单一水平轴风电机组尾迹的模拟方法与流动机理研究综述

为实现碳达峰、碳中和“3060”目标, 风能将在我国能源体系发挥重要作用. 风力机尾迹是影响风电性能和度电成本的关键因素, 需在风力机布置和控制设计中充分考虑. 本文首先介绍风力机尾迹的数值模拟方法, 包括解析模型、低阶模型、大涡模拟和来流湍流生成方法. 解析模型和低阶模型可快速计算风力机尾迹, 但依赖于模型参数, 且不能或不能准确预测尾迹湍流特性. 结合风力机参数化模型的大涡模拟可准确预测尾迹蜿蜒等湍流特征, 是流动机理研究的有力工具, 可为发展快速预测模型提供数据和理论支撑. 接着, 本文介绍了叶尖涡、中心涡和尾迹蜿蜒并讨论其产生机理. 对于湍流来流, 叶尖涡主要存在于近尾迹. 蜿蜒是远尾迹的主要特征, 影响下游风力机的来流特征. 尾迹蜿蜒的产生有两种机制: 来流大尺度涡和剪切层失稳. 数值和观测结果显示两种机制共同存在. 机舱和中心涡对尾迹蜿蜒有重要影响. 采用叶片和机舱的致动面模型可准确预测尾迹蜿蜒. 研究显示不同风力机尾迹间的湍流特征存在相似性, 为发展尾迹湍流的快速预测模型提供了理论依据. 当前研究多关注平坦地形上的风力机尾迹, 复杂地形和海洋环境下的大气湍流和风力机尾迹的机理复杂, 现有工程模型无法准确预测, 有待深入研究.      

一种基于湍动能方程的转捩判定方法简

转捩现象是阻碍阻力高精度求解的主要问题之一. Menter 和Langtry 所提出的γ-θ转捩模型通过引入涡量雷诺数和间歇因子输运方程来驱动转捩,但是其中很多经验公式的理论立足点有待商榷. 驱使层流转变到湍流依赖的仍然是平均速度的一阶和二阶相关量,它们组合构成了湍动能方程的耗散尺度. 在湍动能方程中做合适的耗散平衡后,仅仅依靠湍动能方程可以有效地捕捉转捩现象. 采用自然转捩和旁路转捩测试算例进行了验证,结果证明该方法与试验值匹配较好,具有一定的工程实用价值.     

一种适用于超音速边界层的湍流转捩模式

建立一种合理反映扰动模态和可压缩性影响的新型k-ω-γ转捩模式.其主要特点为:(1)假设脉动动能k 由湍流脉动部分和非湍流脉动部分组成,且在模化后者时采用了稳定性分析的结果; (2)在间歇因子γ方程的源项中,构造了具有``自动判断转捩起始位置'功能的函数; (3)通过构造新型的物面法向长度尺度,保证了模式中所有的表达式均由当地变量构成,可以方便地应用于现代CFD程序之中. 该模式在亚音速、超音速和高超音速条件下的边界层流动中进行了验证. 计算结果表明,该模式可应用于较宽马赫数范围内的自然转捩以及旁路转捩过程,所具有的捕捉流动转捩的性能优于国际上的现有模式.      

圆柱尾迹影响旁路转捩末期发卡涡涡包的研究

基于TR-PIV技术, 通过侧视和俯视两种情况对圆柱尾迹影响下旁路转 捩末期发卡涡涡包的结构及特征尺寸进行了实验研究. 结合二维空间子波变换和\lambda _{ci}准则, 运用线性随机估计方法对速度信号进行条件平均. 在侧视情况下, 条件平均结 果显示, 在边界层中一系列发卡涡涡头与壁面构成17^{\circ}的倾角, 并且被尾迹涡所占据的低 速区域出现在涡包上方的主流区中. 在俯视的结果中, 沿流向方向拉伸(流向尺度 3\delta, 展向尺度0.55δ)的低速条带结构出现在法向高 度为y/δ =0.2的流向-展向平面中, 并且在该低速条带的两侧对称地出 现了沿流向分布的反向旋转的涡结构. 可以得出: 在圆柱尾迹影响下旁路转捩的末期, 由于 尾迹涡诱导作用的影响, 发卡涡涡包在形态上显示出了更大尺度的特征.     

一种基于湍动能方程的转捩判定方法

转捩现象是阻碍阻力高精度求解的主要问题之一. Menter 和Langtry 所提出的γ-θ转捩模型通过引入涡量雷诺数和间歇因子输运方程来驱动转捩,但是其中很多经验公式的理论立足点有待商榷. 驱使层流转变到湍流依赖的仍然是平均速度的一阶和二阶相关量,它们组合构成了湍动能方程的耗散尺度. 在湍动能方程中做合适的耗散平衡后,仅仅依靠湍动能方程可以有效地捕捉转捩现象. 采用自然转捩和旁路转捩测试算例进行了验证,结果证明该方法与试验值匹配较好,具有一定的工程实用价值.      

返回舱高雷诺数再入过程底部流动稳定性

返回舱高雷诺数再入过程中存在肩部高热流、底部阻力无法准确预测以及非定常振动等问题,解决此类问题的关键是分离和转捩等物理现象的准确识别. 本文采用大涡模拟方法细致刻画了返回舱类钝体外形在高雷诺数再入过程中的分离和转捩等物理现象,获得了返回舱底部流动形态以及稳定性特征. 从肩部剪切失稳、底部流动结构失稳、尾迹发展区以及远尾迹区的耦合失稳等多个角度分析了返回舱外形的底部流动失稳机制.研究发现, 返回舱类外形底部流动稳定性主要存在两类失稳模式即肩部剪切失稳模式以及底部流动结构失稳模式,二种模式存在耦合效应, 同时在远尾迹湍流区域存在类卡门涡街的振荡行为.这些认识为理解返回舱外部扰动因素对底部流动的作用机理及返回舱稳定性控制提供了基础理论支撑.      

叶轮机内附面层流动与分离的某些研究进展

通过总结有关叶轮机内附面层的研究工作认为,周期性扫掠的上游尾迹改变了附面层的转捩方式,给附面层流动带来了强烈的非定常性,不同的尾迹强度和扫过频率决定了附面层内不同的时空结构; 此外,雷诺数、叶片负荷、表面粗糙度和来流条件等因素均能影响尾迹对附面层的非定常效应.随后总结了尾迹对附面层作用的机理, 并介绍了转捩模型的发展过程.最后提出了关于附面层研究方向的两个疑问,认为附面层分离后的复杂旋涡流场值得重点关注.      

k-ω-γ模式对转捩影响因素的预测性能研究

高超声速边界层转捩的准确预测对飞行器的防热、减阻至关重要,而影响高超声速边界层转捩的因素众多.从模式角度出发研究边界层转捩的影响因素,采用k-ω-γ 转捩模式对5°圆锥的边界层转捩进行了数值分析,计算了不同头部钝度、来流雷诺数和湍流度情况下的边界层转捩,并与实验结果进行了对比. 研究结果表明:k-ω-γ 转捩模式基本能够反映头部钝度、来流雷诺数、来流湍流度对高超声速圆锥边界层转捩的影响规律,但对转捩后的热流峰值预测不准;从模式构造角度分析发现,雷诺数越高或头部钝度越小,层流区边界层越薄,k-ω-γ 转捩模式中第一、第二模态时间尺度增大,因此转捩起始位置提前;来流湍流度越大,等效脉动动能初值越大,导致层流区发展过程中等效脉动动能越大,因此转捩易于发生.      

结构振动对湍流近尾迹的影响

研究了圆柱绕流中流体与结构的相互作用,侧重结构振动对湍流尾迹的影响,用激光测振仪测量圆柱在升力方向的位移;用热线和LDA（二维）测量湍流的近尾迹,通过变化自由流的速度和圆柱体直径（特征尺寸）来变化雷诺数,用两个振动特性不同的（一个相对刚性,一个相对弹）圆柱来产生尾迹,研究固体结构振动对湍流近尾迹的平均速度场和湍流场的影响,结果表明,结构自由振动对湍流近尾迹场影响明显,该影响随雷诺数的变化不明显。     

剪切流及分离流结构和层流-湍流转捩的研究(成就和问题)

本文介绍了剪切层(边界层,Poiseujlle.流、分离流、尾流)中层流向湍流转捩过程的连续各阶段的实验研究结果,包括:给定流动对外部扰动的敏感性,产生的波(拟序结构)的线性发展,以及引起有序的层流流态破坏的非线性过程.     

Extension of an algebraic intermittency model for better prediction of transition in separated layers under strong free-stream turbulence

A constitutive law describing the Reynolds stresses in boundary layers undergoing laminar-to-turbulent transition, constructed in previous work by elastic-net regression on an experimental data base, is used to improve an algebraic intermittency model for cases with transition in a separated layer influenced by a high level of free-stream turbulence. The intermittency model is combined with a k-ω turbulence model and the basic version, developed in previous work, functions well for bypass transition in attached boundary layers and for transition in separated boundary layers under a low free-stream turbulence level. The basic model version is extended by an additional production term in the transport equation for turbulent kinetic energy. A sensor detects the front part of a separated layer and activates the production term. The term expresses the effect of Klebanoff streaks generated upstream of separation on the Kelvin-Helmholtz instability rolls in the separated part of the layer. The Klebanoff streaks cause faster breakdown by the combined effects of a large adverse pressure gradient and an elevated free-stream turbulence level. The extended model does not alter the results of the basic model version for bypass transition in an attached boundary layer and for transition in a separated boundary layer under a low free-stream turbulence level. The extended model significantly improves the predictions of the previous model version for transition in a separated boundary layer under a high free-stream turbulence level.     

Boundary-layer disturbances subjected to free-stream turbulence and simulation on bypass transition

The phenomena associated with the entrainment of free-stream turbulence(FST) into boundary-layer flows are relevant for a number of subjects.It has been believed that the continuous spectra of the Orr-Sommerfeld(O-S)/Squire equations describe the entrainment process,and thus they are used to specify the inlet condition in simulation of bypass transition.However,Dong and Wu(Dong,M.and Wu,X.On continuous spectra of the Orr-Sommerfeld/Squire equations and entrainment of free-stream vortical disturbances.Journal of Fluid Mechanics,732,616–659(2013)) pointed out that continuous spectra exhibit several non-physical features due to neglecting the non-parallelism.They further proposed a large-Reynolds-number asymptotic approach,and showed that the non-parallelism is a leading-order effect even for the short-wavelength disturbance,for which the response concentrates in the edge layer.In this paper,the asymptotic solution is verified numerically by studying its evolution in incompressible boundary layers.It is found that the numerical results can be accurately predicted by the asymptotic solution,implying that the latter is adequate for moderate Reynolds numbers.By introducing a series of such solutions as the inflow perturbations,the bypass transition is investigated via the direct numerical simulation(DNS).The transition processes,including the evolution of streaks,the amplification of secondary-instability modes,and the emergence of turbulent spots,agree with the experimental observations.     

Free-stream density perturbations in a reflected-shock tunnel

Focused laser differential interferometry is used to quantify the free-stream density perturbations in the T5 reflected-shock tunnel. The investigation of reflected-shock tunnel disturbances is motivated by the study of hypervelocity boundary-layer instability and transition. Past work on hypersonic wind-tunnel noise is briefly reviewed. New results are reported for hypervelocity air flows at reservoir enthalpies between 5 and 18 MJ/kg at Mach ≈ 5.5. Statistical analysis finds no correlation of RMS density perturbations with tunnel run parameters (reservoir pressure, reservoir mass-specific enthalpy, free-stream unit Reynolds number, free-stream Mach number, and shot number). Spectrograms show that the free-stream disturbance level is constant throughout the test time. Power spectral density estimates of each of the experiments are found to collapse upon each other when the streamwise disturbance convection velocity is used to eliminate the time scale. Furthermore, the disturbance level depends strongly on wavelength. If the disturbance wavelength range of interest is between 700 μm and 10 mm, the tunnel noise is measured to be less than 0.5 % with the focused laser differential interferometer.     

Boundary-layer transition on a cylinder with and without separation bubbles

Boundary layer transition with and without transitional separation bubbles was investigated on a cylinder in cross flow. Measurements of the pressure distribution and hot-wire measurements within the boundary layer were carried out at two free-stream velocities and with different flow disturbances. The separation bubble reacts very sensitively to changes in inlet turbulence. Tollmien-Schlichting waves were observed in the separated shear layer just before transition, and their frequencies were in good agreement with stability theory. However, correlations concerning bubble length which were fitted using airfoil data are apparently not suitable for describing separation bubbles on cylinders. Finally, measurements in periodically disturbed flow show how the bubble reacts to this type of disturbance.     

Numerical simulation of boundary-layer transition at Mach two

The linear and early nonlinear stages of boundary-layer transition at free-stream Mach numberM ==2.0 are investigated by direct numerical simulation of the compressible Navier-Stokes equations. Results from simulations with a large computational box and small-amplitude random initial conditions are compared with linear stability theory. The growth rates of oblique waves are reproduced correctly. Two-dimensional waves show a growth that is modulated in time, indicating the presence of an extra unstable mode which moves supersonically relative to the free stream. Further simulations are conducted to investigate the nonlinear development of two- and three-dimensional disturbances The transition due to oblique disturbance waves is the most likely cause of transition at this Mach number, and is found to lead to the development of strong streamwise vortices.     

From Streaks to Spots and on to Turbulence: Exploring the Dynamics of Boundary Layer Transition

“...an eerie type of chaos can lurk just behind a facade of order, and yet deep inside the chaos lurks an even eerier type of order.” Douglas Hofstadter Bypass transition to turbulence in boundary layers is examined using linear theory and direct numerical simulations (DNS). First, the penetration of low-frequency free-stream disturbances into the boundary layer is explained using a model problem with two time scales, namely the shear and wall-normal diffusion. The simple model provides a physical understanding of the phenomenon of shear sheltering. The second stage in bypass transition is the amplification of streaks. Streak detection and tracking algorithms were applied to examine the characteristics of the streak population inside the boundary layer, beneath free-stream turbulence. It is demonstrated that simple statistical averaging masks the wealth of streak amplitudes in transitional flows, and in particular the high-amplitude, relatively rare events that precede the onset of turbulence. The third stage of the transition process, namely the secondary instability of streaks, is examined using secondary instability analysis. It is demonstrated that two types of instability are possible: An outer instability arises near the edge of the boundary layer on the lifted, low-speed streaks. An inner instability also exists, and has the appearance of a near-wall wavepacket. The stability theory is robust, and can predict the particular streaks which are likely to undergo secondary instability and break down in transitional boundary layers beneath free-stream turbulence. Beyond the secondary instability, turbulent spots are tracked in DNS in order to examine their characteristics in the subsequent non-linear stages of transition. At every stage, we compare the findings from linear theory to the empirical observations from direct solutions of the Navier-Stokes equations. The complementarity between the theoretical predictions and the computational experiments is highlighted, and it leads to a detailed view of the mechanics of transition.     

Transition Modeling Using Two Different Intermittency Transport Equations

The accurate numerical simulation of the flow through turbomachinerydepends on the correct prediction of boundary-layer transitionphenomena. Especially heat transfer and skin friction investigationsdemand a reliable simulation of the transition process. Therefore, in this work two different one-equation transport modelsfor a transitional weighting factor are selected and modified for theimplementation into a Reynolds-averaged Navier–Stokes solver. Thisfactor is used to modify the eddy viscosity obtained from a turbulencemodel to simulate the transition process. The first model was originallydeveloped by Steelant and Dick [1] to simulate by-passtransition for high free-stream turbulence. The second model wasproposed by Huang and Suzen [2] as a blending of two modelsfor near-wall intermittency and cross-stream variation of intermittency.In contrast to one-dimensional transition models, the new approachesmodel the transition process not only in flow direction but also acrossthe boundary-layer and thus provide a more realistic prediction of thetransition process. Whereas the Steelant and Dick model (SD) allowsturbulent quantities in the free-stream prior and after transition, thesecond model by Huang and Suzen (HS) sets the free-stream turbulence tozero in the whole flowfield. The models are validated on transitional skin friction experimentson a flat plate (T3 test cases of ERCOFTAC SIG 10), on heat transfermeasurements in a linear turbine cascade done at the VKI and on laservibrometer measurements of a linear turbine cascade. Both models showgood agreement with the skin friction data, but the heat transfer canonly be predicted correctly by the Steelant and Dick model due to itsability to consider the free-stream turbulence.     

Large eddy simulation on the effect of free-stream turbulence on bypass transition

The effect of free-stream turbulence (FST) on bypass transition in a zero-pressure-gradient boundary layer is investigated by means of Large Eddy Simulation (LES). The broadband turbulent inflow is synthesized to validate the feasibility of LES. Both a zero-thickness plate and one with super-ellipse leading-edge are addressed. The calculated Reynolds-averaged fields are compared with experimental data and decent agreement is achieved. Instantaneous fields show the instability occurs in the lifted low-speed streaks similar to earlier DNS results, which can be ascribed to outer mode. Various inflows with bi-/tri-mode interaction are specified to analyze effects of particular frequency mode on the instability pattern and multifarious transition or non-transition scenarios are obtained. Outer instability is observed in the cases with one low-frequency mode and one high-frequency mode inflow as reported by Zaki and Durbin (2005), and with one more high-frequency mode appended. Inner instability is observed in the case with a low-frequency dominant inflow, while the high-frequency mode is indispensable to induce the secondary instability. Furthermore, the results show that the transition onset is highly sensitive to low-frequency mode while the transition rate is highly sensitive to high-frequency mode. Finally, the formational frequency of turbulent spot (FFTS) is counted and the frequency of laminar streaks is demonstrated by spectral analysis.     

Analysis and modelling of the relation between the shear rate and Reynolds stress tensors in transitional boundary layers

A non-linear eddy-viscosity transition model is presented, tuned by a large experimental data set describing transitional boundary layers. Data have been acquired by TR-PIV on a flat plate placed in a 2D converging-diverging channel with variable opening angle, allowing variation of the adverse pressure gradient, the free-stream turbulence intensity and the flow Reynolds number. Overall, 48 different combinations of these flow parameters encompass different modes of transition from bypass to separated-flow mechanisms, thus allowing fine tuning of the model, spanning significantly different conditions. The model is tuned locally as a function of the turbulent kinetic energy, a Reynolds number based on the wall distance and the ℓ₂-norm of the shear rate tensor. A first correlation determines the rotation for alignment of the principal axes of the shear and stress tensors. By a second correlation, the eigenvalues of the stress tensor are obtained. The non-linear eddy-viscosity relation reproduces the anisotropy of the turbulence field observed for both bypass and separated-flow transitional cases. The relation has been applied to another experimental data set that did not participate to the fitting of the model and that is characterized by a different range of Reynolds number and turbulence intensity and a significantly stronger adverse pressure gradient with respect to the tuning dataset. Such application further strengthens the capability of the proposed correlations, that can easily be implemented in existing CFD solvers.     

Modelling of Unsteady Transition in Low-Pressure Turbine Blade Flows with Two Dynamic Intermittency Equations

A transition model for describing wake-induced transition is presented based on the SST turbulence model by Menter and two dynamic equations for intermittency: one for near-wall intermittency and one for free-stream intermittency. In the Navier-Stokes equations, the total intermittency factor, which is the sum of the two, multiplies the turbulent viscosity computed by the turbulence model. The quality of the transition model is illustrated on the T106A test cascade for two Reynolds numbers, using experimental results by Stieger and Hodson for transition mainly due to kinematic wake impact on a separation bubble. The quality of the model is also revealed on the T106D test cascade using experimental results from Hilgenfeld, Stadtmuller and Fottner for wake turbulence induced transition. The test cases differ in pitch to chord ratio, Reynolds number and inlet free-stream turbulence intensity, causing different transition mechanisms. The unsteady results are presented in space-time diagrams of shape factor and wall shear stress on the suction side. The results show the capability of the model to capture the physics of unsteady transition in separated state. Inevitable shortcomings are revealed as well.