Localized Coherent Structures in the Boundary Layer

A Blasius laminar boundary layer and a steady turbulent boundary layer on a flat plate in an incompressible fluid are considered. The spectral characteristics of the Tollmien—Schlichting (TS) and Squire waves are numerically determined in a wide range of Reynolds numbers. Based on the spectral characteristics, relations determining the three–wave resonance of TS waves are studied. It is shown that the three–wave resonance is responsible for the appearance of a continuous low–frequency spectrum in the laminar region of the boundary layer. The spectral characteristics allow one to obtain quantities that enter the equations of dynamics of localized perturbations. By analogy with the laminar boundary layer, the three–wave resonance of TS waves in a turbulent boundary layer is considered.     

Research on Coherent Structures in a Mixing Layer of the Fene-P Polymer Solution

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混合层流动拟序结构的大涡模拟

采用大涡模拟方法对空间发展的二维平面混合层进行了数值模拟 ,动量方程采用分步投影法求解 ,亚格子项采用标准Smagorinsky亚格子模式模拟 ,压力泊松方程采用修正的循环消去法快速求解 ,同时求解了标志物输运方程以实现数值流场显示。模拟结果给出了混合层流动的瞬态发展过程以及流动中拟序结构的发展演变过程 ,成功地模拟了混合层发展中的各种瞬态细节过程 ,如涡的卷起、增长 ,涡与涡之间的配对、合并过程 ,以及大涡破碎为小涡的级联过程 ,为各种以混合层流动为原型流动的射流、尾流等工业流动的控制和优化提供了理论基础。     

Coherent Structures Formation During Wake-Boundary Layer Interaction on a LP Turbine Blade

Particle Image Velocimetry (PIV) measurements have been analyzed in order to characterize the dynamics of coherent structures (eddies and streaks) within the suction side boundary layer of a low pressure turbine cascade perturbed by impinging wakes. To this end, the instantaneous flow fields at low Reynolds number and elevated free-stream turbulence intensity level (simulating the real condition of the blade row within the engine) were investigated in two orthogonal planes (a blade-to-blade and a wall-parallel plane). Proper Orthogonal Decomposition (POD) has been employed to filter the instantaneous flow maps allowing a better visualization of the structures involved in the transition process of the boundary layer. For the unsteady case properly selected POD modes have been also used to sort the instantaneous PIV images in the wake passage period. This procedure allows computing phase-averaged data and visualizing structures size and intensity in the different parts of the boundary layer during the different wake passage phases. The contributions to the whole shear stress due to the largest spanwise oriented scales at the leading and trailing boundaries of the wake-jet structures and those associated with streaky structures observed in the bulk of the wake are discussed. Instantaneous images in the wall-parallel plane are filtered with POD and they allow us to further highlight the occurrence of low and high speed traveling streaks (Klebanoff mode). The periodic advection along the suction side of the high turbulent content regions carried by the wakes anticipates both formation and sinuous instability of the streaks inside the boundary layer as compared with the steady case. The dynamics driving the breakdown of the streaks and the consequent formation of nuclei with high wall-normal vorticity have been found to be almost the same in the steady and the unsteady cases. Auto-correlation of the instantaneous images are also presented in order to highlight analogies and differences in the size and spacing of streaks in the two cases. These results are also compared with the available literature concerning simplified geometries (i.e flat plate) operating under steady inflow.     

Decomposition of Mixing Layer Turbulence into Coherent Structures and Background Fluctuations

The eduction of coherent structures from cross-wire rake data in a fully turbulent incompressible mixing layer confirms the feasibility of a decomposition of a turbulent flow field, first suggested by Farge, as non-periodic non-equilibrium coherent structures interacting with a ‘thermalized’ broad-band turbulence. A simple wavelet coefficient decimation algorithm and orthogonalization yields non-periodic dominant flow structures and a background field that has a Gaussian distribution of velocities at the centerline. The coherent structures are classified in terms of their topology. The non-coherent background field has flat energy spectra and normal distribution of velocity components. Most background field statistics depend only weakly on the type of structure on which they are superposed. It may be possible to adapt existing subgrid scale models to this decomposition. This revised version was published online in July 2006 with corrections to the Cover Date.     

Large-Eddy Simulation of Turbulent Flow Around a Finite-Height Wall-Mounted Square Cylinder Within a Thin Boundary Layer

The paper describes the results of a computational study of the auto-ignition of a fuel spray under Exhaust Gas Recirculation (EGR) conditions, a technique used to reduce the production of NO_x. Large Eddy Simulation (LES) is performed, and the stochastic field method is used for the solution of the joint sub-grid probability density function (pdf) of the chemical species and energy. The fuel spray is n-heptane, a diesel surrogate and its chemical kinetics are described by a reduced mechanism involving 22 species and 18 reaction steps. The method is applied to a constant volume combustion vessel able to reproduce EGR conditions by the ignition of a hot gas mixture previously introduced into the chamber. Once the prescribed conditions are reached the fuel is then injected. Different EGR conditions in terms of temperature and initial ambient chemical composition are simulated. The results are in good overall agreement with measurements both regarding the ignition delay times and the lift-off heights.     

Review of Coherent Structures in Turbulent Free Shear Flows and Their Possible Influence on Computational Methods

A review of selected experiments on coherent structures in turbulent shear flows is performed. Different experimental approaches (conditional averages, filtering techniques, wavelets, linear stochastic estimation and proper orthogonal decomposition, etc.) are illustrated and their links with computations (LES, DNS, SDM, etc.) is emphasized. It is particularly shown that some kind of universal behavior of the background turbulence can be retrieved from these various experimental methods. This revised version was published online in August 2006 with corrections to the Cover Date.     

湍流边界层相干结构剩余脉动雷诺应力项的实验测量

用IFA300恒温热线风速仪和X形二分量热线探针以采样间隔小于湍流耗散时间尺度的分辨率精细测量了风洞中平板湍流边界层不同法向位置的瞬时流向、法向速度分量的时间序列信号。用条件采样和相位平均技术提取了相干结构猝发过程中相干结构剩余脉动雷诺应力和随机脉动对相干结构贡献的雷诺应力的条件相位平均波形。基于理论上对湍流相干结构动量方程中随机脉动对相干结构贡献的雷诺应力和相干结构剩余脉动雷诺应力项的分析,对两种雷诺应力项进行了对比研究。研究发现,相干结构剩余脉动雷诺应力项在数值上具有和随机脉动对相干结构贡献的雷诺应力相同的数量级,表明在相干结构动力学模型方程中,相干结构剩余脉动雷诺应力项并不像以前估计的那样可以忽略不计。     

Scalar Mixing and Large-Scale Coherent Structures in a Turbulent Swirling Jet

The paper presents large eddy simulations of co-annular swirling jets into an open domain. In each of the annuli a passive scalar is introduced and its transport is computed. If the exit of the pilot jet is retracted strong coherent flow structures are generated which substantially impact on the transport and mixing of the scalars. Average and instantaneous fields are discussed to address this issue. A conditional averaging technique is devised and applied to velocity and scalars. This allows to quantify the impact of the coherent structures on the mixing process.     

Experimental and Theoretical Sensitivity Analysis of Turbulent Flow Past a Square Cylinder

The Siemens SGT-800 3^rd generation DLE burner fitted to an atmospheric combustion rig has been numerically investigated. Pure methane and methane enriched by 80 vol% hydrogen flames have been considered. A URANS (Unsteady Reynolds Averaged Navier-Stokes) approach was used in this study along with the k ? ω SST and the k ? ω SST-SAS models for the turbulence transport. The chemistry is coupled to the turbulent flow simulations by the use of a laminar flamelet library combined with a presumed PDF. The effect of the mesh density in the mixing and the flame region and the effect of the turbulence model and reaction rate model constant are first investigated for the methane/air flame case. The results from the k ? ω SST-SAS along with flamelet libraries are shown to be in excellent agreement with experimental data, whereas the k ? ω SST model is too dissipative and cannot capture the unsteady motion of the flame. The k ? ω SST-SAS model is used for simulation of the 80 vol% hydrogen enriched flame case without further adjusting the model constants. The global features of the hydrogen enrichment are very well captured in the simulations using the SST-SAS model. With the hydrogen enrichment the time averaged flame front location moves upstream towards the burner exit nozzle. The results are consistent with the experimental observations. The model captures the three dominant low frequency unsteady motion observed in the experiments, indicating that the URANS/LES hybrid model indeed is capable of capturing complex, time dependent, features such as an interaction between a PVC and the flame front.     

Transient Characterization of the Reattachment of a Massively Separated Turbulent Boundary Layer Under Flow Control

The transient dynamics of a high Reynolds number separated flow over a two-dimensional ramp submitted to pulsed fluidic control is investigated. A spanwise array of 22 round jets, located upstream of the flap leading edge, is used as actuator to generate co-rotating vortical structures. Simultaneous measurements of wall friction using hot-film anemometry and phase-averaged velocity using 2D2C PIV are conducted. The PIV plane encompasses the incoming boundary layer upstream the flap leading edge, the separation bubble and the natural reattachment region. The dynamics of the separated flow is studied for successive sequences of pulsed actuation. Pockets of turbulence are periodically generated by the separation process and pushed downstream. After the transition period, the controlled flow shows large amplitude oscillations around a steady mean, particularly for the separation area. The transient dynamics of the flow at the actuation activation is also studied. The separated flow is strongly modified by the actuation from the first pulse. Characteristic times of the transient dynamics can be determined by fitting a first-order model with delay on the data. For the reattachment, the dimensionless characteristic rising times defined as \(\tau _{r}^{+} = \tau _{r} ~ U_{0} ~/~ L_{sep}\) of 11.7 for the friction gain, 4.8 for the separation length and 4.1 for the first mode of a Conditional Proper Orthogonal Decomposition analysis of the phase-averaged velocity fields were found. These values are in good agreement with previous studies and are of particular interest for modeling the transients and for further closed-loop control applications.     

功能梯度界面相对周期分布纤维增强复合材料反平面剪切的影响

基于复变函数理论和边界配点法,探索了功能梯度界面相在周期均匀分布纤维增强复合材料反平面剪切问题中所起的作用.由于纤维在复合材料基体中的周期分布是均匀的,将其简化成含一功能梯度界面相夹杂的方形单胞.采用分层均匀化方法,将功能梯度界面相离散成K层界面层.当K足够大时,每个界面层可视为匀质材料,同时计算得到的复合材料宏观性能趋于定值.根据单胞内的基体、界面相和夹杂的几何外形特点,分别给出复势函数的级数形式,这些复势函数在各组分的相邻界面应满足连续性条件,在单胞的外边界应满足周期性边界条件和远场加载条件,从而确定复势函数中的待定系数,进而根据平均场理论确定复合材料有效模量.主要探讨了夹杂体积分数、各组分模量、功能梯度界面相的模量渐变形式等因素对纤维增强复合材料性能的影响.结果表明:不管基体模量相对于夹杂模量是大还是小,都有对应的界面相模量渐变形式可使夹杂周围的等效应力集中系数减小;另外还发现仅当夹杂模量较大时,功能梯度界面相模量的变化方式对复合材料有效模量产生不可忽视的影响.     

Magnetic Resonance Measurement of Transient Shear Wave Propagation in a Viscoelastic Gel Cylinder

A magnetic resonance measurement technique was developed to characterize the transient mechanical response of a gel cylinder subjected to angular acceleration. The technique employs tagged magnetic resonance imaging (MRI) synchronized to periodic impact excitation of a bulk specimen. The tagged MRI sequence provides, non-invasively, an array of distributed displacement and strain measurements with high spatial (here, 4 mm) and temporal (6 ms) resolution. The technique was validated on a cylindrical gelatin sample. Measured dynamic strain fields were compared to strain fields predicted using (1) a closed-form solution and (2) finite element simulation of shear waves in a three-parameter “standard” linear viscoelastic cylinder subjected to similar initial and boundary conditions. Material parameters used in the analyses were estimated from measurements made on the gelatin in a standard rheometer. The experimental results support the utility of tagged MRI for dynamic, non-invasive assays such as measurement of shear waves in brain tissue during angular acceleration of the skull. When applied in the inverse sense, the technique has potential for characterization of the mechanical behavior of gel biomaterials.     

Coherent structures and their influence on the dynamics of aeroelastic panels

A panel forced by a supersonic unsteady flow is numerically investigated using a finite difference method, a Galerkin approach, and proper orthogonal decomposition (POD). The aeroelastic model investigated is based on piston theory for modeling the flow-induced forces, and von Karman plate theory for modeling the panel. Structural non-linearity is considered, and it is due to the non-linear coupling between bending and stretching. Several novel facets of behavior are explored, and key aspects of using a Galerkin method for modeling the dynamics of the panel exhibiting limit cycle oscillations and chaos are investigated. It is shown that multiple limit cycles may co-exist, and they are both symmetric and asymmetric. Furthermore, the level of spatial coherence in the dynamics is estimated by means of POD. Reduced order models for the dynamics are constructed. The sensitivity to initial conditions of the non-linear aeroelastic system in the chaotic regime limits the capability of the reduced order models to identically model the time histories of the system. However, various global characteristics of the dynamics, such as the main attractor governing the dynamics, are accurately predicted by the reduced order models. For the case of limit cycle oscillations and stable buckling, the reduced order models are shown to be accurate and robust to parameter variations.     

Thermal Non-equilibrium Natural Convection in a Square Enclosure with Heat-Generating Porous Layer on Inner Walls

Differentially heated enclosure with heat-generating porous layer on inner walls is studied computationally for non-Darcy flow and thermal non-equilibrium models. In this study, this problem is investigated for different internal and external Rayleigh numbers, Darcy numbers, porosity-scaled thermal conductivity ratio, solid-/fluid-scaled heat transfer coefficient and dimensionless thickness of the porous layer. The results indicate that the dimensionless thickness of the porous layer has an important effect on the heat transfer in the enclosure. It was found that the thermal non-equilibrium model is needed for small values of the porosity-scaled thermal conductivity ratio and the solid-/fluid-scaled heat transfer coefficient. It is shown that the convection of heat due to internal heat generation is increased in the enclosure when the ratio of internal Rayleigh number to external Rayleigh number is larger.     

Optical Analysis and Qualitative Discrimination of Vortex-Flame Interaction in a Plane Premixed Shear Layer

To obtain practical schemes of vortex–flame interactions, a series of organized eddies formed in the plane premixed shear layer is investigated, instead of a single vortex ring or a single vortex tube. The plane premixed shear layer is first formed between two parallel uniform propane–air mixture streams. For getting clear qualitative pictures of vortex–flame interactions in the plane premixed shear layer, two extreme ignition points are assigned; one is assigned at the center of an organized eddy where the vortex motion plays an important role, the other at the midpoint between two adjacent organized eddies where the rolling-up motion prevails. A premixed flame is initiated by an electric discharge at one of the two assigned points and propagates either in the large scale organized eddy or along the interface between two uniform mixture streams. Propagation and deformation processes of the flame are observed using the simultaneously two-directional and high-speed Schlieren photography. The tangential velocity of organized eddy and the equivalence ratio of premixed shear flow are varied as two main parameters. The outline of propagating flame after the midpoint ignition is numerically analyzed by superposing the flame propagation having a constant burning velocity on the vortex flow field simulated with the discrete vortex method. The results obtained show that there exists another type of vortex–flame interaction in the plane shear layer in addition to the vortex bursting, and that it is caused by the rolling-up motion particular to the coherent structure in the plane shear layer and is simply named the vortex boosting. It is qualitatively concluded therefore that, in the ordinary turbulent premixed flames formed in the plane premixed shear layer, these two fundamental vortex-flame interactions get tangled with each other to augment the propagation velocity. An empirical expression which qualitatively takes into account of the effects of both vortex and chemical properties is finally proposed.     

圆筒弯扭组合实验中弯曲切应力的测量

分析了圆筒弯扭组合实验中弯曲切应力的主要测量误差,提出了改进的贴片方式,在此基础上,重新设计了弯曲切应力的实验方案.     

方柱尾流流态对床面冲淤形态的影响

应用平面二维悬沙数学模型对方柱尾流区的泥沙输运及床面调整进行了数值模拟，并对重要物理参数做了分析说明。数值计算采用时间分裂一全隐式有限差分格式，流场的计算基于沿水深平均的RANS方程。通过系统的数值模拟，揭示了不同来流情况下，方柱尾流区的流态变化，及与之相应的床面变化规律，并对不同流态下的泥沙运动作了分析。计算结果表明在方柱尾流区，不同的流场流态对尾流区的床面调整有明显影响。流场较弱的情况下，尾流区中湍动强度相应较弱，此时悬浮泥沙由于流速减小而普遍落淤，床面应力的减小也致使床面冲刷量减小。随着流场强度的逐渐增大，尾流区中湍动强度相应增强，床面应力增大，同时湍流的强烈交换作用增强了对泥沙的输运作用，床面变化不再是简单的淤积状态，部分区域出现了冲刷。