;DMD则将流场的主要特征运动提取为一些单频模态的组合,同时能够给出模态的稳定性.      

槽道流POD重构及湍流动能耗散率分析

采用二维粒子图像测速仪（2DPIV）对槽道内涡波流场进行实验研究,用POD技术对2DPIV瞬态速度矢量场进行主导模态重构,得到槽道内的平均流速和湍流动能分布;采用大涡PIV方法对湍流动能耗散率分布进行计算.结果表明：重构流场表征了原始流场的主导结构,剔除了噪声等干扰信息;大涡PIV方法能有效地估算动能耗散率的分布;湍流动能在壁面附近较小,在接近槽道中心区域湍流动能越来越大,呈现出射流的特征;动能耗散率的峰值出现在壁面附近和槽道中心区域,动能耗散率随着远离壁面程度的增加先降低后逐渐增加直至达到峰值.     

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.     

前缘形状对钝三角翼边界层稳定性及转捩的影响

选择典型高速流动条件,基于线性稳定性理论研究了不同前缘几何特征对典型大后掠角平板钝三角翼外形高速边界层流动稳定性及转捩的影响.研究表明,椭前缘(截面为椭圆)形状的变化仅影响前缘附近的流场特征和边界层流动稳定性;前缘截面长短轴比(形状因子)变大,前缘形状变尖,则横流速度变大,扰动波增长率变大;对于横流模态和第1模态,不同...     

Coherent structures over riblets in turbulent boundary layer studied by combining time-resolved particle image velocimetry(TRPIV),proper orthogonal decomposition(POD),and finite-time Lyapunov exponent(FTLE)

Time-resolved particle image velocimetry(TRPIV) experiments are performed to investigate the coherent structure's performance of riblets in a turbulent boundary layer(TBL) at a friction Reynolds number of 185. To visualize the energetic large-scale coherent structures(CSs) over a smooth surface and riblets, the proper orthogonal decomposition(POD) and finite-time Lyapunov exponent(FTLE) are used to identify the CSs in the TBL. Spatial-temporal correlation is implemented to obtain the characters and transport properties of typical CSs in the FTLE fields. The results demonstrate that the generic flow structures, such as hairpin-like vortices, are also observed in the boundary layer flow over the riblets, consistent with its smooth counterpart. Low-order POD modes are more sensitive to the riblets in comparison with the high-order ones,and the wall-normal movement of the most energy-containing structures are suppressed over riblets. The spatial correlation analysis of the FTLE fields indicates that the evolution process of the hairpin vortex over riblets are inhibited. An apparent decrease of the convection velocity over riblets is noted, which is believed to reduce the ejection/sweep motions associated with high shear stress from the viscous sublayer. These reductions exhibit inhibition of momentum transfer among the structures near the wall in the TBL flows.     

旋翼尾流时空发展二维速度场的实验研究

旋翼作为直升机的主要升力和操纵部件, 具有复杂的流场结构, 如非定常性, 桨-涡干扰和桨尖涡等, 导致旋翼流场研究十分困难. 针对这一问题, 结合锁相技术和粒子图像测速(particle image velocimetry, PIV)技术开展了悬停状态下旋翼流场的实验研究, 并通过本征正交分解(proper orthogonal decomposition, POD)提取主要含能模态, 刻画流场时空演化. 结果显示, 旋翼尾流发展过程中向旋转轴靠近, 二维流场结构呈现倒三角结构, 即扩展到三维流动中会呈现倒锥型结构的特性; 通过POD进行含能模态分析, 旋翼尾流中对湍动能贡献最大的为桨叶涡结构, 其次是桨尖涡结构.      

风力机翼型气动计算的降阶模型研究

将基于POD的降阶模型应用于风力机翼型的气动研究。首先应用CFD数值模拟得到一系列快照结果;应用基于本征正交分解(POD)的方法得到流场的一组基模态,认为对于所研究的问题,任一流场可以由这些基模态通过线性表达得到;对控制方程进行Galerkin投影,得到降阶模型,将离散求解N-S方程的问题转化为一组只有十几个自由度的常微分方程,从而减少计算时间,提高计算效率。并对二维翼型的绕流的定常和非定常问题进行了分析,计算结果表明,降阶模型可以较好地捕捉流动的特征,与直接CFD模拟相比计算精度相当,但大幅有效地提高了计算效率。     

Characteristics and generation of elastic turbulence in a three-dimensional parallel plate channel using direct numerical simulation

Direct numerical simulations(DNSs) of purely elastic turbulence in rectilinear shear flows in a three-dimensional(3D) parallel plate channel were carried out,by which numerical databases were established.Based on the numerical databases,the present paper analyzed the structural and statistical characteristics of the elastic turbulence including flow patterns,the wall effect on the turbulent kinetic energy spectrum,and the local relationship between the flow motion and the microstructures' behavior.Moreover,to address the underlying physical mechanism of elastic turbulence,its generation was presented in terms of the global energy budget.The results showed that the flow structures in elastic turbulence were 3D with spatial scales on the order of the geometrical characteristic length,and vortex tubes were more likely to be embedded in the regions where the polymers were strongly stretched.In addition,the patterns of microstructures' elongation behave like a filament.From the results of the turbulent kinetic energy budget,it was found that the continuous energy releasing from the polymers into the main flow was the main source of the generation and maintenance of the elastic turbulent status.     

Helical modes in low- and high-swirl jets measured by tomographic PIV

We report on a parallel study on properties of large-scale vortical structures in low- and high-swirl turbulent jets by means of the time-resolved tomographic particle image velocimetry technique. The high-swirl jet flow is featured by a well-established bubble-type vortex breakdown with a central recirculation zone. In the low-swirl flow, the mean axial velocity, while intermittently acquiring negative values, remains positive in the mean but with a local velocity defect immediately downstream from the nozzle exit, followed by a spiralling vortex core system and its eventual breakdown. Measurements of the 3D velocity fields allowed direct analysis of the azimuthal/helical modes via Fourier transform over the azimuthal angle and proper orthogonal decomposition (POD) analysis in the Fourier space. A precessing vortex core is detected for both swirl cases, whereas the POD analysis showed that the one originating in the bubble-type vortex breakdown is much more energetic and easier to detect.     

基于自适应本征正交分解混合模型的跨音速流场分析

采用非线性模型替代线性回归模型响应本征正交分解(POD)基函数的系数,并采用自适应抽样方法确定快照集合,实现了基于自适应POD混合模型的跨音速叶片复杂流动分析及流场拟合.首先通过比较基于线性回归模型和非线性回归模型的基函数系数响应精度,验证非线性回归模型的收敛性和精确性;之后通过与静态抽样方法进行对比,研究分析自适应抽样技术的优越性;最后开展基于自适应POD混合模型的全三维跨音速流场分析及流动拟合.结果表明,采用自适应POD混合模型,不仅能够清晰地识别三维跨音速流场中的敏感流动特征,还能精确地拟合设计空间内任意状态的流场及出口气动参数.     

集输-S型立管内气液两相流流型的PSD特征

在集输-S型柔性立管系统中实验研究了空气-水两相流的流型,研究了不同流型下立管段压差波动信号的频域特征,分析了压差波动信号的功率谱特性(PSD)。结果表明,随着流型由严重段塞流向稳定流动的转变,立管压差波动信号的能量值逐渐降低,能量值的频率范围逐渐升高。不同流型的功率谱能量值及主峰所在频率存在很大差别,压差信号的功率谱密度函数可作为识别S型柔性立管系统中流型的有效手段。     

POD analysis of near-wake structures of an elliptic cylinder adjacent to a free surface

The POD (Proper Orthogonal Decomposition) method was used to investigate the near-wake behind an elliptic cylinder located under a free surface. For two different depths of cylinder submergence, the first four eigenmodes of the flow field were calculated and their structures were analyzed. The first four eigenmodes reveal the details about the global mean flow structure, with the large-scale structure being mainly related to the most energetic flow motion. The convergence of accumulated eigenvalues shows a slight dependence on the number of instantaneous velocity fields used in POD analysis.     

Proper orthogonal decomposition of atomistic flow simulations

An adaptive proper orthogonal decomposition based on time windows (WPOD) for analysis of velocity fields from atomistic simulations is presented. The method effectively separates the field into ensemble average and fluctuation components, and can be applied to both stationary and non-stationary flows in simple and complex geometries. The criteria to distinguish POD modes representing ensemble average and fluctuation components are based on adaptive examination of eigenvalue decomposition, specifically on the rate of decay of POD eigenvalues and analysis of POD eigenvectors. The WPOD method is efficient and its superior accuracy leads to smooth field gradients that can be used effectively in multiscale formulations. The method has been applied in molecular dynamics and dissipative particle dynamics simulations. We demonstrate the method for several cases including steady and unsteady flow and red blood cell flow, but the same approach can be used in atomistic simulations of materials.     

Projection-free proper orthogonal decomposition method for a cantilever plate in supersonic flow

This paper explores the use of the proper orthogonal decomposition (POD) method for supersonic nonlinear flutter of a cantilever plate or wing. The aeroelastic equations are constructed using von Karman plate theory and first-order piston theory. The two-dimensional POD modes (POMs) in xy plane are determined from the chaotic results given by the traditional Rayleigh–Ritz (RR) approach. For a specific structure, the POMs need to be calculated once and then can be used for various parameters of interest. The derivatives of the POMs are calculated numerically to avoid the complex projection from the POMs to the Rayleigh–Ritz modes (RRMs). Numerical examples demonstrate that the POD method using 4 POMs can obtain accurate limit cycle oscillation (LCO) results with substantial computational cost savings, compared with 12 RRMs by the Rayleigh–Ritz method. The POD method is employed for the analysis of the chaotic oscillations. It is also demonstrated that the POD modes are robust over a range of flight parameters.