Propulsive performance and vortex shedding of a foil in flapping flight

Summary. The propulsive performance and vortex shedding of an oscillating foil, which mimics biological locomotion, are investigated based on a computational fluid dynamics analysis. The objectives of this study are to investigate unsteady forces, in particular a thrust force, for the foil in pitching and plunging motion, and to deal with the relations of the propulsive performance with leading-edge vortex structure and vortex shedding in the near wake. The two-dimensional incompressible Navier–Stokes equations in the vorticity and stream-function formulation are solved by fourth-order essentially compact finite difference schemes for the space derivatives and a fourth-order Runge-Kutta scheme for the time advancement. To reveal the mechanism of the propulsive performance, the unsteady forces and the shedding of the leading- and trailing- edge vortices of the foil in the pitching and plunging motion are analyzed. Based on our calculated results, three types of the leading-edge vortex shedding, which have an effective influence on the vortex structures in the wake of the oscillating foil, are identified. The effects of some typical factors, such as the frequency and amplitude of the oscillation, the phase difference between the pitching and plunging motions, and the thickness ratio of the foil, on the vortex shedding and the unsteady forces are discussed.     

侧壁式压水室离心泵小流量非稳态旋转失速特性

用数值计算方法研究具有特殊结构的侧壁式压水室离心泵,分析小流量工况时模型泵的非稳态旋转失速特性,用快速傅里叶变换(FFT)获得压力脉动信号的频谱特征。结果表明,小流量工况时模型泵的扬程曲线呈驼峰状,压水室不同位置处压力分布不均;受叶轮旋转产生的非稳态作用影响,叶轮不同叶片流道内流动结构差异较大。不同流量下,叶轮内部分离涡结构诱发的激励频率各异,0.4ФN工况时模型泵压力脉动频谱图出现0.5fR及高次谐波频率,压力脉动最大幅值出现于4fR频率处;0.2ФN流量时非定常流动结构会诱发0.18fR及高次谐波频率;0.05ФN流量时压力脉动频谱图同时出现0.1fR、0.28fR两种激励频率。旋转失速现象出现时,频谱图中叶频处压力脉动幅值不再起主导作用。     

矩形柱二维尾流的稳定性研究

采用实验和数值模拟两种方法对长宽比为3∶1的矩形柱尾流进行了稳定性研究.实验发现当雷诺数接近临界值时,未加扰动的矩形柱尾流呈二维定常状态.当在柱体上游一定位置和下游靠近柱体的位置上沿垂直于来流方向施加一个宽度很小的短时脉冲射流扰动以后,扰动随时间放大,并出现旋涡脱落现象,并且这种扰动激发的旋涡脱落可以持续很长时间,不会衰减.而在下游较远处施加同样的扰动时,扰动将会衰减,不能激发出持续的旋涡脱落.数值模拟再现了上述实验结果,并且表明,当扰动强度(脉冲射流出口速度)较小时,不会出现持续的规则旋涡脱落,只有当扰动强度达到一定阈值时,旋涡脱落才能被激发.     

高压降迷宫套筒组合调节阀涡激振动仿真研究

针对高压降类调节阀涡激振动现象,设计多通道迷宫盘片和多级套筒组合高压降调节阀。基于计算流体动力学(CFD)和流固耦合模态分析法,计算调节阀内部瞬态流场及结构模态,得到其三种典型开度的流体流动情况,升力系数时域和频域特性曲线,以及模态频率和模态振型。分析结果表明:4s时刻,随调节阀开度减小,流体最大流速相应减小,第四级套筒外部流道区域的大涡逐渐形成小涡。调节阀全开时流体升力系数波动比70%和40%开度时更为剧烈,一阶模态频率、二阶模态频率均随开度的增大而增大。调节阀模态频率没有落在漩涡脱落主频率范围内,调节阀不会发生涡激振动锁定现象。     

Numerical Study on the Suppression of the Oscillating Wake of a Square Cylinder by a Traveling Wave Wall

Traveling waves generated on the side surfaces of a square cylinder are employed to suppress the oscillating wake for improving the flow behavior around a square cylinder; this method is termed the traveling wave wall (TWW) method. This study aimed to evaluate the influence of the key parameters of TWW on the control of aerodynamic forces and the oscillating wake of the flow around a square cylinder. Unsteady numerical analyses at a low Reynolds number (Re) of 100 were performed using a two-dimensional CFD simulation. First, the grid independence and time step independence tests of the simulation were conducted to verify the rationality of the solving parameter settings, and the validation of flow around the fixed square cylinder at Re =100 was carried out. Subsequently, the lift and drag coefficients and the vortex shedding modes under different combinations of three TWW control parameters, including wave velocity, wave amplitude, and wavenumber, were analyzed in detail. The results show that TWW can remarkably reduce the mean value of drag coefficient and the RMS value of the lift coefficient by more than 12% compared to the method involving a standard square cylinder. Two peaks occur in the lift coefficient spectrum, with the low frequency corresponding to the vortex shedding frequency in the wake of the flow around the square cylinder and the high frequency corresponding to the traveling wave frequency. The vorticity contours show that the alternating vortices in the wake of the square cylinder are not completely suppressed under the selected control parameters.     

直径30cm圆柱的气动力参数和绕流特性研究

为研究光滑圆柱的气动力系数和绕流特性,在均匀流中进行不同风速下的测压风洞试验,试验获得了阻力系数、升力系数、表面风压分布、风压相关性系数、斯托罗哈数等随雷诺数的变化特征,并将试验结果与以往结果进行比较。研究表明:升力系数的脉动值大于阻力系数的脉动值,说明涡脱造成的横风向激励比顺风向紊流激励剧烈;雷诺数位于临界区域时,圆柱表面风压分布呈现出对称-不对称-对称的变化过程,反映了由层流分离转化为湍流分离的全过程;在雷诺数为352000时呈现一侧为层流分离、另一侧为湍流分离的临界流态,风压呈现出左右不对称的单边泡形式;获得层流分离和湍流分离时的表面风压相关性分布特征,层流分离时圆柱同一侧的风压测点均呈较强的正相关,而湍流分离时在分离点前的区域相关性较强,分离点之后的区域相关性较弱;层流分离的升力系数谱有显著的峰值,表明尾流是规则的漩涡脱落,而湍流分离的升力系数谱没有明显峰值,表明尾流是随机的漩涡脱落。     

调制FFT及其在离散频谱校正技术中的应用

提出了一种调制快速傅立叶变换(FFT),通过时域调制对实信号进行移频,打破频域内的对称性,再进行传统的FFT.该算法克服了直接进行FFT计算结果有一半冗余的缺点.将频率分辨率提高了一倍.提高了频率定位的精确度,从而减小了最大的幅值和相位误差,并进一步提高了抗噪性能.以比值校正法和相位差法为例,将调制FFT应用到离散频谱校正技术中,解决了基于FFT的离散频谱校正方法由于噪声影响而出现的一些问题,进一步提高了校正精度和抗噪性能.理论分析和Monte Carlo计算机模拟实验验证了上述结论的正确性.     

喷射方法对尾流旋涡脱落的抑制

采用数值模拟的方法研究了尾部喷射对波动来流绕圆柱流动旋涡脱落的抑制,进而研究圆柱尾流控制机理.研究流场的无量纲波频范围为0².8,来流波动的无量纲幅值为0.2,雷诺数=200.在圆柱尾部沿圆柱母线开宽度为0.04倍柱体直径的缝隙,从缝隙中射出流体对尾流进行抑制,寻找可有效抑制旋涡脱落的喷射速度范围,进而求出雷诺数=200,无量纲振幅为0.2时有效抑制范围.当喷射速度在一定范围内时,可有效抑制旋涡脱落,并且随着无量纲频率的增大,有效抑制范围逐步减小.     

高层建筑风致动力干扰效应机理的试验研究

在风洞中用高频动态天平研究了两个高层建筑间的风致干扰效应，通过分析上游拖扰模型的尾流和动态天平试验中受扰模型的基底弯矩谱，得到了上游建筑的旋涡脱落，上游建筑对受扰建筑自身涡脱特性的影响以及上游建筑尾流中紊流的增强这三种基因是建筑物风致动力干扰效应的机理，最后分析了风场对动力干扰效应的影响。     

微椭圆形截面斜拉索临界雷诺数区气动特性试验研究

为了研究微椭圆形截面斜拉索临界雷诺数区的气动特性,以标准圆形截面斜拉索模型和微椭圆形截面斜拉索模型为研究对象,开展了考虑截面变形和风攻角变化的风洞试验,得到了不同情况下雷诺数对模型气动力系数的影响规律,同时通过对升力时程和升力频谱分析,得到了临界雷诺数及附近区域的标准圆形截面和微椭圆形截面模型尾流旋涡脱落的变化。结果表明:微椭圆形截面模型在雷诺数亚临界区时,升力系数基本不受雷诺数的影响;在临界区时,升力系数随雷诺数逐渐增大;风攻角0°~50°时,微椭圆形截面模型升力系数最大值对应的雷诺数随风攻角同步增大;微椭圆形截面模型的升力时程在TrBL0向TrBL1阶段和TrBL1向TrBL2阶段过渡中会出现双稳态现象;微椭圆形截面变形会影响斜拉索尾流区旋涡脱落情况,进而影响不同雷诺数下的St数值变化。     

Influence of the wake model on the thrust of oscillating foil

Trust generation by flapping wing is a complex fluid phenomenon involving unsteady effects. The work discusses a Boundary Element Method (BEM) based computer model for the analysis of hydrodynamic forces on flapping foil. The specific focus is on the wake model and its effects on the generated thrust. An unsteady formulation of the Kutta condition, assuming finite pressure difference at the trailing edge of the moving foil, was implemented in the numerical procedure to account for the shedding of trailing-edge vortical structures. It is shown that the numerical results depend strongly on the choice of the wake model, especially at large oscillation frequencies. However, the model of the thrust-producing jet (due to the trailing-edge pressure differences) predicts accurately the thrust trend as a function of the oscillation frequency. In other words, the numerical results show the need of experimental data in order to choose the appropriate wake model. An experimental validation of the numerical method is proposed on the basis of recent experimental results that clearly show the time lag between the foil vertical acceleration and generated thrust as predicted by the model. This proves the point that the unsteady BEM approach to these problems is physically sound.     

Precise and fast phase wraps reduction in fringe projection profilometry

Phase wraps in a 2D wrapped phase map can be completely eliminated or greatly reduced by frequency shifting. But it usually cannot be optimally reduced using conventional fast Fourier transform (FFT) because the spectrum can be shifted only by a integer number in the frequency domain. In order to achieve a significant phase wrap reduction, we propose a fast and precise two-step method for phase wraps reduction in this paper, which is based on the iterative local discrete Fourier transform (DFT). Firstly, initial estimate of the frequency peak is obtained by FFT. Then sub-pixel spectral peak with high resolution is determined by iteratively upsampling the local DFT around the initial peak location. Finally, frequency shifting algorithm that operates in the spatial domain is used to eliminate phase wraps. Simulations and experiments are conducted to demonstrate the superb computing efficiency and overall performance of the proposed method.     

不同立面收缩形式圆角弧边三角形超高层建筑的气动力特性

为探索立面变化对圆角弧边三角形超高层建筑气动力特性的影响,采用风洞试验方法对6种立面收缩形式的建筑物进行了表面风压测试,研究了不同立面收缩形式、不同地貌和风向角情况下建筑三分力系数、横风向弯矩系数功率谱以及层脉动升力系数功率谱的变化特性。结果显示：锥度化和同向退台均可有效降低平均阻力系数、平均升力系数以及脉动升力系数,且降幅随锥度增大而增大,相比之下退台的效果更佳;两种退台旋转可更大程度减小三个平均分力系数和脉动阻力系数,并改变脉动升力系数和脉动扭矩系数峰值所对应的风向范围。随着锥度增大,横风向弯矩系数功率谱峰值频率右移,谱峰降低,采用退台则影响程度更大。退台旋转会使功率谱部分频段的能量明显超过同向退台情况,因为当旋转部分圆角迎风时,在两侧风弧面上加速的气流可对上方相邻退台的漩涡脱落起增强效应。     

汽轮机叶顶汽封泄漏涡诱导下的压力脉动研究

以某汽轮机级为研究对象,采用三维非定常数值模拟的方法,研究了动叶顶部汽封泄漏涡的涡动特性和泄漏涡影响下汽封腔室内的压力脉动规律。结果表明:叶顶汽封腔室内存在周向螺旋状涡与壁角涡组成的多尺度涡系,涡系的位置和影响范围会随时间经历复杂的变化。叶顶汽封腔室内的压力脉动频率中兼具叶轮旋转的周期性频率和溃散为小尺度涡结构后的高频率。间隙内的多尺度涡结构和动静叶间交替变化的压力场在汽封间隙内的传播是引起汽封腔室内周向压力不均的主要原因。     

双矩形拱肋间的气动干扰效应研究

以某拱桥为例,通过数值模拟研究了串列双矩形拱肋的气动干扰效应,及其对两截面气动力系数的影响。在对计算模型进行验证的基础上,进一步研究了截面宽高比、间距比和来流风攻角对拱肋周围流场的影响,并结合压力云图和湍动能云图解释了气动力系数的变化规律,讨论了不同宽高比截面的漩涡脱落频率与结构自振频率之间的关系,分析了两拱肋升力时程的差异对整体扭矩可能产生的增大效应。结果表明：串列拱肋间的气动干扰效应显著。受上游截面尾流的影响,下游截面的阻力系数明显减小,其值与漩涡的形态、能量大小、移动轨迹等因素密切相关。上、下游截面的升力时程在幅值和相位上存在明显差异,导致拱肋整体的力矩增大,其效应随宽高比或间距比的增大而明显加强,随风攻角的增大而有所降低。漩涡脱落频率随宽高比的增大呈先增大后减小的趋势,而受间距比、风攻角的影响有限。对漩涡脱落频率与宽高比的变化进行多项式拟合,结合结构的模态频率可为拱肋的气动外形设计提供参考。     

Fluctuations of DC atmospheric double arc argon plasma jet

An original DC double anode plasma torch has been devised and tested, which generates a long-time and highly stable argon plasma jet at atmospheric pressure. The arc unsteadiness and dynamic behaviour are investigated through the fluctuations of electrical signals combined with the classical tools, such as the statistic method, fast Fourier transform (FFT) and correlation analysis. The current-voltage characteristics of the argon plasma jet show an increasing tendency. The takeover mode is identified as the typical fluctuation behaviour of the double arc argon plasma jet in our experiment. The FFT and correlation calculation results of electrical signals exhibit the only characteristic frequency of 150 Hz, whereas the high frequency fluctuations are totally disappeared. Such low frequency fluctuation (150 Hz) can be attributed to the inherent characteristic of the torch power source and is independent of any change in the operating parameters. These results indicate that the nature of fluctuation in an argon plasma jet is mainly induced by the undulation of tri-phase rectified power supply. In addition, both arcs are characterized as the one and only system, and each arc root attachment is rather evenly diffused than located at a fixed position on the anode surface. The time shift derived from the cross correlation between the arc voltage and current intensity can be used as an independent parameter to reflect the fluctuating behaviour in the plasma jet.     

一种基于充气气囊的垂尾抖振抑制新方法研究

通过数值模拟探索了一种运用充气气囊抑制双垂尾抖振的新方法。该文方法利用充气气囊可迅速充气变形的特点,在三角翼上翼面靠近顶点沿涡核的位置设置气囊。在小迎角下气囊不凸起,从而保证机翼前缘涡的强度以产生非线性涡升力;当大迎角抖振现象较严重时,迅速对气囊充气形成凸起,该凸起通过对前缘分离涡的强度和涡空间位置的影响,减弱涡破裂对双垂尾的非定常气动载荷激励,达到抑制抖振的目的。对某三角翼双垂尾布局模型的计算结果表明:气囊可以使前缘涡的涡核弯曲、扭转,减弱了前缘涡的强度,使前缘涡破裂点位置提前,在大迎角范围可将垂尾绕翼根的弯矩值显著减小,并且减小了垂尾表面压力脉动的幅度和对应的功率谱密度的峰值。因此,该文所探索的利用充气气囊抑制抖振的方法是一种简单可靠,并且值得进一步研究的技术途径。     

Shear flow past a square cylinder near a wall

A numerical study on the wake behind a square cylinder placed parallel to a wall has been made. The cylinder is considered to be within the boundary layer of the wall, so that the outside flow is taken to be due to uniform shear. Flow has been investigated in the laminar Reynolds number (based on the cylinder height) range. The interaction of wall boundary layer on the vortex shedding at Reynolds number up to 1400.0 has been investigated for cylinder to wall gap height 0.5 and 0.25 times the cylinder height. The gap flow between the cylinder and wall during a period of vortex shedding has been obtained. The governing unsteady Navier–Stokes equations are discretised through the finite volume method on staggered grid system. An algorithm SIMPLE has been used to compute the discretised equations iteratively. Our results show that at the gap height 0.5 times the cylinder height the vortex shedding occurs at a Strouhal number greater than for an isolated cylinder. Vortex shedding suppression occurs and wake becomes steady up to a certain value of Reynolds number at gap height 0.25 time the cylinder height. At higher Reynolds number the formation of a single row of negative vortices behind the cylinder when it is in close proximity to wall is noteworthy. Due to the shear, the drag experienced by the cylinder is found to decrease with the reduction of gap height.     

HEV动力切换过程车内振动与噪声试验分析

为对某混合动力汽车动力切换过程中车内振动与噪声进行研究,测试正副驾驶地板的振动加速度、正驾驶双耳噪声、副驾驶右耳以及正驾驶后排右耳的噪声、发动机转速、蓄电池电量(SOC)。数据处理采用小波包变换、时域以及频域分析法,分析动力切换过程中车内振动与噪声。试验结果表明,动力切换时的振动与噪声都呈现明显的非稳态特征且能量集中分布在低频段,与稳定车速纯电动模式下的幅值相比较差异明显;在558 Hz处振动幅值明显加强,与起动电机拖动发动机起动快速上升的转矩有关系;在800 Hz处振动幅值明显加强,与动力耦合装置动力输出不平稳有关系。     

抗噪性联合变换相关器

我们对联合变换相关器的输入图象进行Roberts梯度处理,同时对参考图象采取相移技术,并在频域中将功率谱二值化。与传统的联合变换相关器（CJTC）相比,新型的联合变换相关器（NJTC）消除了功率谱的直流分量,锐化了相关峰强度,提高了衍射效率和识别能力,同时增强了联合变换相关器的抗噪性能。用计算机模拟出预言结果。