尾流激励的叶片气动力降阶模型

气动力降阶模型是研究叶片气动弹性振动快速高效的新方法。现有气动力降阶模型的研究主要集中在叶片颤振方面,没有涉及更为常见的上游尾流激励的叶片振动问题。本文提出基于Volterra级数的尾流激励叶片气动力降阶模型,为尾流激励下叶片振动和动静叶干涉振动研究提供了新的思路。采用行波法简化尾流的参数个数,用阶跃信号法识别降阶模型的核函数。二维叶片的算例结果表明,本文方法可以较准确地描述尾流激励引起的叶片气动力振荡,而且计算效率极高。     

大长细比导弹的气动弹性降阶模型

对于大长细比导弹,需要在设计阶段准确计算气动弹性/气动伺服弹性,但其复杂的气动力给计算带来困难,因此气动力降阶模型是突破大长细比导弹跨音速气动弹性分析与控制瓶颈的关键技术.虽然气动力模型降阶方法已在预测二维机翼结构的气动弹性方面取得重要进展,但几乎未见关于全机模型的气动力降阶模型研究报道.本文基于递归Wiener模型的气动力降阶方法,利用CFD计算的气动力作为模型辨识数据,用鲁棒子空间和Levenberg-Marquardt算法辨识降阶模型参数,建立了大长细比导弹气动力降阶模型.在此基础上与大长细比导弹有限元模型相结合,构造出气动弹性降阶模型,并在数值仿真中测试气动弹性降阶模型在不同马赫数下的适用性.数值仿真结果表明,该气动弹性降阶模型能够精确预测导弹模型在不同飞行条件下的非定常气动力和导弹模型的气动弹性频率响应特性.     

变厚度翼型的非定常气动力模型降阶及影响分析

建立了基于Kriging代理和递归算法的变厚度翼型气动力降阶模型,将滤波的高斯白噪声作为输入信号,采用计算流体力学(CFD)方法获得不同厚度下翼型的非定常气动力并将其作为降阶模型的训练样本。该降阶模型不仅大大提高了非定常气动力的计算效率,而且其预测得到的非定常响应的精度不低于92.01%。通过蒙特卡洛法的变厚度翼型全局灵敏度分析表明,俯仰运动是影响气动力波动的主要因素,俯仰与沉浮位移的耦合对气动力影响很小。考虑翼型变厚度时对翼面压力及分离点的影响,本文建立了压强系数的降阶模型。通过与CFD结果的对比,得到其精度为99.9995%,验证了降阶模型的正确性和有效性。     

跨音速极限环型颤振的高效数值分析方法

事先建立一个低阶的非线性、非定常气动力模型是开展非线性流场中气动弹性问题研究的一个捷径. 基于CFD方法, 通过计算结构在流场中自激振动的响应来获得系统的训练数据. 采用带输出反馈的循环RBF神经网络, 建立时域非线性气动力降阶模型.耦合结构运动方程和非线性气动力降阶模型, 采用杂交的线性多步方法计算结构在不同速度(动压)下的响应历程, 从而获得模型极限环随速度(动压)变化的特性. 两个典型的跨音速极限环型颤振算例表明, 基于气动力降阶模型方法的计算结果与直接CFD仿真结果吻合很好, 与后者相比其将计算效率提高了1~2个数量级.      

气动力⁃安培力⁃谐波激励作用下载流导线的非线性振动特征

研究了气动力-安培力-受迫激励联合作用下载流新月形覆冰导线的振动特征,在传统只考虑安培力的载 流导线振动方程中引入了气动力和受迫激励项,使得模型更加符合实际工程．首先建立了载流新月形覆冰输电 导线模型,接着推导了载流覆冰导线模型的振动方程,利用Galerkin方法将该振动方程转变为有限维度的常微 分方程,采用多尺度法进行摄动求解,求得幅-频响应函数,并对比分析主共振情况下考虑气动力与不考虑气 动力时载流、间距、风速、张力、激励幅值对振动幅值的影响．研究结果表明：考虑气动力的振动幅值较未考 虑气动力时小．各线路参数对导线的幅值都有一定程度的影响,并且使系统表现出明显的软弹簧特征和复杂的 动力学行为．本文研究成果能为实际工程提供一定参考价值     

涡激响应尾流双振子模型的参数反演方法

尾流双振子模型是研究圆柱结构涡激振动响应的重要模型,模型参数的准确确定对悬浮隧道设计理论具有重要意义.首先通过降阶法将多变量二阶非线性常微分方程组的尾流双振子模型变换为一阶方程组.然后给出一种新型的圆柱结构水槽试验设计方案,其中试验模型的刚度能够较好反映悬浮隧道等实际工程结构的刚度,基于相机动态捕捉和视频识别计算机程序,获取圆柱结构在水平和竖直方向的位移试验数据.基于试验结果和龙格-库塔方法求解一阶方程组,采用BP神经网络智能算法对模型参数进行反演,同时利用遗传算法对神经元的初始权值和阈值进行优化,所得结果平均误差仅为5.50％,优于遗传算法和未优化的BP神经网络模型.结果表明,基于遗传算法优化的BP神经网络智能算法能够精确实现尾流双振子模型的参数确定,为圆柱结构涡激振动响应分析提供理论基础.     

基于边界元方法的气动弹性降阶模型及仿真

传统气动弹性的时域计算耗费了大量时间,为了提高计算效率,本文发展了基于边界元方法的降阶模型技术。首先基于边界元方法建立非定常流场的求解模型,结合特征值分析技术建立了非定常气动力的低阶模型;然后,利用边界元方法建立了气动网格和结构网格之间的信息转换矩阵;最后将非定常气动力降阶模型和结构动力学方程联合,建立了气动弹性系统的低阶状态空间模型。将所发展的降阶模型方法应用于NACA0012翼型的非定常气动力求解中,结果表明降阶模型可以在保证原系统计算精度的同时提高了计算效率;将降阶模型技术应用到三维机翼的气动弹性响应计算中,在系统阶数仅为12阶的情况下可以得到与原系统一致的极限环响应,说明降阶模型技术在求解气动弹性问题中的巨大优势。     

基于气动力辨识的ASE模型降阶研究

CFD/CSD耦合计算能够准确预测跨音速段飞行器弹性振动的非定常气动力, 但其带来的巨大 计算量及高阶维数不利于气动弹性系统的分析与综合. 针对于此,采用系统辨识及 均衡截断技术对高阶气动伺服弹性模型进行降阶处理,并利用所得到的低阶模型进行系统综 合：(1) 基于Volterra级数气动力辨识技术,得到非定常气动力的时域降阶模型(ROM), 耦合结构动力学模型及控制机构动力学模型获得气动伺服弹性(ASE)状态空间方 程;(2) 利用均衡截段法对时域ASE模型进行进一步降阶,得到能够较真实反映所关心频域内系统响应 的低阶ASE模型;(3) 针对建模误差和降阶误差存在造成的系统不确定性问题,结合降阶模型 采用混合灵敏度$H_{\infty}$控制方法设计颤振主动抑制鲁棒控制律,保证其作用 于真实系统的有效性;对控制器进行 均衡阶段降阶并保持其鲁棒性,得到低阶鲁棒的颤振抑制控制器. 最后利用典型的BACT模型 进行气动伺服弹性的降阶及主动颤振抑制控制,仿真结果表明,基于ROM建立的低阶气动弹 性模型能够较真实地反应系统的颤振特性;而基于截断后的降阶模型所设计的低阶鲁棒控制 器能够有效应用于存在不确定性摄动的实际系统,并将系统颤振速度提高36%.     

一种高效的叶轮机叶片气动阻尼计算方法

运用叠加原理, 发展了一种可以运用于小振幅运动的叶轮机叶片非定常气动力降阶模型, 并将该模型与传统的能量法相结合, 提出了一种叶轮机叶片气动阻尼的高效求解方法. 运用该方法求解叶轮机叶片的气动阻尼系数, 对某个频率、某个模态只需要进行一次非定常计算, 就可以求出所有叶间振动相角下的气动阻尼系数, 提高了气动阻尼的求解效率. 在STCF4和NASA Rotor67两个算例上运用非定常雷诺平均N-S(RANS)方程和提出的降阶模型进行了对比计算.算例表明, 在小振幅下该方法的计算结果与RANS方程计算得到的气动阻尼系数能很好地吻合, 而计算效率相比多通道非定常RANS方程计算提升了近一个数量级, 并且该方法还可以运用于有失谐情况的颤振分析, 在工程上有较高的应用价值.      

桁架系统的模型降阶及其主动控制研究

对桁架结构系统的模型降阶和主动控制进行了研究。系统模型采用有限元方法进行建模;模型降阶分别采用模态价值分析方法和内平衡降阶方法;控制设计采用最优控制方法。同时详细给出了模态价值分析方法和内平衡降阶方法的降阶过程,并将两者的结果进行了数值对比。仿真结果显示,两种降阶方法均能够有效地对桁架系统进行模型降阶,本文采用的最优控制律能够有效地抑制结构的振动。     

An aerodynamic ROM of the blade subjected to wake based on Fourier method for flow

A reduced-order model (ROM) is presented based on Fourier method for flow to predict aerodynamic forces of blades subjected to periodic time-varying upstream wakes. In the method, a time-varying wake is decomposed into harmonic waves by fast Fourier transformation. Using the Fourier method for flow and neglecting the cross-coupling between harmonics, the aerodynamic forces caused by the wake are represented by a linear combination of harmonics with the same frequencies as the wake. The coefficients of the aerodynamic force harmonics are interpolated at the per-fitted curves of the normalized Fourier coefficients (coefficients of aerodynamic forces harmonics corresponding to a unit simple harmonic excitation)–frequency relationship. A blade example is used to show the ability of the proposed method. The results indicate that the ROM method can predict the aerodynamic forces of blades caused by wakes efficiently and accurately. The amplitude levels of wakes have a linear impact on the accuracy of the ROM. Neglecting the higher-order cross-coupling between the harmonics in the ROM method is acceptable.     

Aeroelastic analysis of large horizontal wind turbine baldes

A nonlinear aeroelastic analysis method for large horizontal wind turbines is described. A vortex wake method and a nonlinear finite element method (FEM) are coupled in the approach. The vortex wake method is used to predict wind turbine aerodynamic loads of a wind turbine, and a three-dimensional (3D) shell model is built for the rotor. Average aerodynamic forces along the azimuth are applied to the structural model, and the nonlinear static aeroelastic behaviors are computed. The wind rotor modes are obtained at the static aeroelastic status by linearizing the coupled equations. The static aeroelastic performance and dynamic aeroelastic responses are calculated for the NH1500 wind turbine. The results show that structural geometrical nonlinearities significantly reduce displacements and vibration amplitudes of the wind turbine blades. Therefore, structural geometrical nonlinearities cannot be neglected both in the static aeroelastic analysis and dynamic aeroelastic analysis.     

Dynamic wind loads and wake characteristics of a wind turbine model in an atmospheric boundary layer wind

An experimental study was conducted to characterize the dynamic wind loads and evolution of the unsteady vortex and turbulent flow structures in the near wake of a horizontal axis wind turbine model placed in an atmospheric boundary layer wind tunnel. In addition to measuring dynamic wind loads (i.e., aerodynamic forces and bending moments) acting on the wind turbine model by using a high-sensitive force-moment sensor unit, a high-resolution digital particle image velocimetry (PIV) system was used to achieve flow field measurements to quantify the characteristics of the turbulent vortex flow in the near wake of the wind turbine model. Besides conducting “free-run” PIV measurements to determine the ensemble-averaged statistics of the flow quantities such as mean velocity, Reynolds stress, and turbulence kinetic energy (TKE) distributions in the wake flow, “phase-locked” PIV measurements were also performed to elucidate further details about evolution of the unsteady vortex structures in the wake flow in relation to the position of the rotating turbine blades. The effects of the tip-speed-ratio of the wind turbine model on the dynamic wind loads and wake flow characteristics were quantified in the terms of the variations of the aerodynamic thrust and bending moment coefficients of the wind turbine model, the evolution of the helical tip vortices and the unsteady vortices shedding from the blade roots and turbine nacelle, the deceleration of the incoming airflows after passing the rotation disk of the turbine blades, the TKE and Reynolds stress distributions in the near wake of the wind turbine model. The detailed flow field measurements were correlated with the dynamic wind load measurements to elucidate underlying physics in order to gain further insight into the characteristics of the dynamic wind loads and turbulent vortex flows in the wakes of wind turbines for the optimal design of the wind turbines operating in atmospheric boundary layer winds.     

Unsteady aerodynamic reduced-order modeling of an aeroelastic wing using arbitrary mode shapes

Computational fluid dynamics (CFD) based unsteady aerodynamic reduced-order model (ROM) can offer significant improvements to the efficiency of transonic aeroelastic analysis. To construct a ROM based on mode shapes, one run of CFD solver is needed to compute aerodynamic responses corresponding to mode excitations. When mode shapes change with structure, another run of the CFD solver is required to construct the new ROM. The typically large computational cost associated with repeated runs of the CFD solver impedes the application of existing unsteady aerodynamic reduced-order modeling methods to transonic aeroelastic design optimization and aeroelastic uncertainty analysis. This paper demonstrates a method that can replace the CFD solver used in the process of existing unsteady aerodynamic reduced-order modeling. It can produce aerodynamic responses corresponding to mode excitations for arbitrary mode shapes within a few seconds. Computational cost can be reduced by two orders of magnitude using the mode excitations and the corresponding aerodynamic responses computed by the method to construct the ROMs used for flutter analyses in aeroelastic design optimization or aeroelastic uncertainty analysis in transonic regime compared with the existing unsteady aerodynamic reduced-order modeling methods. Results show that the method can accurately produce the aerodynamic responses corresponding to the mode excitations and predict the flutter characteristics of AGARD 445.6 wings root-attached in three different ways.     

基于涡尾迹方法的风力机气动性能分析

基于(势流)涡尾迹方法开发了水平轴风力机叶片气动性能分析程序,采用固定尾迹涡模型和自由尾迹模型分别对气动设计性能进行计算分析,得到风力机设计工况下的涡位置、诱导因子、功率系数及扭矩系数等气动性能参数,并与设计结果对比。结果表明,涡尾迹方法能够快速准确地计算风力机叶片气动性能参数,对风力机叶片气动分析,固定尾迹涡模型较自由尾迹模型计算时间短,具有较好的实用性。     

Flow field studies on a micro-air-vehicle-scale cycloidal rotor in forward flight

This paper examines the flow physics and principles of force production on a cycloidal rotor (cyclorotor) in forward flight. The cyclorotor considered here consists of two blades rotating about a horizontal axis, with cyclic pitch angle variation about the blade quarter-chord. The flow field at the rotor mid-span is analyzed using smoke flow visualization and particle image velocimeV are compared with flow fields predicted using 2D CFD and time-averaged force measurements acquired in an open-jet wind tunnel at three advance ratios. It is shown that the experimental flow field is nearly two dimensional at μ = 0.73 allowing for qualitative comparisons to be made with CFD. The incoming flow velocity decreases in magnitude as the flow passes through the retreating (upper) half of the rotor and is attributed to power extraction by the blades. A significant increase in flow velocity is observed across the advancing (lower) half of the rotor. The aerodynamic analysis demonstrates that the blades accelerate the flow through the lower aft region of the rotor, where they operate in a high dynamic pressure environment. This is consistent with CFD-predicted values of instantaneous aerodynamic forces which reveal that the aft section of the rotor is the primary region of force production. Phase-averaged flow field measurements showed two blade wakes in the flow, formed by each of the two blades. Analysis of the blades at several azimuthal positions revealed two significant blade-wake interactions. The locations of these blade-wake interactions are correlated with force peaks in the CFD-predicted instantaneous blade forces and highlight their importance to the generation of lift and propulsive force of the cyclorotor.     

Recent improvements of actuator line-large-eddy simulation method for wind turbine wakes

In a large wind farm,the wakes of upstream and downstream wind turbines can interfere with each other,affecting the overall power output of the wind farm.To further improve the numerical accuracy of the turbine wake dynamics under atmosphere turbulence,this work proposes some improvements to the actuator line-large-eddy simulation(AL-LES)method.Based on the dynamic k-equation large-eddy simulation(LES),this method uses a precursor method to generate atmospheric inflow turbulence,models the tower and nacelle wakes,and improves the body force projection method based on an anisotropic Gaussian distribution function.For these three improvements,three wind tunnel experiments are used to validate the numerical accuracy of this method.The results show that the numerical results calculated in the far-wake region can reflect the characteristics of typical onshore and offshore wind conditions compared with the experimental results.After modeling the tower and nacelle wakes,the wake velocity distribution is consistent with the experimental result.The radial migration velocity of the tip vortex calculated by the improved blade body force distribution model is 0.32 m/s,which is about 6%different from the experimental value and improves the prediction accuracy of the tip vortex radial movement.The method proposed in this paper is very helpful for wind turbine wake dynamic analysis and wind farm power prediction.     

A nonlinear model for aerodynamic configuration of wake behind horizontal-axis wind turbine

Determination of the aerodynamic configuration of wake is the key to analysis and evaluation of the rotor aerodynamic characteristics of a horizontal-axis wind turbine.According to the aerodynamic configuration, the real magnitude and direction of the onflow velocity at the rotor blade can be determined, and subsequently, the aerodynamic force on the rotor can be determined. The commonly employed wake aerodynamic models are of the cylindrical form instead of the actual expanding one. This is because the influence of the radial component of the induced velocity on the wake configuration is neglected. Therefore, this model should be called a "linear model". Using this model means that the induced velocities at the rotor blades and aerodynamic loads on them would be inexact. An approximately accurate approach is proposed in this paper to determine the so-called "nonlinear" wake aerodynamic configuration by means of the potential theory,where the influence of all three coordinate components of the induced velocity on wake aerodynamic configuration is taken into account to obtain a kind of expanding wake that approximately looks like an actual one. First, the rotor aerodynamic model composed of axial(central), bound, and trailing vortexes is established with the help of the finite aspect wing theory. Then, the Biot-Savart formula for the potential flow theory is used to derive a set of integral equations to evaluate the three components of the induced velocity at any point within the wake. The numerical solution to the integral equations is found,and the loci of all elementary trailing vortex filaments behind the rotor are determined thereafter. Finally, to formulate an actual wind turbine rotor, using the nonlinear wake model, the induced velocity everywhere in the wake, especially that at the rotor blade,is obtained in the case of various tip speed ratios and compared with the wake boundary in a neutral atmospheric boundary layer. Hereby, some useful and referential conclusions are offered for the aerodynamic computation and design of the rotor of the horizontal-axis wind turbine.