Effects of hysteretic and aerodynamic damping on supersonic panel flutter of composite plates

The governing equation for the finite element analysis of the panel flutter of composite plates including structural damping is derived from Hamilton's principle. The first order shear deformable plate theory has been applied to structural modelling so as to obtain the finite element eigenvalue equation. The unsteady aerodynamic load in a supersonic flow is computed by using the linear piston theory. The critical dynamic pressures for composite plates have been calculated to investigate the effects of structural damping on flutter boundaries. The effects are dependent on fiber orientation because flutter mode can be weak or strong in the fiber orientation of composite plates. Structural damping plays an important role in flutter stability with low aerodynamic damping but would not affect the flutter boundary with high aerodynamic damping.     

The effect of damping and mass ratio on the stability of a tube bank

Experiments have been conducted in a wind tunnel on a parallel triangular tube array with a pitch ratio of 1·375. The aerodynamic component of damping was determined as a function of flow velocity. This damping was found to increase linearly with flow to about 50% of the stability threshold and then to decrease. Additionally, six different sets of tubes were tested to examine the effect of mass ratio on fluid-elastic stability. The results were compared with similar experiments in which damping alone was varied. It is seen that damping and mass ratio do not appear to be linearly dependent parameters which is contrary to the assumption used commonly in the stability analysis of tube banks. More importantly, however, the stability threshold is substantially less dependent on both damping and mass ratio than indicated by conventional theory.     

Oscillating reed valves--an experimental study

The results of experiments on the threshold behavior and large-amplitude oscillation of "outward-swinging door" vibrating flap valves in an air environment are reported and compared with the predictions of a simple nonlinear theory that parametrizes aerodynamic effects by means of a simple damping coefficient together with a contraction coefficient for the flow. The agreement is acceptably good for the threshold blowing pressure for valve oscillation, the large-signal vibration amplitude, the pressure jump in the transition from threshold to large-signal behavior, and the variation in vibration frequency, all as functions of reservoir volume. The calculated pressure waveform in the reservoir has the observed phase and magnitude but fails to reproduce finer details. It is concluded that the simple theory provides an adequate account of the behavior of such valves. There are just two parameters in the theory, describing jet contraction and aerodynamic damping, respectively. Since these may depend significantly upon the detailed geometry, valves with different shapes may behave in quantitatively different ways.     

A theoretical investigation of flow-induced instabilities in compliant coatings

The flow-induced instabilities of a fairly general class of compliant coatings are investigated theoretically. The coatings are of finite length and consist of elastic plates or membranes stretched over a fluid substrate having a density which may be different from the main flow. Provision is also made for the plate to be backed by an elastic foundation of arbitrary spring stiffness. Fairly standard aeroelastic methods are followed. The aerodynamic forces generated by the main flow are calculated by using thin aerofoil theory with a correction factor to allow for the presence of a boundary layer. The pressure induced in the substrate fluid is calculated by assuming potential flow and applying the method of images. A single-mode analysis shows that coatings with laminar boundary layers suffer a divergence-type instability in contrast to turbulent boundary layers which always give rise to a flutter-type instability with a higher critical velocity. The order of the most dangerous mode is calculated and found to rise with an increase in equivalent spring stiffness for fixed tension or flexural rigidity. Results are presented for plates and membrane coatings with an air stream over air and water substrates. Taking account of the substrate fluid dynamics reduces the growth rate of the instability by an order of magnitude and completely suppresses flutter with water substrates. Single-, double- and triple-mode analyses are carried out and the results compared. The critical velocity is adequately predicted by single-mode analysis but a coupling of odd and even modes can lead to flutter even with a laminar boundary layer.     

ABSOLUTELY UNSTABLE WAVES IN INVISCID HYDROELASTIC SYSTEMS

The effect of inviscid plug flow on the stability of several hydroelastic systems is investigated by determining the absolute or convective nature of the instability from the linear dispersion relation. The fluid-structure systems consist of plates and membranes with bounded and unbounded flow. A method is proposed to derive systematically in parameter space the boundary between convective and absolute instability, based on the particular symmetries of the dispersion relation as originally noted by Crighton and Oswell. This method is then applied to the case of plates with superimposed tension, thick plates with rotary inertia and walls made of plates or membranes bounding channel flow, oscillating in a sinuous or varicose mode of deformation. A relation is drawn with solutions by previous authors for plates, for pipes and for the Kelvin-Helmholtz instability with surface tension. To illustrate these results some temporal evolutions are calculated by using an integration in the wavenumber space. Based on the large set of new cases solved in the paper some general trends are discussed as to the influence of flow velocity, confinement and structural stiffness on the existence of absolutely unstable waves in inviscid hydroelastic systems.     

Noise Sources of Aerodynamic Origin in Air Blowers

The paper presents the results of theoretical and experimental studies of the occurrence and locations of aerodynamic noise sources in air blowers related to air flow around stationary and moving elements inside a machine body. These studies were based on basic research by Lighthill and Curle and used a developed method for measuring pressure pulsations on rotating blades and stationary elements of a machine body. The most significant sources of discrete and broadband components of aerodynamic noise were revealed. The role of blades in an impeller in the emission of discrete noise components was studied. It was established that broadband peaks in the emitted noise are associated with acoustic resonances of the internal volume of the air blower. It was shown that the turbulence and velocity of the incoming flow influence the intensity of aerodynamic sources inside the body. Our studies spurred both deeper research into the nature of aerodynamic noise sources that form in air blowers and recommendations for reducing the noise produced by these sources.     

等离子体气动激励机理数值研究

基于介质阻挡与准直流电弧放电的物理过程, 分析了它们的气动激励机理, 建立了各自的气动激励模型, 并分别研究了它们对低速和超声速流动的激励效果. 结果显示: 介质挡板放电等离子体气动激励机理是改变了连续流体中的三种力, 即由牛顿内摩擦引起的剪切应力、由电动力学引起的体积力及由压力突变引起的冲击力, 其中基于电动力学的体积力效应占主导地位; 临近空间环境中体积力的作用效果更强, 诱导速度更大; 超声速来流下准直流电弧放电气动激励机理主要是等离子体的热阻塞效应, 本文所建立的爆炸丝传热模型可以用于仿真其控制激波的过程; 热电弧对于超声速来流而言就像一个具有一定斜坡角度的虚拟突起, 可用于高超声速飞行器前体激波的控制.     

交变流动中突变截面局部损失特性分析

从突变截面流道内流体满足的方程组出发,给出交变流动中突变截面阻力系数的定义以及考察方法,采用量纲分析法获得影响局部阻力特性的四个无量纲影响参数：动态雷诺数与幅值雷诺数之比、幅值雷诺数、变截面面积比、声场压力、速度相位差。通过PW（粒子成像测速仪）测量,分析了流场结构特征,并与CFD计算结果对比,验证了CFD计算结果的可...     

交变流动中压力速度之间相位差对突变截面损失的影响分析

从弱非线性热声理论出发,给出交变流动中突变截面阻力系数的定义以及考察方法。通过PIV(粒子成像测速仪)测量与CFD计算结果的对比,验证了CFD计算结果的可靠性。进而利用CFD模拟考察了交变流动中压力与速度之间相位差对突变截面局部损失的影响,观察到产生这种影响的流场内部流动机制。分析表明,阻力特性确实对声场压力与速度之间的相位差存在依赖性,但这种依赖性会随着非线性的增强而减少。     

振荡射流改善翼型气动性能的实验研究

本文针对采用振荡射流控制流动分离改善大攻角下翼型气动特性的问题,在NACA0015翼型上进行了多种工况的风洞实验。结果表明:在失速攻角附近,振荡射流抑制流动分离提高翼型升力系数的作用十分明显,可将翼型失速攻角推迟2°左右。存在最佳的振荡射流频率段、射流动量范围和射流位置,使得翼型性能的改善最大。实验还得到了振荡射流的频率、动量和施加位置等参数对翼型气动性能的影响规律。     

宽弦高速跨音风扇颤振特性研究

颤振是航空发动机、燃气轮机等运行安全的重要威胁,但颤振稳定性与流动结构之间的关系尚不清晰。本文使用行波法和影响系数法,对某宽弦复合掠型高速跨音风扇转子的一阶模态进行了颤振特性研究,计算了在100%转速下从堵塞点到近失速点的颤振表现。使用影响系数法时,分析了不同通道数的计算域对气动阻尼计算的影响,并与行波法得到的结果进行了对比。研究了流动结构与叶片表面气动阻尼之间的关系,旨在提高对流动致颤机理的认识。结果表明影响系数法和行波法均能对叶片的气动阻尼进行较好的预测;流动结构方面,激波、激波附面层分离、叶尖泄漏流以及吸力面前缘叶顶附近的非定常压力波动,对叶片的气动阻尼分布有较大的影响。     

振荡液滴内部流态

在外界来流作用下,液滴在固体表面上呈现周期性振荡特性.利用数值方法模拟平板上二维液滴在气流剪切作用下的界面及内部流动特性,重构二维液滴内部流场,着重认识液滴内部速度分布和压力分布.     

The study of hydrodynamic processes of a coolant flow in FA-KVADRAT PWR with various mixing spacer grids

The article presents the results of experimental studies of the influence of different designs of mixing spacer grids on the coolant flow in FA-KVADRAT PWR. The investigations were carried out by simulating the coolant flow in the core on the experimental air stand being an aerodynamic open loop through which air is pumped. To measure the local hydrodynamic characteristics of the coolant flow, special pneumometric sensors were used for measuring the total velocity vector and the flow rate value. During the investigations of the local hydrodynamics of the coolant, transverse flow velocity, as well as the coolant flow rates in the cells of the experimental model FA-KVADRAT, was measured. The analysis of spatial distribution of projections of absolute flow velocity allowed studying and detailing the coolant flow pattern behind the mixing spacer grids with different designs of deflectors as well as selecting the optimal design of the deflector. The accumulated database on the coolant flow in FA-KVADRAT has formed the basis for engineering assessment of active zones’ structures of PWR. The results of experimental studies are used to verify CFD codes (in both foreign and domestic development) as well as programs for detailed cell-by-cell calculation of active zones in order to reduce conservatism when assessing heat engineering reliability.     

微尺度振荡Couette流的格子Boltzmann模拟

采用有效多松弛时间-格子Boltzmann方法(Effective MRT-LBM)数值模拟了微尺度条件下的振荡Couette和Poiseuille流动. 在微流动LBM中引入Knudsen边界层模型,对松弛时间进行修正. 模拟时平板或外力以正弦周期振动,Couette流中考虑了单平板振动、上下板同相振动这两类情况. 研究结果表明,修正后的MRT-LBM模型能有效用于这类非平衡的微尺度流动模拟;对于Couette流,随着Kn数的增大,壁面滑移效应变得越明显. St越大,板间速度剖面的非线性特性越剧烈;两板同相振荡时,若Kn,St均较小,板间流体受到平板拖动剪切的影响很小,板间速度几乎重叠在一起;在振荡Poiseuille流动中,St数增大到一定值时,相位滞后现象减弱;相对于Kn数,St数对振荡Couette 和Poiseuille流中不同位置处速度相位差的产生有较大影响. 关键词： 格子Boltzmann方法 有效MRT模型 Knudsen层 振荡流     

The influence of the wake on the stability of cantilevered flexible plates in axial flow

Cantilevered flexible plates in axial flow lose stability through flutter. Using the inextensibility condition for the cantilevered nonlinear plate equation of motion and the unsteady lumped-vortex model to calculate the fluid loads, a flutter boundary has been obtained. In the time-domain analysis performed to this end, the wake behind the oscillating cantilevered plate is assumed to issue tangentially from the free trailing edge and extend downstream with an undulating form. The influence of the wake on system stability may be characterized in terms of the non-dimensional mass ratio, reduced flow velocity and flutter frequency. For large values of the mass ratio, the plate vibrates with high frequency and high-order mode content. It is shown that the wake has less influence on system stability for long plates than it does for short ones.     

旋转飞行器动稳定导数获取原理

旋转飞行器具有独特的周期性非定常气动现象，其动态特性更加复杂，气流与旋转高度非定常关联，俯仰阻尼导数和Magnus力矩导数同等重要.文章从飞行器气动力建模的理论基础出发，考虑飞行器自转角速度非零的事实，将其作为基态影响参数，重新构建了旋转飞行器气动力数学表达式，并借鉴成熟的常规动导数强迫振动法，采用Fourier级数表征旋转影响，沿用旋转飞行器周期气动等效概念，建立了基于基态旋转流场的周期气动等效平均俯仰动稳定导数、纵向洗流时差导数和滚转动稳定导数获取的原理表达式，并设计了相应的运动模式，可供数值计算和风洞实验使用.      

Hydromagnetic Blood Flow of Sisko Fluid in a Non-uniform Channel Induced by Peristaltic Wave

In this paper, a smooth repetitive oscillating wave traveling down the elastic walls of a non-uniform twodimensional channels is considered. It is assumed that the fluid is electrically conducting and a uniform magnetic field is perpendicular to flow. The Sisko fluid is grease thick non-Newtonian fluid can be considered equivalent to blood. Taking long wavelength and low Reynolds number, the equations are reduced. The analytical solution of the emerging non-linear differential equation is obtained by employing Homotopy Perturbation Method(HPM). The outcomes for dimensionless flow rate and dimensionless pressure rise have been computed numerically with respect to sundry concerning parameters amplitude ratio ?, Hartmann number M, and Sisko fluid parameter b1. The behaviors for pressure rise and average friction have been discussed in details and displayed graphically. Numerical and graphical comparison of Newtonian and non-Newtonian has also been evaluated for velocity and pressure rise. It is observed that the magnitude of pressure rise is maximum in the middle of the channel whereas for higher values of fluid parameter it increases. Further, it is also found that the velocity profile shows converse behavior along the walls of the channel against multiple values of fluid parameter.     

振荡射流提高翼型升力的机理研究

本文数值模拟了施加振荡射流以及相应定常吸气条件下的翼型分离流动。对振荡射流改善翼型升力的机理进行了研究。结果表明,翼型表面施加的振荡射流能够控制流动分离的形态,提高分离区流体的湍流度,增强分离区内部流体,以及与主流的动量和能量交换,增强近壁区流体的动能,降低翼型吸力面压力,而对压力面无显著影响,因而翼型的升力得到提高。     

Measured wavenumber: frequency spectrum associated with acoustic and aerodynamic wall pressure fluctuations

Direct measurements of the wavenumber-frequency spectrum of wall pressure fluctuations beneath a turbulent plane channel flow have been performed in an anechoic wind tunnel. A rotative array has been designed that allows the measurement of a complete map, 63×63 measuring points, of cross-power spectral densities over a large area. An original post-processing has been developed to separate the acoustic and the aerodynamic exciting loadings by transforming space-frequency data into wavenumber-frequency spectra. The acoustic part has also been estimated from a simple Corcos-like model including the contribution of a diffuse sound field. The measured acoustic contribution to the surface pressure fluctuations is 5% of the measured aerodynamic surface pressure fluctuations for a velocity and boundary layer thickness relevant for automotive interior noise applications. This shows that for aerodynamically induced car interior noise, both contributions to the surface pressure fluctuations on car windows have to be taken into account.     

振荡流共轭换热格子——Boltzmann模拟

振荡流共轭换热现象广泛存在于热声热机等工程应用中.基于双分布格子-Boltzmann模型,对平行平板间振荡流共轭换热进行了数值模拟.通过假定共轭界面处流体和固体的未知内能分布函数均为对应的平衡态滑移修正格式,提出了一种处理共轭换热边界的新方法.模拟结果表明,该方法可以保证共轭界面上温度连续和热流连续.分析了不同流体与固体导热系数比情况下振荡流共轭换热的速度场、温度场以及热流分布的特点.